WO1995020180A1

WO1995020180A1 - Display system and method

Info

Publication number: WO1995020180A1
Application number: PCT/US1995/001036
Authority: WO
Inventors: James L. Fergason
Original assignee: Fergason James L
Priority date: 1994-01-25
Filing date: 1995-01-25
Publication date: 1995-07-27
Also published as: AU1733195A

Abstract

A display system which includes a source of polarized light (30), an analyzer (22) for reflecting or transmitting light as a function of wavelength and polarization, and a display (63) which changes the polarization of the light from the analyzer and then directs the light to the analyzer for reflection or transmission. A method of display including providing polarized light, analyzing by reflection and transmission in accordance with polarization and wavelength, generating a pattern by changing the polarization of light from the analyzer, then analyzing the light with the analyzer. A reflective variable birefringence liquid crystal cell with an active matrix substrate, liquid crystal at one substrate having homeotropic alignment and a state with minimal phase retardation at the other substrate. A display including a light modulator and light source, and a control for the intensity of light from the source. A method of displaying an image using passive light modulation including controlling the intensity of a light source as a function of the brightness of the image to be displayed.

Description

Title: DISPLAY SYSTEM AND METHOD

TECHNICAL FIELD The present invention relates generally, as is indicated, to optical illumination systems, displays, especially relatively small size displays, and methods of using displays . The invention is particularly useful in head mounted displays, such as those sometimes referred to as heads-up displays, virtual reality displays, and multimedia displays and in projection displays and in other displays, too.

BACKGROUND In the field of optical display technology it is desirable to provide uniform, bright and efficient illumination. Some displays in the past have been relatively brighter at one or more portions and relatively less bright at others. It would be desirable to provide for substantially uniform illumination brightness across the area of an optical display. Some displays in the past have presented different color effects across the face. It is desirable to provide for substantially uniform color across the area or face of a display.

Some prior optical displays have required relatively large dimensions in order to distribute uniformly over the surface of the display illuminating or incident light. However, it often is desirable to minimize such display dimensions. For example, in the field of head mounted displays, such as those used in the field of virtual reality, heads-up display technology, etc. where the display is mounted on the head of an observer, it is desirable to minimize the size and weight of the display and the illuminating source.

Various techniques have been used in the past to improve efficiency of light illumination systems for displays and for the developing of light output by the display for viewing. For example, in some prior displays the intensity of the output light may be as small as only a few percent of the intensity of the illumination source for the display. It would desirable to increase the amount of output light able to be produced by a display relative to the amount of input light provided the display.

One of the problems in conventional liquid crystal displays, such as twisted nematic liquid crystal displays, is the inefficiency in light usage. Since the light usually needs to go through two polarizers, and additional color filters for color displays, the output light often only is two to four percent of the intensity of the incident light from the illuminating source. In liquid crystal television and computer displays, for example, the display is a transmissive one; the light source is on one side of the display while the viewer views the display from the opposite side. The losses in light in such systems are substantial and, therefore, such systems require a substantial amount of power for the light source to provide adequate illumination for satisfactory viewing. If the light source is supplied by battery power, usually the battery life is relatively short because of such substantial power requirement. It would be desirable to improve illumination efficiency for displays and, therefore, to increase battery life for such displays and systems using them.

Various head mounted displays and optical systems have been developed in the past. The present invention is useful to provide illumination for head mounted displays and for other types of displays.

The present invention may be used in an optical system or in a display system in which a common image is provided to both eyes of a person, and the invention- also may be used in devices and displays in which different images are provided to respective eyes of a person. When different images are provided, they may be coordinated or not; when they are coordinated, though, they may be so coordinated so as to provide three dimensional or stereoscopic imaging. The invention also may be used with projection displays or as part of a projection display.

Liquid crystal alignment usually refers to the alignment of the liquid crystal structure, especially when referring to nematic liquid crystal material. Alignment also may refer to the direction of the liquid crystal director, structural organization of the liquid crystal material, etc., as is well known in the art. Several of the types of liquid crystal material include nematic liquid crystal, smectic liquid crystal and cholesteric liquid crystal. Nematic liquid crystal tends to align directionally with respect to a surface of a liquid crystal cell with which the liquid crystal is in relative proximity (meaning next to or near) or is directly engaged, etc.

The surface treatment referred to above tends to cause that liquid crystal material which is generally in proximity to the particular substrate to align in a preferred direction. Examples of surface treatment include rubbing the surface with cotton, felt, or some other material in a particular direction, which causes the liquid crystal material to align relative to that direction. Another example of suifece treatment includes applying a silicon oxide (SiO) coating to the surface using an evaporation technique; depending on the angle of evaporation relative to the substrate surface and other factors, which are known in the art, the liquid crystal material will tend to align in a particular direction, which usually is generally parallel to the surface but at some tilt angle, as is well known. Other examples of surface treatment includes the applying of a polyvinyl alcohol (PVA) material to the surface or a polyimide coating to the surface. The various coatings also may be rubbed using cotton, felt or some other material to provide the desired alignment characteristics.

Most current displays in head mounted display systems utilize the twisted nematic mode of operation of the liquid crystal cell(s) used in such displays. The speed of this type of display is relatively slow causing the image to tend to smear when rapidly changing video is displayed. The speed of response (switching speed from one light transmissive condition to the other, for example) is too slow to allow color images to be created by color sequential addressing also referred to as field sequential or frame sequential operation. It would be desirable to provide increased switching speed of operation in a liquid crystal display device. The switching speed of at least some variable birefringence liquid crystal cells has been found to be faster than that of twisted nematic liquid crystal cells. (See, for example, U.S. patents Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,396.)

An example of field sequential or frame sequential operation, sometimes also referred to as color sequential addressing, is described in U.S. patent No. 4,582,396. A liquid crystal display sequentially presents respective images at a speed faster than the human eye can separately distinguish or follow. The sequential images are separated in time rather than in space. The images are merged or integrated by the eye to compose an image which in effect is a combination of several sequential images. In this way images having multiple colors can be produced. For example, in the mentioned patent using a series of color filters that are responsive to the direction of plane polarized light a multicolor image can be produced; at one moment in time an image is produced and is filtered by one color filter; and at a subsequent moment in time the same or a correlated image is produced and is filtered by a second color filter; the images are combined (integrated) by the viewer's eye so that a combined image is seen. In the absence of a prescribed input, such as an electric field, to the liquid crystal cell, the alignment of liquid crystal material usually is influenced by the surface and surface treatment, especially for nematic liquid crystal or operationally nematic liquid crystal. Operationally nematic means the liquid crystal operates sufficiently similarly to nematic liquid crystal as to be useful in the present invention described below. However, upon application of an electric field, at least some of the liquid crystal material tends to align with respect to the field, which tends to overcome the influence of the surfaces on liquid crystal alignment. The stronger the electric field, i.e., the greater the magnitude of the field or the voltage causing the field, the greater the amount of the liquid crystal material that tends to align with respect to the field and/or the more accurately the liquid crystal aligns with the field. The electric field may be developed by applying a voltage between an electrode located on one of the substrates and an electrode on the other of the substrates. In some liquid crystal cells known as active matrix or thin film transistor (TFT) devices, a number of electronic components, such as transistors, capacitors, etc., may be provided at or on one of the substrates or surfaces thereof to develop the appropriate electrical energization for liquid crystal material in the liquid crystal cell at one or more locations in the cell.

In an exemplary flat panel display technology an active matrix liquid crystal display is fabricated from substrates of amorphous or polysilicon thin film transistor arrays deposited on quartz or glass. Displays of this type are typically back lit and viewed in transmission. They suffer from several disadvantages. First, to obtain a colored image filters are deposited at each pixel (sometimes referred to in the art as a picture element where a portion of an image is created; the sum of all or many of the pixels of the display can be used to create a total image, as is well known). The fabrication of such filters is a difficult and expensive process and results in a display with a dramatically reduced light transmission as well as 1/3 reduction in image resolution since three filters (e.g., red, green and blue) are required at separate areas of each pixel. Second, the thin film transistors of such arrays must be physically large causing a large fraction of each pixel to be non-functional optically, which reduces the amount of light output capability for each pixel. Also, since the traces of such arrays are opaque and black in transmission, the inter pixel spaces are emphasized. An alternate approach addressing the above deficiencies of the substrates described above has been to fabricate a single crystal silicon array on a wafer. This allows the use of conventional semiconductor processing, and the transistors can be physically smaller than those mentioned above. However, the array must be "lifted" from the wafer and deposited onto the glass substrate. The process to do this introduces additional steps to the wafer fabrication process and adds to the cost of the substrate.

As is described further below, the present invention relates to liquid crystal cells and displays which are fabricated directly on the semiconductor wafer substrate. A disadvantage of an active matrix substrate used in a liquid crystal cell is the non-uniformity of the surface thereof, which usually has various peaks and valleys in the surface due to the electronic components formed therein. Such surface non- uniformity may have a noticeable degrading effect on the quality of images produced by a liquid crystal cell. For example, a change in path length of light in such a liquid crystal cell or a random alignment or misalignment of liquid crystal material due to a peak or a valley in a substrate may uncontrollably change optical phase retardation. This negative impact on the display is compounded if the display is used in a reflective mode because light then transmits through the liquid crystal twice.

To increase the contrast, resolution, and brightness of images created by a liquid crystal cell in a display and to facilitate manufacturing, it would be desirable to use an active matrix drive device for the liquid crystal cell. In the present invention these advantages can be accomplished, for example, by using a display operated in reflective mode. The active matrix transistors then can be located in the substrate beneath the electrodes of the matrix array, if desired, which increases the optical operational area of each pixel. In contrast, in a transmission display the respective active matrix transistors block light in part of each pixel. However, the fact that the surface of an active matrix substrate is rough or unsmooth, which disrupts a uniform liquid crystal alignment, has lead away from using such a substrate, especially in a variable birefringence liquid crystal cell. Nematic liquid crystal and smectic liquid crystal can have characteristics of birefringence, whereby the ordinary index of refraction and the extraordinary index of refraction are different. In a variable birefringence liquid crystal cell, by changing the orientation or alignment of the liquid crystal (or some of the liquid crystal) relative to the direction of light propagating through the liquid crystal, optical phase retardation can be varied correspondingly. Examples of variable birefringence liquid crystal cells in which optical phase retardation can be varied are described in U.S. Patents Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,396, which are incorporated by reference. As is described in those patents, by changing the applied input, such as electric field, the alignment of liquid crystal material in the liquid crystal cells can be altered thereby to alter the effective optical phase retardation of the light transmitted through the liquid crystal material. As also is described in the just-mentioned patents, the liquid crystal material in proximity to the respective substrates has generally homogeneous alignment; these portions of the liquid crystal material or liquid crystal layer sometimes are referred to as the surface layers of the liquid crystal material and it is these layers or at least parts thereof which switch alignment in response to applied field input during operation of the liquid crystal cell to change the optical phase characteristics of the liquid crystal cell in response to say application or removal of electric field. The surface layers or surface portions are separated by a portion of the liquid crystal material or a layer thereof which generally is aligned perpendicularly with respect to the surfaces. Such perpendicularly aligned liquid crystal material tends not to contribute to optical phase retardation (or whatever contribution it has is relatively minimal compared to the possible phase retardation provided by the surface portions). Such generally perpendicularly aligned liquid crystal material also may tend to separate the physical/mechanical interaction of the two surface portions of liquid crystal material during operation of the liquid crystal cell as the surface portions switch from one alignment to the other. The liquid crystal material which tends to separate the surface portions sometimes is referred to as the "bulk" liquid crystal; whether the bulk is more or less quantity of liquid crystal than the surface portions does not deter use of such label "bulk". Various means may be used to align the bulk portion of the liquid crystal material. Those means may be electrical, mechanical, a combination thereof, or some other means, for example, as is described in the aforementioned patents. It would be desirable to provide variable optical phase retardation capability in a reflective liquid crystal cell and display using such a cell, and, especially, to do so in a liquid crystal cell that has an active matrix type substrate. It would be desirable to provide substantial uniformity of operation and optical phase retardation characteristics in a variable birefringence liquid crystal cell while reducing the affect of and/or without regard to disparities in cell thickness due to peaks and valleys in the substrate. The displaying of a dark scene using a display device (sometimes referred to as a passive display), which modulates light received from a separate light source, encounters a disadvantage which ordinarily is not present for displays which produce their own light, such as a cathode ray tube (CRT). The problem has to do with reduced resolution and/or contrast of the displayed image. In a CRT, for example, when it is desired to display a dark scene, the intensity of the output light can be reduced. The different parts of the dark scene, then, all may be output at the reduced brightness or illuminance level. All pixels (e.g., picture elements, phosphor dots in a monochrome display or group of three red, green and blue phosphor dots for a multicolor display, etc.) of the CRT can be active so that resolution is maintained even though intensity of the light produced by the phosphors is reduced.

However, in a passive display device, such as a liquid crystal display, an electrochromic display, etc., whether of the light transmitting type or of the light reflecting type, the usual practice to reduce brightness of a displayed image or scene is to reduce the number of pixels which are transmitting light at a particular moment. Such a reduction reduces the resolution of the display. Also, such a reduction can reduce the contrast of the display.

The human eye has difficulty distinguishing between seeing or recognizing the difference between low and high brightness and contrast ranges. This difficulty is increased when the number of pixels is decreased and resolution is degraded. It would be desirable to improve the contrast and resolution of passive displays.

The present invention relates generally, as is indicated, to variable intensity high contrast passive displays and methods of displaying images.

SUMMARY Briefly, according to an aspect of the invention an illumination system for a display includes a source of polarized light, an analyzer for reflecting light to the display or transmitting light as a function of the wavelength and/or polarization characteristic of incident light, and light from the display being directed to the analyzer for reflection or transmission thereby.

According to another aspect, a method for illuminating a display includes supplying light having a polarization to an analyzer, reflecting or transmitting light by the analyzer as a function of the wavelength and polarization characteristic of incident light, the reflecting including reflecting light to a display for illuminating the display.

According to a further aspect, a display system includes a source of polarized light, an analyzer for reflecting or transmitting light as a function of the polarization characteristic of incident light, and a display receiving light from the analyzer, the display being operable selectively to change a polarization characteristic of incident light, and light from the display being directed to the analyzer for reflection or transmission thereby.

According to another aspect of the invention, input polarized light is supplied to a reflector, the reflector is operative to reflect such incident light as a function of the polarization characteristics and/or wavelength characteristics of such light, the reflected light being directed to a device for utilizing the light, such as an optical display or the like, and light from such utilizing device being directed back to the reflector for transmission or further reflection.

According to still another aspect, a display includes a light source for supplying light having a polarization characteristic, a reflector for reflecting light having one polarization characteristic and transmitting light of a different polarization characteristic, a light modulating display for receiving light reflected by the reflector and for directing light toward the reflector, the display including a means that is selectively operable to change the polarization characteristic of light thereby to determine what light is transmitted and what light is reflected by the reflector.

According to even another aspect of the invention, an optical device includes a source of polarized light, an analyzer, a folded optical system coupling the source and the analyzer, the folded optical system including a reflector reflecting light toward the analyzer, the analyzer including a selective analyzer for transmitting light having one polarization characteristic and for reflecting toward the reflector light having a different polarization characteristic. According to yet another aspect, a high intensity folded illumination system includes a source of light, a first polarizer for polarizing light from the source with a circular polarized characteristic, an analyzer for reflecting incident light from the light source that has the same direction of polarization as the mentioned polarizer, a reflector for reflecting light received from the analyzer back toward the analyzer for transmission or reflection by the analyzer as a function of the polarization characteristic of such light, and a display operative to change the polarization characteristic of light directed to the analyzer in order to determine what light is transmitted and what is reflected by the analyzer. Still a further aspect of the invention relates to a high intensity folded illumination system for an optical display including a source of polarized light having a particular polarization characteristic, a display operative to alter the polarization characteristic of light directed thereto, an analyzer for receiving light from the source and directing the light to the display, the analyzer being operative to reflect light at a particular call polarization characteristic and to transmit light that does not have such polarization characteristic, and a reflector for reflecting light from the display to the analyzer for selective transmission by the analyzer as a function of the polarization characteristic of light.

Another aspect relates to a cholesteric liquid crystal reflector having a configuration to improve uniformity of distribution of light to a display.

A further aspect relates to providing a white or multicolor liquid source for a display.

An additional aspect relates to frame sequential or field sequential illuminating of a display using different color light sources. Still another aspect relates to displays using illumination sources and various features hereof.

Another aspect relates to illumination and output techniques for head mounted displays and the like.

Another aspect relates to illumination and output techniques for projection displays.

An aspect relates to a method of display including providing a source of polarized light, analyzing polarized light by selectively reflecting or transmitting light as a function of the wavelength and polarization characteristic of incident light, creating a desired light pattern from light received from said analyzing step by selectively changing a polarization characteristic of incident light, and selectively transmitting light according to such pattern. According to an aspect of the invention, a liquid crystal cell includes a birefringent liquid crystal material, a pair of surfaces containing the birefringent liquid crystal material therebetween, the birefringent liquid crystal material in proximity to one surface providing relatively minimal optical phase retardation, the birefringent liquid crystal material being preconditioned to switch between and/or in a range of retardations to provide relatively minimal optical phase retardation and increased optical phase retardation.

According to another aspect, a birefringent liquid crystal apparatus includes a pair of surfaces; liquid crystal material between the surfaces, the liquid crystal material in relative proximity to one surface exhibiting generally homeotropic alignment, one of the surfaces being reflective, the liquid crystal material in relative proximity to the other surface exhibiting generally homogeneous alignment, and means to alter alignment of at least one of the alignments to change the optical phase retardation characteristic exhibited by the liquid crystal material in the apparatus.

According to a further aspect, a reflective liquid crystal apparatus includes an optically reflective semiconductor substrate having electrical components, a further substrate, liquid crystal between the substrates, electrical means cooperative with electrical components of the semiconductor substrate to apply electric field to liquid crystal material to alter the optical phase retardation characteristics of the liquid crystal, the further substrate being able to transmit light therethrough for transmission through the liquid crystal and reflection by the optically reflective semiconductor substrate.

According to an additional aspect, a liquid crystal apparatus includes first and second substrates, one substrate having a relatively unsmooth surface and the other having a relatively smooth surface, birefringent material between the substrates, the birefringent material comprising liquid crystal material, the liquid crystal material in relative proximity to the relatively unsmooth substrate having generally homeotropic alignment, and the liquid crystal material in relative proximity to the relatively smooth surface having generally homogenous alignment. According to even another aspect, a display apparatus includes a source of polarized light, a liquid crystal display for selectively modulating light, the liquid crystal display comprising a semiconductor substrate and a further substrate with birefringent liquid crystal material between the substrates, the source being operative to provide light to pass into the liquid crystal material through the further substrate toward the semiconductor substrate, and means to reflect light to pass through the liquid crystal material and subsequently through the further substrate, the liquid crystal material being operative in response to an input to change the optical phase retardation characteristics of the liquid crystal material. According to even another aspect, a birefringent liquid crystal optical apparatus includes a pair of substrates and a birefringent liquid crystal material between the substrates, means for applying electric field between the substrates to alter liquid crystal orientation (sometimes referred to as alignment) and therefore optical phase retardation characteristics of liquid crystal subject to such field, one of the substrates including means for causing generally homeotropic alignment of liquid crystal material that is relatively proximate to the substrate, the means for applying electric field comprising mean to buttress such homeotropic alignment.

Another aspect relates to a liquid crystal cell fabricated directly on a semiconductor substrate. Another aspect relates to a method for fabricating a liquid crystal cell directly on a semiconductor substrate.

Another aspect relates to a reflective liquid crystal display in which one of the substrates is a semiconductor substrate.

Another aspect relates to a reflective variable birefringence liquid crystal cell in which one of the substrates is a semiconductor substrate

Another aspect relates to providing fast switching operation of a variable birefringence liquid crystal cell for use in an optical device.

With the foregoing in mind, an aspect of the invention relates to an apparatus for displaying an image including a light modulating passive display, and a control for controlling the intensity of light supplied to the light modulating passive display. Another aspect relates to a display including a light modulator, a source of light, and a control for controlling the intensity of light from the source supplied to the light modulator.

A further aspect relates to a method of displaying an image using a passive light modulating display apparatus including controlling the intensity of light illuminating the display apparatus as a function of a brightness characteristic of the image.

An aspect of the invention relates to an apparatus for displaying an image including a light modulating passive display, and a control for controlling the intensity of light supplied to the light modulating passive display while the display is operated in a field sequential mode.

An aspect of the invention relates to an apparatus for displaying an image including a light modulating passive display, and a control for controlling the intensity of respective lights of different color sequentially supplied to the light modulating passive display while the display is operated in a field sequential mode. Another aspect relates to a display including a light modulator, a source of light, and a control for controlling the intensity of light from the source supplied to the light modulator while the modulator is operated in a field sequential mode.

Another aspect relates to a display including a light modulator, sources of light of different respective colors, and a control for controlling the intensity of light from the respective sources supplied to the light modulator while the modulator is operated in a field sequential mode.

A further aspect relates to a method of displaying an image using a passive light modulating display apparatus including controlling the intensity of Ught __ uminating the display apparatus as a function of a brightness characteristic of the image while the display apparatus is operated in a field sequential mode.

A further aspect relates to a method of displaying an image using a passive light modulating display apparatus including controlling the intensity of plural lights of different respective colors illuminating the display apparatus as a function of a brightness characteristic of the image while the display apparatus is operated in a field sequential mode.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Although the invention is shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the claims. BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

Fig. 1 is a schematic view of a head mounted display system including a pair of display subsystems, each with an illumination system of the invention;

Fig. 2 is a schematic elevation view of an illumination system in accordance with the present invention used in a display subsystem;

Fig. 3 is a graph depicting reflectivity vs. wavelength for an aligned cholesteric liquid crystal film used in the reflector of the illumination system of the invention;

Fig. 4 is a graph of reflectivity vs. wavelength as a function of angle of incidence of light on an aligned cholesteric liquid crystal film of the reflector of the illumination system of the invention;

Figs. 5A and 5B are schematic elevation views of the illumination system of the invention showing operation thereof;

Fig. 6 is a schematic elevation view of the illumination system of the invention showing an exemplary lighting pattern; Fig. 7 is a schematic elevation view of the illumination system of the invention using a cholesteric liquid crystal polymer reflector;

Fig. 8 is a schematic elevation view of the illumination system of the invention using plural cholesteric liquid crystal reflectors;

Fig. 9 is a schematic elevation view of the illumination system of the invention using plural cholesteric liquid crystal reflectors and plural light sources of respective colors to provide a multicolor or full color display; Fig. 10 is a schematic side elevation view of a projector using the illumination system of the invention;

Fig. 11 is a schematic side elevation view of a reflective variable birefringence liquid crystal cell having an active matrix substrate, the cell being aligned ready for operation;

Fig 12. is a schematic side elevation view of a reflective variable birefringence liquid crystal cell similar to that of Fig. 11;

Figs. 13A and 13B are, respectively, graphs depicting light transmission characteristics and applied electrical signal for achieving such transmission characteristics for the liquid crystal cell of Fig. 11;

Fig. 14 is a schematic side elevation view of a display system using the display of Fig. 11 employing a reflective variable birefringence liquid crystal cell;

Fig. 15 is a schematic elevation view of a reflective variable birefringence liquid crystal cell using homeotropically aligned liquid crystal material that has negative dielectric anisotropy;

Fig. 16 is a schematic illustration of an alternate form of liquid crystal cell and display using a plane polarizer;

Fig. 17 is a schematic illustration of a light transmissive display system according to the invention; Fig. 18 is a schematic illustration of a light reflective display system according to the invention;

Fig. 19 is a schematic view of a reflective field sequential display and illumination system using plural cholesteric liquid crystal reflectors and plural light sources of respective colors to provide a multicolor or full color display, similar to Fig. 9; and

Figs. 20-25 are schematic graphical illustrations depicting operation of the invention.

DESCRIPTION

Referring to the drawings, wherein like reference numerals designate like parts in the several figures, and initially to Fig. 1, a head mounted display system 10 includes a pair of display subsystems 11, 12 and a control system 13 for creating images intended to be viewed by the eyes 14, 15 of a person. The display subsystems 11, 12 may be positioned in relatively close proximity, for example, at approximately one inch distance, to the respective eyes 14, 15. A mounting mechanism, such as temple pieces 16, 17 and a nose bridge 18 may be provided to mount the display subsystems of the display system 10 on the head of the person. The control system 13 in conjunction with the display subsystems 11, 12 is intended to create images for viewing by the eyes. Those images may be monochromatic. The images may be multicolor. The images may be two-dimensional or they may provide a three dimensional, stereoscopic effect. Stereoscopic effect viewing is obtained when the control system 13 operates the display subsystems 11, 12 to provide, respectively, right eye and left eye images that are sufficiently distinct to provide depth perception. Right eye, left eye imaging and depth perception are techniques used in some stereoscopic imaging and viewing systems which are commercially available.

The display subsystems 11, 12 may be identical. The control system 13 provides control and/or power input to the display subsystems 11, 12 to create images for display to the eyes 14, 15. The display system 10 may be a head mounted display, such as a heads-up display, a virtual reality display, or a multimedia display. The control system 13 may be generally a control system of the type used in known head mounted displays to create such images. Such a control system may provide for control of color, light intensity, image generating, gamma, etc. The display subsystems 11, 12 may include focusing optics so as to focus the image created by the display subsystems for comfortable viewing, for example from a few inches up to a few feet in front of the eyes, say, from about 20 inches to about several feet in front of the eyes. It will be appreciated that the features of the present invention may be used in the display system 10 of the head mounted type in which are they are plural display subsystems. Also, features of the invention may also be employed in other types of display systems. One example is a display system that uses only a single display subsystem of the type described herein. Such display subsystem may be located in proximity to an eye for direct viewing; alternatively, such display subsystem may be used as part of a projection type display in which light from the display subsystem is projected onto a surface where the image is formed for viewing. Various lenses and/other optical components may be used to direct from the display subsystem light to create an appropriate image at a desired location.

For brevity, though, the various features of the invention will be described below with respect to display subsystem 11; however, it will be appreciated that the various features described below are applicable to the display subsystem 12 and to other types of display subsystems and display systems in which the features of the invention may be employed.

Display Subsystem 11 With Illumination System 20. Turning to Fig. 2, an illumination system 20 in accordance with the present invention used in a display subsystem 11 is illustrated. The illumination system 20 is folded. The folded illumination system reduces the thickness or depth requirement for the display subsystem 11 while still allowing for substantially uniform illumination in the display subsystem. As is seen in Fig. 2, the illumination system 20 includes a source of polarized light 21 and a reflector 22 (sometimes referred to as an analyzer) which is able to transmit or to reflect light as a function of the wavelength of the light and/or the polarization characteristic of the light. Light from the illumination system 20 is directed to the front or face of a display 23. Light from the display 23 is reflected toward the reflector/analyzer 22 for transmission thereby to be viewed, projected, or for reflection, etc. A lens 24 may be included in the illumination system 20 to help distribute light substantially uniformly across the area of the display 23. The lens 24 also may be part of an optics system generally designated 25, which may include one or more lenses, mirrors, and/or other optical elements, one such lens 26 being shown in Fig. 2. The lens 24 and optics system 25 may cooperate to provide an image focused at a comfortable distance relative to and for viewing and focusing by the eye of a viewer, to project an image onto a screen, etc. The various components of the illumination system 20 and display 23, which form the display subsystem 11 may be mounted in a frame or other support 27. In the illumination system 20, the source of polarized light 21 is shown in Fig.

2 as a light source 30 and a circular polarizer 31. An exemplary light source 30 is a light emitting diode. The light emitting diode may have a light output of a specific wavelength or it may have a light output over a band or range of wavelengths. In the latter case, it may be desirable to filter the light from the light emitting diode 30 by a conventional color filter (not illustrated) to block one or more wavelengths or wavelength bands of light in order to coordinate the wavelength or wavelength band of the light incident on the reflector 22 with respect to the wavelength-determined reflection characteristics of the reflector.

The light source 30 may be a plurality of light emitting diodes located about the outer perimeter of the display 23, for example, outside the viewing area or active area of the display, located either in front of or laterally adjacent an edge of the display, for example. Two of such light emitting diodes 30 are shown in Fig. 2. Alternatively, a single light emitting diode light 30 may be used in the illumination system 20 to provide light to the display 23.

Other exemplary light sources which may be used in connection with the invention include fluorescent light sources, incandescent light sources, and light channeled by fiber optics from a source internal or external of the frame 27 of the display subsystem 11. The light source may be monochromatic. Alternatively the light source may be multicolor and even may be white. A white light usually includes all colors or substantially all colors. The light source may include red, green and blue components that can be used to provide a full color system, as is described in greater detail below. The light source 30 may also be a metal halide lamp, such as one which produces light having red, green and blue spectral peaks. Thus, it will be appreciated that various devices may be used as the light source 30. For brevity, reference below to the light source 30 will be with respect to light emitting diodes, however, it will be appreciated that other light sources alone or in combination with light emitting diodes or in combination with each other also may be used in the invention.

The polarizer 31 is a circular polarizer, the handedness of which is coordinated with the characteristics of the reflector 22. The polarizer 31 may be a left hand circular polarizer or a right hand circular polarizer. Alternatively, the polarizer 31 may be composed of plural polarizers, which cooperate to form a circular polarizer; an example is a linear polarizer (sometimes to referred to as a plane polarizer) in optical series with a quarter wave plate, the appropriate axis of which is oriented at 45 degrees relative to the transmission axis of the linear polarizer, as is known. It will be appreciated that light sources and polarizers other than those described herein, but which functionally are equivalent to provide a source of polarized light 21 may be used in the illumination system 20 of the invention.

The reflector 22 is formed of cholesteric liquid crystal material or equivalent providing the equivalent functions described herein. Several properties of cholesteric liquid crystal material make such material particularly suitable for the present invention. Cholesteric liquid crystal material tends to reflect circular polarized light that has the same direction, i.e., left hand or right hand circular polarization, or same handedness, as the left or right handedness of the twist characteristic of the cholesteric liquid crystal material, provided the wavelength of the incident light on the cholesteric liquid crystal material is of a wavelength or within a wavelength band that is determined by the composition, pitch, or other characteristic of the cholesteric liquid crystal material. Therefore, in the case of cholesteric liquid crystal material having a right hand twist and being operative to reflect green light of right hand polarization, upon the directing of green right hand polarized light from the light source 21 onto the reflector 22, such light will be reflected by the reflector to illuminate the display 23. Light that is left hand circular polarized and/or that is not green or within a suitable wavelength band that includes green light, is transmitted by the reflector 22.

The combination of the light source 21 and the reflector 22 provide a folded optical path for light travel from the source to the display. The folded path helps reduce the thickness requirement for the display subsystem 11 and also provides bright efficient illuminating of the display. Since a cholesteric liquid crystal reflector is an efficient reflector, often more efficient than a specular reflector for the particular wavelength and polarization characteristic of incident light, bright, substantially uniform illumination of the display 23 is possible by the illumination system 20.

The display 23 may be a liquid crystal display or some other type of display. The display may include an active matrix substrate and drive or some other type of drive. The display 23 is operative, preferably is selectively operative, in response to an input, such as electrical, magnetic, or temperature input, to receive incident light from the illumination system 20 and to direct light to the reflector 22 and may alter or not alter a characteristic of the incident light thereby to determine whether the light is to be reflected or transmitted by the reflector 22. Such alteration preferably is of polarization characteristic; however, such may be wavelength or may be some other characteristic that determines whether or not the light is to be transmitted or reflected by the reflector 22. The display 23 may be about one inch square in area, for example, or may be larger or smaller, as may be desired. The display 23 may be a twisted nematic liquid crystal display, a variable birefringence liquid crystal display, examples of which are known. A folded variable birefringence liquid display useful as display 23 is disclosed in concurrently filed, commonly owned, U.S. patent application Serial No. , the entire disclosure of which hereby is incorporated by reference. Other displays that provide the indicated functions of being able to alter the polarization and/or wavelength characteristic of light also may be used in the display subsystem 11 in conjunction with the illumination system 20 of the invention.

Cholesteric Liquid Crystal Reflector 22. Referring to Fig. 3, a graph depicts reflectivity vs. wavelength for an aligned cholesteric liquid crystal film used in the reflector 22. The cholesteric liquid crystal film is operable to reflect green light having a specified circular polarization, left or right hand circular polarization. In the graph of Fig. 3 the X-axis represents wavelength in nanometers (nm); and the Y-axis represents percent reflection of incident light directed to the cholesteric liquid crystal film of the reflector 22, for example.

In Fig. 3, curve 40 represents percent reflection as a function of wavelength by the mentioned cholesteric liquid crystal film when unpolarized white light is incident on that film. A portion of the spectrum of the unpolarized white light is transmitted by the film without alteration; that portion does not fall under the curve 40 shown in Fig. 3. However, there is a wavelength band over which the cholesteric liquid crystal film does affect the light. The cholesteric liquid crystal film will reflect the circular polarized component of incident light that corresponds to the twist direction (say right hand) of the liquid crystal material and falls within the specified reflection wavelength band of the cholesteric liquid crystal material. The cholesteric liquid crystal film will transmit the other, say left hand, circular polarized component of such light both within and outside of such wavelength band. If the cholesteric liquid crystal film had a left hand twist, it would reflect the left hand circular polarized component and transmit the right hand circular polarized component.

The wavelength band over which such polarized reflection occurs is determined by the following equation or relationship:

λ_FmLι_ _ ΔN_AVG

N A, VG

ΔNAVG = the average birefringence of the cholesteric liquid crystal NAVG = the average index of refraction of the cholesteric liquid crystal λo = the center wavelength of the reflective band for normally incidence light ΔXFWHM = the wavelength width of the reflection band

The center wavelength and the width of the wavelength band over which polarized light is reflected (the reflection band) can be tailored to a specific wavelength and range in the spectrum, respectively, by a proper choice of the cholesteric liquid crystal material. These characteristics are a function, for example, of the composition of the cholesteric liquid crystal material, the pitch thereof, and so forth, as is known. In Fig. 3, the reflection band of the cholesteric liquid crystal material of reflector 22 is centered at 540 nm and has a wavelength width or band of about 70 nm. Within the wavelength band the polarizing efficiency is very high, that is, the light reflected (e.g., green light which is 540 nm) or the light transmitted by the cholesteric liquid crystal film is almost perfectly polarized with circular polarization characteristic. Transmitted light is circular polarized opposite the twist direction of the cholesteric liquid crystal material; and reflected light is the same handedness circular polarization as the twist direction of the cholesteric liquid crystal material. Reference now is made to Fig. 4. The graph of Fig. 4 shows reflectivity vs. wavelength as a function of angle of incidence of light on an aligned cholesteric liquid crystal film of reflector 22, for example. On the X-axis is shown wavelength of incident light in nanometers (nm), and on the Y-axis is shown percent reflection of incident right hand circular polarized light on the cholesteric liquid crystal film that has a right hand twist.

An optical characteristic of the cholesteric liquid crystal film used in the reflector 22 is that the center wavelength of the reflection/transmission band shifts toward a shorter wavelength as the angle of incidence of the incoming light increases. This characteristic is illustrated in Fig. 4. The relationship between the center wavelength at normal incidence (incident on the film at an angle that is 90 degrees to the plane of the film, or is parallel to a normal to the film) and the center wavelength at an off-axis angle of incidence is given by the following relationship: λ=λ sj-v- -_j-i-STi θ) ]

^■"AVG λ = the center wavelength of the reflective band at the off axis reflection angle θ = the angle between the normal and the reflected ray The solid line in Fig. 4 represents the curve 40 shown in Fig. 3. Light under the curve 40 is right hand circular polarized (or in any event the .same hand circular polarization as the twist direction of the cholesteric liquid crystal material in the film of the reflector 22). Therefore, substantially all light falling below the projection of the line 41 of the curve 40 onto the X-axis will be reflected by the cholesteric liquid crystal reflector 22. The curve 40 in Fig. 4 represents the reflection/ transmission band of the reflector 22 for light that is at normal incidence (incident at an angle of 90 degrees to the plane of the reflector 22). Curve 42 shown in dotted line in Fig. 4 represents the wavelength range over which the cholesteric liquid crystal film of the reflector 22 is reflective when the incident light is at an angle other than normal incidence, e.g., at 10 degrees to normal, 20 degrees to normal, etc., (in any event, other than at 90 degrees to the plane of the cholesteric liquid crystal film of the reflector 22).

The arrow 43 represents the wavelength range over which the cholesteric liquid crystal reflector 22 is reflective over a given angular range of incidence from normal incidence to an angle other than normal incidence at which the curve 42 was determined. The wavelength of the light produced by the illumination system 20, for example, by a light emitting diode 30, should fall within the range depicted by the arrow 43, which is the case in Fig. 4 where the wavelength is shown at approximately 565 nm.

For a given range of the angle of incidence of light from the illumination system 20 in the display subsystem 11, for example, there will be a wavelength band which remains within the reflection/transmission band, and that wavelength band is depicted by the arrow 43 in Fig. 4. The illumination system 20 should be designed so that the wavelength of the illumination system falls at the short wavelength side of the normal incidence reflection band depicted by curve 40. In this way, the wavelength of the illumination source 20 will fall within the reflection band when the light is incident at an acceptable angle that is off the normal angle (90 degrees to the plane of the reflector 22). Such acceptable angle of incidence will be a function, for example, of the angular field of view of the display subsystem 11, usual positioning of the display subsystem 11 relative to the eye 14, angle over which light is to be projected by the display subsystem 11 when used in a projection type display, thickness of the display subsystem 11 for acceptable distribution of incident light from source 20 on the display 23, etc. In currently available cholesteric liquid crystal materials, the value

ΔXFWHM can be made as large as about 100 nm. To reflect the entire spectrum of a white light illumination source 20, it is expected that more than one, perhaps two, three or more, cholesteric liquid crystal reflectors in optical series would be required for the reflector 22. Each such cholesteric liquid crystal reflector would reflect a portion of the spectrum as a function of the characteristics of the cholesteric liquid crystal material thereof.

As is seen in Figs. 1 and 5, the reflector 22 is formed of a cholesteric liquid crystal material 50 contained between two relative rigid, flat, transparent substrates or plates 51, 52. Those plates may be glass, plastic or some other material. The reflector 22 may be made of other types of material. In designing the reflector 22, alignment of the cholesteric liquid crystal material

50 also may be considered. An exemplary cholesteric liquid crystal material is sold by Wacker Chemical as Cholesteric Liquid Crystal Silicones. If a homogeneous alignment layer is placed on the inner surfaces of both substrates 51, 52., i.e., the surfaces confronting the cholesteric liquid crystal material 50, then the cholesteric liquid crystal material would adopt a planar texture. A planar texture reflects incident light rays as would a specular mirror. However, in the illumination system 20 of the present invention including the source of polarized light and the reflector 22, a more uniform distribution of light intended to be reflected to the display 23 could be obtained if the reflection from the cholesteric liquid crystal reflector were somewhat more diffuse than that obtained by specular reflection. This is accomplished by applying an alignment layer (such alignment material being known), which tends to align the cholesteric liquid crystal material parallel to the substrate face to only one of the substrates 51, 52. By this means, the axis of the cholesteric liquid crystal material and the planes of reflection become somewhat randomized and the reflectivity becomes somewhat more diffuse. The following are three exemplary mixtures R-l, R-2, and R-3 of cholesteric liquid crystal material which may be used for reflecting red light. Also presented below are three exemplary mixtures B-l, B-2, and B-3 and G-l, G-2 and G-3 of cholesteric liquid crystal material which may be used for reflecting blue (designated prefix "B") or for reflecting green light (designated prefix "G"). For a full three color reflector system using three reflectors as is described below in the multicolor embodiment, one reflector may be made of red material indicated below; one of blue, and one of green. Percentages are by weight. R-l: CN: 33.59% OCC: 21.21 % CC: 21.20% E-49: 24.00% R-2: CN: 39.90%

CC: 14.25% CCin: 2.85% E-63: 43.00% R-3: CN: 22.10% OCC: 13.95%

CC: 13.95% E-7: 50.00% CN is cholesteryl nonanoate; CC is chloride; OCC is Oleyl carbonate, and CCin is Cinnamate. E-49 and E-63 are derived from E-14 of E. Merck. B-l OCC: 60%

E-49: 40% B-2: OCC: 60%

PPMeOB 40% B-3: OCC: 50% PPMeOB 10%

PPPOB: 40% G-l: CN: 37.92% OCC: 23.94% CC: 23.94% E-49: 14.20%

G-2: OCC: 50%

PPMeOB: 38.2% PPPOB: 15.0% G-3: BL-009: 60.5%

CB-15: 39.5% PPMeOB is 4-pentylphenyl-4'-methoxybenzoate; PPPOB is 4-pentylphenyl-4'- pentyloxybenzoate; and BL-009 is sold by E. Merck.

Operation of Display Subsystem 11 using the Illumination System 20.

Referring to Figs. 5A and 5B, the operation of the illumination system 20 in the display subsystem 11 is illustrated. A single light emitting diode light source 30 is shown. It will be appreciated that there may be more than one such light emitting diode or that the light source may be some other type of device. Preferably the light emitting diode is outside the viewing area or viewing angle of the display subsystem 11 , for example being located laterally adjacent a side edge of the display 23, as is seen in Fig. 5A, or being located in front of the display at an or adjacent an edge of the display 23, as is shown in Fig. 2.

In Fig. 5 A the polarization characteristics of a light ray as it travels from the light source 30 to the eye 14 of a viewer is shown. Light symbolized by light ray 60 emitted by the light emitting diode 30 is unpolarized. The polarizer 31 placed in front of the light emitting diode 30 converts the light ray 60 to right hand circular polarization light 61. The light ray 61 is refracted by the lens 24 and is reflected by the cholesteric liquid crystal layer 50 of the reflector 22. (The wavelength of the light ray 61 and the angle of incidence of the light on the reflector 22 is consistent with the reflectivity vs. wavelength graph of Fig. 4 falling within the range designated by the arrow 43 of that graph). The reflected light ray 61 remains right hand circular polarized, as is depicted at light ray 61a.

The light ray 61a is designated 62 after it enters the front face 63 of the display 23 at 64. In the display 23 the polarization characteristic of the light ray 62 may or may not be changed. Such a change would be a function of the characteristics of the display 23, e.g., whether or not it retards light transmitted therethrough. In the example illustrated in Fig. 5 A, the display 23 includes liquid crystal material 65, for example nematic liquid crystal, and a reflector 66. The reflector 66 may be, for example, a specular reflector. An example of such a specular reflector may be a metal surface, a reflective active matrix substrate, silicon material, or other light reflecting material. Other types of displays 23 and reflectors 66 also may be used. The display 23 is intended selectively to affect the light transmitted therethrough, for example, to create an image of an object, alphanumeric information, etc. The affect at varying locations of the display 23 may be a function of the characteristics of the liquid crystal material 65 and the operation or energization of the display 23 and, in particular, the various portions thereof used to create an image, for example. The display 23 may include a wave plate 67, such as a quarter wave plate, to provide dark field and/or other functions, e.g., as is described in the above mentioned patent application. For example, a 1/4 wave plate 67 may introduce a phase shift in the light transmitted therethrough to cooperate with the phase shift characteristic of a variable birefringent liquid crystal type display 23 to obtain the desired affect on the light.

As the light ray 62 travels through the display 23 it may encounter some amount of retardation which in general converts the light to right hand elliptical polarization (or maintains such right hand elliptical polarization characteristic). The light ray 62 then is reflected by the rear reflector 66 as ray 68 and as a result of that specular reflection the phase of the light is reversed, as is well known, for example, the right circular (or elliptical) polarized light converted to left hand elliptical polarization. Traveling back through the display, in particular through the liquid crystal 63 thereof, the ray 68 may encounter further retardation but remains left hand circular or elliptically polarized.

For convenience of illustration, the ray 68 is shown in the large scale illustration of Fig. 5A shifted at 69 to facilitate showing its travel path in the display subsystem

11 without requiring a further drawing figure. The ray 68 is refracted by the lens 24.

Part of the light ray 68 may be transmitted by the cholesteric liquid crystal reflector 22. The extent to which the ray 68 is transmitted, as compared to the extent it is reflected by the reflector 22, is determined by its ellipticity. Specifically, elliptically polarized light can be thought of as composed of a left hand circular polarized component and a right hand circular polarized component. The two components would, in general, have different amplitudes. Only the left hand circular polarized component would be transmitted by the cholesteric liquid crystal reflector 22; the right hand circular polarized component would be reflected back to the display 23. If the display 23 does not convert the incident light to circular polarized light or elliptically polarized light, but rather operates solely on plane polarized light selectively to alter the plane of polarization, the quarter wave plate 67 and/or other optical components may be used to provide the light output from the display 23 with suitable circular polarization (or elliptically polarized characteristics) or wavelength characteristics to enable the reflector 2 to function as a polarization analyzer, or the like, for example, selectively to transmit or to reflect light as a function of such characteristic.

Briefly referring specifically to Fig. 5B, the parts of the subsystem 11 including illumination system 20 are shown in schematic form. Light from the LED 30 is circularly polarized by circular polarizer 31. According to the wavelength and circular polarization direction of such light the cholesteric reflector 22 reflects the light toward the display 23. A wave plate, such as a quarter wave plate 67, may be provided in the light path. The wave plate 67 may be omitted, if desired. Light transmitted through the display 23 is reflected by the reflector 66 back through the liquid crystal of the display toward the reflector 22. The cholesteric reflector 22 then serves an analyzer function, transmitting only that light received from the display 23 which has the correct circular polarization. Optics 25, including optics 26 can deliver the light for viewing to form an image, e.g., at about 20 inches to about a few feet, even though the lens 26 may be located only approximately one inch in front of the eye of a person.

The Lighting Pattern in the Illumination System 20.

Turning to Fig. 6, the lighting pattern in the illumination system 20 used in an exemplary display subsystem 11 is shown. The cholesteric liquid crystal reflector 22 is spaced from the display 23 by a distance equal to about half the diagonal of the display 23. In the illustrated example of Fig. 6, the display 23 is approximately square in viewable area. The shape of the lens 24 and the emission pattern of the light emitting diode light source 30 are adjusted so that each of the illustrated two light emitting diodes 30 would illuminate about half the face 63 of the liquid crystal display 23. In this way the angle of incidence of the light rays on the lens 24 and hence the cholesteric liquid crystal reflector 22 will not be too oblique. The more oblique, the narrower ordinarily will be the range represented by the arrow 43; the less oblique, the larger the range of the arrow 43, and, therefore, the larger the viewing angle of the display 23 without degradation of the viewed image by allowing the wavelength of the light emitted by the diodes 30 to remain within the reflective band of the cholesteric liquid crystal material. The lens 24 may be part of a lens system of a virtual reality display or other heads-up display in order to place the image created by the display subsystem 11 at a convenient viewing distance from the eye. The illustrated plano-convex lens 24 on the display side of the cholesteric liquid crystal reflector 22 as part of the illumination system also may serve as part of the optics required to adjust the image viewing system. A second plano-convex lens 26 and/or other optical components generally designated 25 may be placed on the other side of the cholesteric liquid crystal reflector 22 to complete the optics in a very compact and convenient form.

An example of a type of display 23 useful in the display subsystem 11 is a variable birefringence liquid crystal display. Examples are disclosed in U.S. patents Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,296, which are incorporated by reference. For a variable birefringence nematic liquid crystal display, a quarter wave plate may be provided between the reflector 22 and the face 63 of the display to convert the circular polarized light to plane polarized light for delivery to the display. In the display the polarization is affected, e.g., undergo retardation of one quadrature component relative to another, or not, depending on whether or not the display is energized, for example. Such retardation may have the effect of rotating plane of polarization of the light. The light is specularly reflected by the reflector 64 and is directed back to the quarter wave plate. The output light from the quarter wave plate will have a circular (or elliptical) polarization characteristic that is a function of whether or not the display has rotated the plane of polarization. Depending on that characteristic, light will transmitted or not by the reflector 22.

Cholesteric Liquid Crystal Polymer Reflector Embodiment.

In Fig. 7 a display subsystem 111 generally of the type described above has a modified cholesteric liquid crystal reflector 22'. The reflector 22' is formed of a cholesteric liquid crystal polymer that has the same twist direction as the polarizer 31, for example, right hand twist and right circular polarized light, respectively. The reflector 22' is curved generally in the manner illustrated to improve the uniformity of distribution of light from the light source 30 across the face 63 of the display 23. Thus, the curvature of the reflector 22 is such that the light emitted by the light emitting diode 30 is distributed uniformly across the face 63.

Multiple Reflectors 22.

In Fig. 8 an embodiment of the invention using plural cholesteric liquid crystal reflectors as part of an illumination system 20 in a display subsystem 211 is shown. The various components of the display subsystem 211 are substantially the same as those in the described above with respect to the display subsystems 11 and 111. However, in the display subsystem 211, particularly in the illumination system 20 thereof, the light source 30 provides light of more than one wavelength. Although the reflector 22 may be able to reflect light of one of those wavelengths, it is not able to reflect light of the other wavelength. However, a second one of the cholesteric liquid crystal reflectors 22a, for example, is able to reflect light of the other wavelength. Therefore, the two reflectors 22, 22a would be able to reflect both wavelengths produced by the light source 30.

The light source 30 may produce more than two wavelengths of light, for example, red, green and blue wavelengths. In such case, three cholesteric liquid crystal reflectors 22, 22a, 22b may be used respectively to reflect a particular wavelength of light in the manner described above. The respective cholesteric liquid crystal reflector which reflects a particular wavelength is transparent to other wavelength.

Thus, it will be appreciated that the illumination system 20 shown in Fig. 8 may include a white light source 30. The several cholesteric liquid crystal reflectors used in the illumination system 20 of Fig. 8 as part of the display subsystem 211 are operative to reflect the various wavelengths of such white light to the display 23. The display subsystem 211 of Fig. 8 using a white light illumination system 20 is able to produce output images that are black and white. This output is distinguished from the color of the output images produced by the display subsystems 11 and 111 described above wherein the color of the output light is a function of the wavelength produced by the light source of the respective illumination system. Full Color Frame Sequential Illumination System and Display.

Turning to Fig. 9, a full color display subsystem 311 including illumination system 320 is shown. The various portions of the display subsystem 311 are substantially the same as the portions of the above described display subsystem 211. However, in the display subsystem 311 the illumination system 320 includes several sources of light, each having a different wavelength. For example, three separate light sources 30r, 30g, 30b provide red, green and blue wavelength light, respectively, or light that is in respective wavelength bands or ranges that include red, green and blue, respectively. The light sources may be respective light emitting diodes or they may be other sources of red, green and blue light or other respective wavelengths of light, as may be desired for use in the display subsystem 311. The cholesteric liquid crystal reflector 22 is able to reflect green light; the reflector 22a is able to reflect red light; the reflector 22b is able to reflect blue -light. Such reflection occurs, as was described above, when the circular polarization characteristic of the light is the same direction as the twist direction of the cholesteric liquid crystal material in the respective reflector. The reflectors 22, 22a, 22b are transparent to the other polarizations of incident light and to the other wavelengths of incident light.

The illumination system 320 is intended sequentially to illuminate the display 23 (or respective portions of the display) with respective wavelengths of light. For example, for a period of time the display 23 (or portion thereof) is illuminated with red light; subsequently illumination is by either green or blue light; and still subsequently illumination is by the other of green or blue light. Such sequential illumination may be carried out sufficiently rapidly so that respective red, green and blue images created by the display 23 when illuminated by the respective colors of light are output from the display subsystem 311 and are integrated by the human eye. As a result, the human eye effectively sees a multicolor image. Other examples of frame sequential switching to provide multicolor and/or full color outputs are known in the art. Various advantages inure to a frame sequential multicolor display, including the ability to provide high resolution with approximately one-third the number of picture elements required for a full color r, g, b display system in which respective pixels are red, green or blue. The sequential delivering of red, green and blue light to the display 23 is coordinated by the control system 13 with the driving of the display 23. Therefore, when a red image or a portion of a red image is to be produced by the display 23, it is done when red light is incident on the display 23; and the similar type of operation occurs with respect to green and blue images.

If the respective light sources 30r, 30g, 30b are light emitting diodes, they may be sequentially operated or energized to provide light in coordination with the operation of the display 23 under direct control and/or energization by the control system 13. Alternatively, the control system 13 may be coordinated with whatever other means are used to provide the respective red, green and blue color lights of the light source.

Another example of frame sequential or field sequential operation of a displays subsystem line that shown at 11 herein is described in the above-referenced patent application. Another example of field sequential operation is described in U.S. patent No. 4,582,396, which is mentioned above and incorporated by reference. Briefly referring to Fig. 10, a projection type display according to the invention is shown at 400. The display 400 includes the various parts of the display subsystem 11 described above with respect to Fig. 5B, for example. However, in the display 400 there is a projection lens as part of the optics 25, for example, which is able to project the image formed by the display, which is illuminated by the illumination system 20, for viewing. The image may be projected onto a screen, for example, which is viewed in transmission mode or in reflection mode.

For a display subsystem 11 used in an exemplary head mounted display 10, the viewing area of each liquid crystal cell 23 and display system 11 may be on the order of approximately 0.75 inch diagonal. A display 23 according to the invention may have a pixel resolution of, for example, 360 columns by 256 rows, with a pixel pitch at 40 microns by 40 microns, an array size of 0.906 inch by 0.660 inch and 0.708 inch diagonal. The display may be operable sufficiently fast to provide images at the speed of approximately 180 frames per^' second. Power to the circuitry of the display may be less than 2 watts with voltage being supplied at between about 30 V DC and about 35 V DC. Signal formats may be interlaced or non-interlaced. Also, one may use 30 frames per second operation. One also may use 1 micron geometry or 3 micron geometry for the pixels. These values are provided by way of example and are not intended to be limiting.

In. Fig. 11 a reflective variable birefringence liquid crystal cell having an active matrix substrate in accordance with the present invention is indicated at 110. The liquid crystal cell 110 includes a pair of substrates 111, 112 and liquid crystal material 113 in the space 140 between the substrates. A seal 115 (Fig. 12) is provided about the perimeter of the liquid crystal cell to retain the liquid crystal material therein preventing leakage.

Of the liquid crystal material 113, the liquid crystal material 113a relatively proximate (meaning relatively near or at) the substrate 111 generally has homogeneous alignment. The liquid crystal material 113b relatively proximate the substrate 112 has generally homeotropic alignment. During operation of the liquid crystal cell 110 optical phase retardation of light 116 (represented by a light ray line shown) traveling through the liquid crystal material can be controlled as a function of the applied electric field across the liquid crystal material, as is described in greater detail below.

The substrate 111 may be glass, plastic, or some other material, as may be desired. The surface 120 of the substrate 111 facing the liquid crystal material 113 is relatively smooth or flat. A transparent conductor 121 is on and/or supported by the surface 120 of the substrate 111. The conductor may be indium tin oxide or some other material that has electrically conductive characteristics suitable for use in the energization of the liquid crystal cell, for example application of electric field. The conductor 121 may be applied by a variety of conventional techniques. Surface treatment 122 is provided the substrate 111 to obtain the desired homogeneous alignment of liquid crystal material 113a. The surface treatment may be a polyimide coating (applied, for example, by evaporation or some other technique), which may or may not be rubbed, a coating of polyvinyl alcohol, which preferably is rubbed, or a direct rubbing of the exposed surface of the conductor 121. Rubbing may be done in conventional manner using cotton, felt or other material, as is well known. Other types of surface treatment also may be used to obtain the desired alignment. The liquid crystal material 113 preferably is nematic liquid crystal material. A characteristic of nematic liquid crystal is that it tends to have directional orientation and not positional orientation, this being in contrast to smectic liquid crystal material which tends to have both directional and positional orientation. Another characteristic of nematic liquid crystal is that it tends to align with respect to a surface that is engaging or bounding the liquid crystal material, and the nature of such alignment may be governed by surface treatment. The alignment referred to herein is sometimes referred to as structural alignment, direction of liquid crystal axis or optical axis, direction of the liquid crystal directors, liquid crystal orientation, etc. Nematic liquid crystal also may be birefringent; desirably the nematic liquid crystal material 113 is birefringent to cause the desired optical phase retardation as a function of liquid crystal alignment or liquid crystal cell energization. An example of nematic liquid crystal material 113 is that sold under the designations E7, E22 and E54 (each sold by E. Merck).

The liquid crystal cell 110 will function with many other different nematic liquid crystal materials. In the embodiment of liquid crystal cell 110 the liquid crystal material should have positive dielectric anisotropy and be birefringent. The actual nematic liquid crystal material used may be selected as a function of speed of response desired by the liquid crystal cell 110; that speed may be a function of the elastic constants of the liquid crystal material and of the thickness of the liquid crystal cell, i.e. , of the liquid crystal material layer in the cell. Cell thickness and the birefringence of the liquid crystal material collectively would be characteristics that affect the total amount of optical phase retardation that is possible by the cell. For example, for a liquid crystal cell that is to be about 6 microns thick and to provide approximately 1/4 lambda optical phase retardation on each pass of light 116 through the cell 110 a liquid crystal material, such as 1840 sold by E. Merck, which has a birefringence of from about .10 to about .12 would be useful. If such liquid crystal cell were to have a thickness of about 3 microns the birefringence of the liquid crystal material may be on the order of about .22, which is that for the liquid crystal material commonly designated E54. Liquid crystal material E7 is more suited to a liquid crystal cell which has a thickness or spacing of about 3 to 4 microns. Other liquid crystal materials suitable for these devices are identified in the catalog book "Merck Liquid Crystals, Merck Liquid Crystal Mixtures For Electro-Optic Displays" published by E. Merck September, 1992, which is hereby fully incorporated by reference.

In selecting a liquid crystal material for use in the cell 110 consideration also can be given to the field of view expected of the display using such cell. Usually the greater the birefringence of the liquid crystal material the narrower the angle of or field of view of the display over which an image of acceptable quality would be produced. Therefore, it is desirable that the birefringence be as low as possible consistent with the desired speed of response and thickness of the cell. Usually, the higher the birefringence of the liquid crystal material or the thinner the liquid crystal cell, the faster the switching speed of the cell.

In an example of the invention the liquid crystal cell 110 is able to provide a maximum of more than 1/4 lambda retardation. During operation of the cell 110 it would be able to be switched between approximately or precisely zero retardation and 1/4 lambda retardation. Preferably, as a function of the magnitude of electric field applied across the liquid crystal material 113 in the cell 110 the cell would be able to provide any amount of retardation between zero and 1/4 lambda. The precise amount of retardation may vary slightly as a function of the wavelength of the light, as is known on account of the phenomenon of color dispersion; however, preferably the cell 110 is operated approximately at sufficiently low order, most preferably in the zero order range, so that the effects of color dispersion will be minimal if they exist at all.

The actual amount of retardation provided by the cell 110 operated in the reflective mode will be twice that provided by the liquid crystal layer 113 since the light passes through the cell twice. Therefore, a liquid crystal cell 110 which provides 1/4 lambda retardation on the passage of light from one substrate to the other will provide that amount of retardation again as the light is reflected back to the first substrate. Accordingly, a cell 110 operated between 0 and 1/4 lambda retardation will actually provide between 0 and 1/2 lambda retardation.

The substrate 112 is an active matrix semiconductor substrate. More particularly, the substrate 112 is a silicon substrate 123 having therein a plurality of electrical and/or electronic components 124 (the terms electrical and electronic regarding the substrate 123 may be used interchangeably below) formed by various solid state techniques that are known in the art. The substrate may be a single crystal silicon material. Most TFT display devices use amorphous silicon or polysilicon. Either may be used in the case of the present invention. However, single crystal silicon is preferred because of the relatively uncomplicated standard processing techniques available to make devices from such material, exemplary devices being conventional RAM, DRAM and other semiconductor devices. Therefore, single crystal silicon substrates are readily available and reasonably inexpensive. Also, virtually any circuit configuration can be incorporated into the silicon semiconductor substrate material; and such circuits can work with a wide range of voltages. Although single crystal silicon usually is not transparent, it is useful in the present invention in which the liquid crystal cell 110 is operated in the reflective mode. If desired, other materials can be used for the substrate 112, such as gallium arsenide, germanium and/or other materials used in the semiconductor field. On and/or in the silicon substrate 123 are a plurality of such components 124, such as transistors, electrodes, capacitors, diodes, and/or other components. A representative electrode is indicated at 125. The electronic components 124 are functional in response to an applied input to provide a voltage on or to electrodes 125 so that there is potential difference between the electrode 125 and the electrode 121 on the substrate 111. Such voltage and potential difference create an electric field between the electrodes 121, 125. Such electric field causes liquid crystal material to align relative to that field. By increasing or decreasing the magnitude of the field, more or less of the liquid crystal material located between the respective electrode 125 and the electrode 121, for the most part in a direct path there between, will align or not relative to the field. An exemplary active matrix substrate is manufactured and sold by a number of companies including Teleview Research, Inc., Palo Alto, California.

The liquid crystal material may be operationally nematic. Operationally nematic liquid crystal may or may not be nematic liquid crystal material; however, the operational properties of such material, e.g., being responsive to surface interaction to undergo alignment, having desired birefringence or other optical characteristics, being responsive to application of a prescribed input such as a field input, for example, an electric field, should be sufficiently similar to nematic liquid crystal to function in the cell 110 generally as is described herein.

The liquid crystal material may be smectic liquid crystal. Smectic liquid crystal tends to have memory or to hold alignment after having been aligned by a particular input, such as the application of a field. However, such alignment can be altered, for example, by application of heat to cause the smectic liquid crystal be become as nematic liquid crystal such that alignment would be a function of surface interaction. Therefore, under appropriate circumstances it is possible that the liquid crystal may be of the smectic liquid crystal type.

Preferably the liquid crystal is birefringent, as was mentioned above. The ordinary index of refraction and the extraordinary index of refraction of birefringent liquid crystal material are different. Therefore, as a result of alignment of the liquid crystal material, one component of plane polarized light, for example, may be retarded relative to the other component; and in this way the state of polarization can be altered. For example, the plane of polarization can be rotated or the light can be changed from right circular polarized to left circular polarized light (or vice versa).

There are a number of electronic components 124 on and/or in the substrate 112, as is seen in Fig. 11, and those components tend to cause the surface 130 of the substrate to be relatively rough or unsmooth having a multitude of peaks 130p and valleys 130v. Surface treatment 132 is provided the surface 130 of the substrate 112. The surface treatment 132 is intended to tend to cause homeotropic alignment of liquid crystal material that is located relatively proximate the surface 130 of the substrate 112.

Various techniques are available for providing the surface treatment 132 to obtain generally homeotropic alignment. For example, the surface 130 may be coated with a Langmuir film using known processing techniques. The Langmuir film is a surfactant, which forms a monomolecular film. Another surface treatment may be provided by applying barium stearate to the surface 130. A further example is to use a steryl silicone material on the surface 130. Still another coating material for the surface 130 is silane. A silane material, for example, provides fatty acid molecules standing on their ends or tails, which tend to cause the nematic liquid crystal structure to align homeotropically. The foregoing are examples of techniques conventionally used to obtain homeotropic alignment of nematic liquid crystal material. Other techniques also may be used to achieve the structure and functions of the invention describe herein.

Absent a specific surface treatment to obtain homeotropic alignment of the liquid crystal material in relative proximity to the surface 130, the liquid crystal structure at the surface 130 would tend to follow the contour of the surface somewhat in parallel thereto. Due to the roughness, unevenness, waviness, etc. of the surface 130, a relatively random orientation of the liquid crystal structure there would tend to occur, which would be undesirable for a liquid crystal cell whose operation in response to the presence or absence of an input, or a variation in the magnitude of the input, should be predictable. The rubbing surface treatment mentioned above with respect to obtaining homogeneous surface treatment and surface alignment of liquid crystal material may in some instances cause static electrical charge. It may be undesirable to have such a static charge on a semiconductor substrate. If the surface 130 has homeotropic alignment, such rubbing may be avoided. The liquid crystal material U3a directly in contact with the substrate 111 tends to align and parallel, for example, in a direction from the left hand side of Fig. 11 toward the right hand side of Fig. 11 (or vice versa). Such liquid crystal material usually tends to have a tilt angle relative to the surface 120 of the substrate 111. In many instances such tilt angle is as small as a fraction of a degree up to on the order of 5 degrees and may be even larger. The liquid crystal 113a located further and further away from the surface 120 also tends to have generally homogeneous alignment but may be less and less parallel, i.e., more tilted, relative to the surface 120, as is shown schematically in Fig. 11.

Of the liquid crystal material 113, the liquid crystal material 113b directly engaged with the surface 130 of the substrate 112 tends to have homeotropic alignment. The liquid crystal material has positive dielectric anisotropy. Therefore, the liquid crystal 113 tends to align with respect to or in the direction of an electric field. As is described further below, such homeotropic alignment can be buttressed by application of a relatively low voltage electric field and continues as the field is increased. The homeotropic alignment surface treatment 132 also may be used in a variable birefringence liquid crystal cell in which the substrate is relatively smooth or flat. An example of such a substrate would be a substrate made of glass material, plastic material, or a solid state material in which the electronic components thereof do not cause a rough, undulating, uneven, etc. surface 130. Although the invention is described having homogeneous alignment at the substrate Ul and homeotropic alignment at the substrate 112, it also is possible that such alignments are the reverse, i.e., homeotropic at substrate Ul and homogeneous at substrate 112. While not a requirement, this reversal would especially be possible if the substrate 112 were planarized to reduce the undulations, peaks, valleys, etc. in the surface 130 or if the substrate 112 were a particularly good specular reflector or had thereon a good quality reflector which reflects a high percentage of incident light. The substrate 112 preferably is optically reflective. Therefore, light 116 entering the liquid crystal cell 110 from outside 133 the liquid crystal cell through the substrate 111 travels through the liquid crystal material 113, is reflected by the substrate 112, travels back through the liquid crystal material 113, and exits through the substrate 111 to the externally ambient 133. The reflection characteristic of the substrate 112 may be a natural characteristic of the material of which the substrate 112 is made, including the silicon substrate and/or the electronic components 124 thereof. Alternatively, a separate coating of reflective material generally designated 134 in Fig. 11 on top of the electrode 125 is able to reflect the light 116 may be applied to the substrate 112, for example, above or below the surface treatment 132. Usually the mechanical structure of a liquid crystal cell of a display type device includes in sequential order or a stack a substrate at one side of the liquid crystal cell, a reflector, a surface treatment, and finally the liquid crystal material itself. It is possible that other parts and/or layers may be included in the "stack". Sometimes it may be possible to revise the order so that the reflector is closer to the substrate than the electrode, i.e., the reflector and the electrode reverse positions in the stack; and this is possible particularly if the electrode is transparent, for example. An exemplary coating material may be a metallized coating of aluminum. The reflective function may be provided by a dielectric stack. The reflector material should be compatible with the semiconductor circuit material and/or the electrode thereof. An advantage of aluminum reflector material and/or some other reflector materials, is that the aluminum blocks transmission of light. Therefore, the reflector can protect the semiconductor material from incident light. Many semiconductor materials are photoconducting, and it is desirable to protect the semiconductor circuit especially while writing the image to the semiconductor material, i.e. providing appropriate signals to the semiconductor parts to apply electric field to the liquid crystal thereby to create an image. It also is possible that the substrate 112 is not reflective, but rather is optically transmissive. In such case reflection function can be provided optically beyond the substrate 112 (i.e., optically downstream) or the liquid crystal cell 110 could be used in an optical transmission control device, such as a transmissive display or other device. A display using the liquid crystal cell 110 in a light transmissive mode would require liquid crystal material which has twice the birefringence or twice the thickness of the cell to obtain the same amount of optical retardation as is obtained in a reflective mode of operation cell. Twice the birefringence or twice the actual thickness of the liquid crystal cell would double the effective optical thickness of the cell. Furthermore, if desired the liquid crystal cell 110 could be partially transmitting to project an image to the outside world or to superimpose the image created by the liquid crystal cell 110 on another image from the outside world. In such case, some of the light passing into the cell 110 to create an image by the cell could be reflected to the eye of a viewer and some of the light from a device beyond the cell 110 could be transmitted through the cell for viewing by the viewer. Other arrangements which use such a semireflective cell 110 also can be used in accordance with the invention. Even further, if the substrate 112 were transmissive, the reflector 134 or the reflector function could be provided at the substrate 111.

Briefly referring to Fig. 12, a schematic section elevation view of a variable birefringence liquid crystal cell 110', which is structurally similar to the liquid crystal cell 110, is shown. The liquid crystal cell 110' includes substrates 111, 112. A standard seal 115 may be provided at opposite edges of the liquid crystal cell 110' of Fig. 12. Exemplary seals may be an adhesive material, epoxy material, a gasket, clamping mechanism, etc., as are well known in the art. Electrical members 121', 124' are used to provide electric field across part or all of the liquid crystal material 113. Electric field may be developed by a circuit 140 that provides electrical power and/or control of input electrical power with respect to the electrical members 121', 124'. In the liquid crystal cell 110 example of Fig. 11, the electrical member 121, corresponding to electrical member 121' of Fig. 12, is an electrical coating over substantially the entire surface 120 of the substrate 111. However, if desired, a plurality of electrically conductive strips that are separated from each other may be provided. Alternatively, other types of electrically conducted members may be used for the member 121' that are suitable to cooperate in providing electrical operation of the liquid crystal cell 110'.

The electrical member or layer 124' in combination with the substrate 112 in effect is an active matrix substrate of silicon material having a plurality of electronic components therein or thereon. The substrate 112 may be other than an active matrix substrate, and electrical member 124' may be other than electrical components on an active matrix silicon substrate. For example, the substrate 112 may be glass, plastic or some other material and the electrical layer or member 124' may be thin film transistors on a side of the glass, may be an electrically conductive coating across the entire substrate 112, may be strip electrodes on the substrate 112, etc.

According to the invention a variable birefringence liquid crystal cell depicted in Figs. 1 and 2, for example, has homeotropic alignment at least at the surface of one of the two substrates thereof. In the embodiment of Fig. 11 homogeneous alignment may be at the other substrate; and in the embodiment of Fig. 15 homeotropic alignment is at both substrates.

In the liquid crystal cells 10', specular reflection of light is caused at or in the area proximate the substrate 112. Reflection may be a result of a property of the substrate itself; for example, an active matrix substrate with electronic components on it may have a reflection characteristic such that it reflects approximately 80% of the light incident on it. Alternatively or additionally reflection function may be provided by a separate reflective layer 134 applied to the substrate 112. The reflective layer 142 may be, for example, a metallized coating applied to the electrode 124' or electrical layer. The substrate 112' may be prepared by conventional semiconductor processing techniques including masking, etching and material depositing steps. The reflective layer 142 may be put over the electrode 124' while mask material, e.g. , the photoresist mask used to fabricate the semiconductor device, still is left in place on the electrode. The mask material then insulates the electrode from the reflector.

The power and control circuit 140 may be a conventional electrical power supply and control of the power or voltage to the liquid crystal cell 110', such as those circuits commonly used in liquid crystal display devices of the variable birefringence type and may be the same or similar to that shown at 13 in Fig. 9, for example. For example, the circuit 140 may provide a relatively low voltage to achieve one alignment relationship of liquid crystal material 113 and a relatively larger magnitude voltage to obtain a different alignment relationship. The circuit 140 may be used to provide only a. single level voltage that is sequenced in a desired way to obtain one alignment characteristic and is sequenced in a different way and/or voltage level to achieve a different alignment characteristic of the liquid crystal 113. The circuit 140 may provide for multiplexed operation of the liquid crystal cell 110', if desired. The circuit 140 may provide a range of output voltages intended to obtain a corresponding range of alignment characteristics of the liquid crystal material 113 in the liquid crystal cell 110' and thereby to obtain a corresponding range of optical phase retardation characteristics.

Referring back to Fig. 11, the following description is based on the exemplary liquid crystal cell 110 having a transparent top substrate 111 and a non-planer reflective active matrix semiconductor bottom substrate 112. The top substrate 111 is treated to provide generally homogeneous alignment to the liquid crystal material relatively proximate thereto. The bottom substrate 112 is treated to obtain generally homeotropic alignment of the liquid crystal material relatively proximate thereto.

In Fig. 11 are illustrated numerous oval shapes or ellipses intended generally to depict the nematic liquid crystal material 113 in the cell 110 and the approximate alignment characteristics of that liquid crystal material in the area where the oval is located. Where the major axis of the oval generally is horizontal, i.e., generally in parallel to the substrate 111, homogeneous alignment exists; and where the major axis of the oval is vertical, the alignment of the liquid crystal material in that area is homeotropic. A given oval represents a substantial amount of liquid crystal material, not just a single liquid crystal molecule; such ovals, dashed lines, and other types of representations are used conventionally to depict the liquid crystal material and alignment thereof, as is conventional.

The liquid crystal material U3a is included in a surface layer 143 of liquid crystal 113, and the liquid crystal U3b is included in a bulk layer 144 of liquid crystal material 113. As is seen in Fig. 11, liquid crystal material between the homogeneously aligned liquid crystal 113a and the homeotropically aligned liquid crystal U3b tends to be aligned at an angle other than homeotropic or homogeneous. In Fig. 11 that liquid crystal material which tends to have alignment which is more nearly homeotropic is indicated as a bulk layer 144 of liquid crystal material 113, and that liquid crystal material which tends to be more homogeneously aligned than homeotropically aligned is labeled the surface layer 143. The liquid crystal material is birefringent.

In operation of the liquid crystal cell 110 optical phase retardation is increased when plane polarized light is transmitted across the extraordinary direction of the liquid crystal material. However, optical phase retardation usually is not affected when the plane polarized light travels along the liquid crystal axis, usually referred to as the ordinary axis.

From the foregoing, then, it will be appreciated that the surface layer 143 will have a greater impact on optical phase retardation than will the bulk layer 144 of the liquid crystal material shown in the liquid crystal cell 110. The actual thickness of the surface layer 143 and of the bulk layer 144 may vary and may be selected as a function, for example, of the particular liquid crystal material used, the birefringence characteristic thereof, the indices of refraction thereof, the response thereof to application of electric field, and possibly other characteristics. It is intended that as the magnitude of electric field applied across the liquid crystal cell 110 increases, the amount of optical phase retardation will decrease, and vice versa. Therefore, in a sense, as the magnitude of electric field increases, the thickness of the surface layer 143 decreases and the thickness of the bulk layer 144 increases; and vice versa as the magnitude of the electric field voltage is decreased.

Application of electric field to the liquid crystal cell 110 tends to enhance the uniformity of homeotropic alignment of the liquid crystal material 113b adjacent the surface 130 of the substrate 112 so that peaks and valleys, undulations, etc., in the surface 130 will not cause the liquid crystal material there to follow such uneven surface. Since light 116 traveling through the liquid crystal 113b travels along the ordinary axis of the liquid crystal material and experiences primarily the ordinary index of refraction thereof, increased path length through the liquid crystal material U3b will not increase the optical phase retardation of the light 116. However, as the area 145 between the surface layer and the bulk layer 143, 144 moves toward or away from the substrate 111 to decrease or to increase the thickness of the surface layer 143, optical phase retardation effect on the light 116 correspondingly will be decreased or increased. Alignment of liquid crystal material in the bulk layer 144 tends not to be significantly altered, whereas alignment of liquid crystal material in the surface layer 143, particularly that liquid crystal material that is adjacent the bulk layer 144, tends to alter alignment as the applied field increases or decreases.

Reference to the surface layer, bulk layer, and area therebetween 143-145 is intended to refer to approximate locations in the liquid crystal cell 110 and approximate portions of the liquid crystal material. Usually there is no precise location where the surface layer ends and the bulk layer begins. Rather, there is a somewhat uniform and gradual transition (the transition may be sharper or more gradual than that illustrated) between the surface layer in which a more significant amount of optical phase retardation would be expected to occur and the bulk layer where a less significant amount of optical phase retardation would be expected to occur. That is to say, the alignment of liquid crystal in part or even in all of the bulk layer may be other than perfectly homeotropic and alignment in the surface layer may be other than perfectly homogeneous. However, such alignments are sufficiently different that most of the optical phase retardation occurs in the surface layer.

The liquid crystal cell 110 may be approximately six microns thick, that is the thickness of the liquid crystal material 113 or the distance between the surfaces 120, 130. Since the liquid crystal cell 110 is reflective, whereby light 116 travels through the liquid crystal material 113 twice, the effective thickness of the liquid crystal cell 110 would be about 12 microns. In operation of the liquid crystal cell 110, though, optical retardation primarily is provided by the surface layer 143 of liquid crystal material. The actual thickness of the liquid crystal cell may be greater or less than six microns. The actual thickness dimension selected may be a function of the birefringence characteristics of the liquid crystal material, index of refraction characteristics of the liquid crystal material, dielectric anisotropy characteristics of the liquid crystal material, other electrical or optical characteristics of the liquid crystal material, speed of response desired, energy requirements and availability, and possibly other characteristics or parameters.

In operation of the liquid crystal cell 110, absent application of an electric field or any energization of the liquid crystal cell, the liquid crystal 113a will have generally homogeneous alignment in proximity to the substrate 111. The liquid crystal material 113b in proximity to the surface 130 of the substrate 112 will have generally homeotropic alignment. Due to the undulations in the surface 130, it is possible that some of the liquid crystal material U3b may be aligned in a direction that is altered relative to a direction perpendicular to the generally flat plane of the silicon 123. The liquid crystal material 113 will provide optical phase retardation with respect to a plane polarized light 116. This at rest condition exists when the liquid crystal cell 110 has been at rest for a period of time such that there is no alignment effect due to application of electric field at that time or immediately preceding that time. The cell 110 may provide more than 1/4 lambda retardation in the at rest condition. Therefore, when the cell 110 is operated at the preconditioned and fully energized states described below between maximum desired controlled retardation of, for example, 1/4 lambda (1/2 lambda for the combined incident and reflected passes of light through the cell) or some other desired amount of retardation and minimal retardation (zero retardation or near zero retardation) can be achieved.

Upon application of a minimal level electric field, the tendency of the liquid crystal material U3b to have homeotropic alignment is buttressed by such field. Such electric field, though, would have relatively minimal impact on the liquid crystal U3a in the surface layer 143 due to the relatively stronger influence of the surface 120 and homogeneous alignment layer 122 on the liquid crystal material 113a. In this pre- aligned condition or pre-conditioned state of the liquid crystal cell 110, the liquid crystal material 113 provides optical phase retardation to the plane polarized light 116. The pre-aligned condition is obtained in the illustrated example by applying a relatively low voltage electric field across the liquid crystal cell 110. However, such pre-aligned condition of the liquid crystal material can be obtained by other means, such as by periodically applying a relatively larger electric field to the liquid crystal material and permitting the liquid crystal material subsequentiy to relax, and then repeating application of the field. Also, mechanical or magnetic field means or other means or driving technique may be used to obtain the pre-aligned condition.

By increasing the voltage of the electric field, more of the liquid crystal material in the bulk layer 144 tends to align substantially homeotropically like the liquid crystal 113b, and some of the liquid crystal material in the surface layer 143 tends to align homeotropically, thus reducing the thickness of the surface layer 143. Ideally all of the liquid crystal material in the surface layer 143 would align homeotropically in response to application of a maximum voltage electric field; however, in practice this usually is not the case. Rather, at least a relatively thin layer of liquid crystal material 113a will remain in at least partial homogeneous alignment even when a relatively large voltage electric field is applied. In any event, upon application of such relatively large voltage, the amount of optical phase retardation provided by the liquid crystal material 113 to plane polarized light 116 will be reduced to a minimum. Compensation for residual optical phase retardation due to the liquid crystal material 113a that does not align with respect to a relatively large voltage electric field can be provided, for example, in the manner disclosed in U.S. patent No. 4,385,806. The compensation for residual birefringence can be provided by using an evaporated wave plate. The evaporated wave plate may be applied directly on the glass, quartz or other transparent substrate 111. Alignment of the slow axis of the wave plate preferably is such as to remove or to reduce the affect of residual birefringence of the liquid crystal cell 110. For example, the slow axis of such compensating wave plate may be at 90 degrees to the rub direction of the homogeneously aligned liquid crystal material U3a, for example. The compensating wave plate may be a quarter wave plate, a 1/lOth wave plate, and so on, as may be desired to provide the desired amount of compensation.

In the example described above of liquid crystal cell 110 in Fig. 11, upon application of maximum voltage electric field, zero optical phase retardation occurs (zero optical phase retardation includes the possibility of a relatively small amount of residual optical phase retardation mentioned above). Upon application of a reduced voltage electric field to obtain the pre-alignment condition mentioned above for the liquid crystal material 113, the liquid crystal material 113 provides a specified amount of optical phase retardation. For example, such specified amount of optical phase retardation may be one-quarter wave retardation on each pass of light through the liquid crystal 113. One quarter wave retardation is applied or occurs as the light 116 is directed toward and travels to the surface 130 and one-quarter wave for the light 116 reflected away from the surface 130 and travelling to the substrate Ul, for a total sum of one half wave retardation. Alternatively, the liquid crystal cell 110 may be used to provide more or less than a total of half wave optical phase retardation when the liquid crystal material is in the pre-aligned condition. It is possible to vary the optical phase retardation between minimum (e.g., zero) and maximum (e.g., 1/4 wave) in a step function by adjusting the voltage of the electric field, respectively to the maximum or to the minimum. The amount of optical phase retardation also can be varied substantially continuously over a range between and including the minimum and maximum by fully varying the voltage of the electric field to the liquid crystal between and including the relatively low level that provides the pre-aligned condition and the relatively high level that provides the substantially zero optical phase retardation.

When the liquid crystal cell 110 is in the fully de-energized or rest state absent any electric field, the amount of optical phase retardation ordinarily would be more than that occurring during the low level energization pre-aligned condition. Low level energization sets up the pre-conditioned alignment of the liquid crystal material which may help buttress homeotropic alignment at the substrate 112 and which may establish a prescribed amount of optical phase retardation provided then by the liquid crystal material, for example, by adjusting the effective thickness of the surface layer 143. As is mentioned elsewhere herein, other techniques to provide preconditioning of the liquid crystal material also may be used, such as mechanical, magnetic or some other means and/or technique.

In some circumstances, if the homeotropic alignment provided by the surface treatment 132 is rather precise and does not require buttressing by application of a relatively low voltage electric field, or if the nonuniformity and optical phase retardation across the face of the liquid crystal cell due to the misalignment occurring in the homeotropically aligned liquid crystal 113b and/or the increased path length or decreased path length due to peaks and valleys 130p, 130v for the light ray 116 can be tolerated or accepted, it is possible that the pre-aligned condition and the fully de-energized condition of the liquid crystal cell 110 may be the same. In such case, no separate low voltage electric field is required to obtain the pre-aligned condition; rather, such pre-aligned condition would be acceptably obtained by the surface treatment 132, for example. Substantially maximum optical phase retardation of which the cell 110 is capable would occur at that time. However, upon application of electric field, the amount of optical phase retardation would be reduced.

The liquid crystal cell 110 may be used as a shutter for simultaneously providing substantially the same amount of optical phase retardation to all light incident thereon. Such shutter effect may be provided substantially continuously or may provide a field sequential type operation or effect (sometimes referred to as frame sequential operation or effect) whereby one optical phase retardation condition exists and subsequently a different optical phase retardation condition exists; and such operation is repeated periodically. Other types of field sequential or frame sequential operation of the liquid crystal cell 110 as a shutter also may be employed as will be evident to those who have ordinary skill in the art from the description herein. For example, in a color sequential addressing mode of operation described below, the cell 110 may be used in a display to present respective color parts of an image at different times in sequence and the eye can integrate or add those images to form a full color image that is a still image or has motion.

In Figs. 3A and 3B are waveforms depicting the response time of a variable birefringence liquid crystal cell 110 relative to electrical energization thereof. In Fig. 13A prior to time t_j a relatively high voltage AC signal 148 shown in Fig. 13B, e.g., at 18 volts peak to peak and a frequency of from about 30 Hz to about 10 KHz, is applied to the liquid crystal cell 110; and the liquid crystal cell provides zero (or nearly zero) optical phase retardation, since substantially all liquid crystal material 113 therein is homeotropically aligned and other optics in a display 23 (Fig. 14) in which the liquid crystal cell 110 is used (described further below) blocks light transmission. At time t_j the voltage 148 is reduced to, for example, from about zero to about

5 volts, depending on whether (and to what extent needed) or not voltage is needed to obtain the pre-aligned condition of the liquid crystal material 113. It takes about 5 ms for the liquid crystal cell 110 and display 23 to achieve full transmission thereafter as is shown in the drawing. At time t₂ the voltage of the signal 148 again is increased to relatively high voltage; and the liquid crystal cell and display again assume a light blocking state. It takes approximately 1 ms from the time the relatively high voltage is applied until such light blocking state occurs.

As is usually the case and is depicted in Figs. 3A and 3B, measurement of turn off and turn on times for the liquid crystal cell 110, i.e., removing high voltage and applying high voltage, respectively, to assume respective transmission states of the display 23 is between about ten percent (10%) transmission and about 90 percent (90%) transmission. In the illustrated example, the display 23 using liquid crystal cell 110 is driven to dark condition or minimum transmission condition by applying relatively high voltage electric field; and it assumes the maximum transmission condition at relatively low (or zero) voltage electric field. However, it will be appreciated that the opposite operation can be used, whereby maximum and minimum transmission occur in response to maximum and minimum electric field, respectively. Also, preferably the electrical signal applied to the liquid crystal material is an AC signal, and, as usually is the case, the liquid crystal material responds to the rms value of the applied voltage electric field. However, in some circumstances it is possible to operate the liquid crystal cell 110 by a DC voltage, a pulsed DC voltage, etc.

The voltages, times, parameters and other dimensions described herein are exemplary. It will be appreciated that voltages, times, parameters and other dimensions may be used or selected for use in a liquid crystal cell and display using such cell in accordance with the present invention. The relatively fast operation of the liquid crystal cell 110 described above enables the liquid crystal cell and a display using that cell to operate satisfactorily in a frame sequential or field sequential mode of operation. The invention may be used with a time sequential addressing scheme to create multiple images at different sequential times in order to produce multicolor images. For example, as was mentioned above, at one time one image can be produced; and subsequently another image can be produced; the speed at which such images are produced can be integrated by the human eye, i.e., by a person watching the images being produced. Therefore, the display 23 can be illuminated with red light and a red image can be produced; and at subsequent times blue and green light, respectively, can illuminate the display 23 while the display creates different respective blue and green images. The eye can integrate the respective images to see a full color image. The relatively fast operation of the liquid crystal cell 110 also allows a mouse curser or some other display image to be moved relatively quickly across a display without encountering any smearing or there remaining a residual image. Advantages which inure to field sequential operation to produce a color image by the display include increased resolution and increased brightness of the display compared to conventional displays which use color filters. Each pixel of the liquid crystal cell 110 and display 23 using the cell is able to control each of the colors of light provided to illuminate the display; such control is provided at different time sequenced periods. However, since there is no need for a separate color filter for each color produced by a pixel, all light of a color delivered to the pixel can be controlled, and, if desired, output by the display. Additionally, since the entire pixel is used to control brightness of a particular color at any given time, and there are no color filters used to block light of the other colors at that given time, only one active light control device per pixel is required (rather than three, as in conventional r,g,b color filter devices), thus enhancing resolution. Another advantage of the present invention is the increased optically operative portion of the pixel compared to prior art liquid crystal display devices. Since the substrate 112 is an active matrix semiconductor substrate and the liquid crystal cell 110 is operated in reflective mode, the transistors and other electrically operative portions of the circuitry used to provide voltage to the electrodes 125 and the electric field developed as a result of such voltage, can be located beneath the electrode 125 so the transistor, etc., does not interfere with the light reflection function. In fact the transistor, or part of it, can be located beneath the surface of the semiconductor substrate 123 which forms surface 130 of the substrate 112, which may help in the surface 130 of the substrate being smooth or planar. (If the surface 130 were relative smooth or planar, sometimes referred to as planarized, the alignment of liquid crystal material there may be homogeneous and that at the surface 120 of the substrate Ul homeotropic.) Since each pixel then can control substantially all of the light incident thereon without having substantial "optically dead area" where the transistor is located, both brightness and resolution are increased relative to conventional transmissive liquid crystal display systems.

Increased brightness and resolution improves the output of the display 23 compared to prior art liquid crystal displays. Also, the intensity (brightness) of the light source can be reduced while high quality, good resolution and bright images can be created; and by reducing the light intensity or brightness requirements, the power consumption of a display 23 according to the invention can be reduced compared to prior art displays. Reduced power consumption facilitates using the liquid crystal cell 110 and display 23 in various portable head mounted displays, including those which are battery powered.

Each component 124 of the substrate 112 may be energized to cooperate with the conductor 121 on the substrate Ul to provide an electric field therebetween. Such electric field generally is confined to the area directly between the respective component 124 and the electrode 121. There is relatively little impact of the field in the area directly between the electrode 125 of a component 124 and the electrode 121 and the liquid crystal 113 in the area between a relatively adjacent electrode 125 and the electrode 121. Therefore, the electrical component 124a at the left hand side of the liquid crystal cell 110 illustrated in Fig. 11 can be energized to apply electric field to the liquid crystal in the area 150a while the electrical component 124b adjacent to component 124a is not energized so that electric field is not applied to the liquid crystal material in the area 150b (or a relatively low level voltage electric field is applied by the component 124b to obtain the pre-aligned condition for the liquid crystal material in the area 150b). Thus, each electrical component 124a, 124b, etc. may be selectively energized or not to apply desired electric field or not to the liquid crystal material in the area between the respective component and the conductor 121 at the substrate Ul. Selective control of the electrical components 124a, 124b, etc. enables the liquid crystal cell 110 to provide different amounts of optical phase retardation for light at the respective areas 150a, 150b, etc., thus providing different phase modulation of such light. The respective areas 150a, 150b, etc., and respective corresponding electrical components 124a, 124b and respective portions of electrode 121 may be considered respective picture elements (sometimes referred to in the art as pixels) 151a, 151b, etc. By providing different amounts of phase modulation to light by respective pixels and then decoding the modulation using appropriate analyzing apparatus, such as by a cholesteric liquid crystal reflector, one or more optical plane polarizers, and/or other optical elements, light transmission is controlled and images can be created.

As was mentioned above, the ellipses shown in the drawing of Fig. 11 represent alignment or orientation of a quantity of liquid crystal material in the area of the respective ellipse, such alignment being generally along the direction of the major axis of the ellipse. The actual size of the ellipses is not intended to be indicative of the actual size of liquid crystal molecules. The liquid crystal cell 110 is depicted schematically in Fig. 11. The actual dimensions of the various parts of the liquid crystal cell 110 are not intended to be proportional to the actual dimensions of a liquid crystal cell constructed in accordance with the invention, for example, having a thickness of liquid crystal layer 113 on the order of about six microns. Use of a silicon material as the substrate 112 permits part, or preferably all, of the circuits for developing electric field and driving the liquid crystal material 113 to respective orientations on or preferably in the substrate. The silicon substrate also may include auxiliary circuits used for control, signal conditioning, multiplexing, etc. Memory (RAM) may be included as part of the substrate to increase the performance capabilities of a display using the cell 110, for example, as is the case in computer video cards which include memory on board. Thus, the substrate may be a single "chip" containing all or substantially all the electronics required for operating the liquid crystal cell 110. Other circuitry also may be included as part of the substrate.

In operating the circuitry, for example, including circuitry 140 and components 124, the columns and rows of pixels 151a, 151b, etc. are driven by timing pulses, e.g. , to the columns, and using shift register arrangements, appropriate signals can be delivered to the rows of pixels. Information for operating respective pixels can be applied to the display one line at a time, for example, e.g. , as information is supplied to a conventional RAM. Depending to the nature of the signal applied to respective rows when timing signals are applied to respective columns, a particular pixel will be turned on or not to apply electric field to the liquid crystal between the respective electrode 125 and electrode 121. Since the electrical operation of the circuitry 140 and components 124 can be quite fast and since operation of the liquid crystal cell 110 also can be quite fast in switching from one liquid crystal orientation to the other, the illumination of the cell 110 can be turned off while the cell switches to a desired state and then the illumination can be turned on. This fast operation capability of the liquid crystal cell 110 and of the circuitry associated with it facilitates use in the color sequential addressing operation described above. Thus, using the just described operation the cell 110 would be illuminated by one color light and would produce an image of that color; subsequently the cell would be illuminated with a different color light and would produce another part of the same image; and so on as was described further above. The actual color operation may be according to computer graphics or NTSC convention.

Although liquid crystal cell 110 is shown as a single cellin Fig. 11, for example, it will be appreciated that a display 23 or other device may use a single cell 110 or a plurality of cells 10. If a plurality of cells 10 are used in a display, for example, the cells may be located in relation to each other to form an image whereby one part of the image is formed by one of the cells and another part of the image is formed by another cell, and so on.

It will be appreciated that the features of the liquid crystal cell 110 of the present invention may be used in the display 10 of the head mounted type. Also, features of the invention may also be employed in other types of display systems. One example is a display system that uses only a single display system of the type described herein. Such display system may be located in proximity to an eye for direct viewing. Alternatively, such display system may be used as part of a projection type display in which light from the display system is projected onto a surface where the image is formed for viewing. Various lenses and/other optical components may be used to direct from the display system light to create an appropriate image at a desired location.

The folded paths used in the liquid crystal cell 110 and in the illumination system 20 of the display 23 minimize the size of the display system, for example. Such minimization tends to reduce the weight of the display system and facilitates using it in a head mounted display. In an exemplary head mounted display, the viewing area of each liquid crystal cell and display system may be on the order of approximately 0.75 inch diagonal. A display 23 according to the invention may have a pixel resolution of, for example, 360 columns by 256 rows, with a pixel pitch at 40 microns by 40 microns, an array size of 0.906 inch by 0.660 inch and 0.708 inch diagonal. The display may be operable sufficiently fast to provide images at the speed of approximately 180 frames per second. Power to the circuitry of substrate 12 may be less than 2 watts with voltage being supplied at between about 30 V DC and about 35 V DC. Signal formats may be interlaced or non-interlaced. Also, one may use 30 frames per second operation. One also may use 1 micron geometry or 3 micron geometry for the pixels. These values are provided by way of example and are not intended to be limiting. The high resolution and brightness capabilities of the invention cooperate to provide for an efficient head mounted display as to size, weight, quality of image, minimizing power requirements, and so on. The liquid crystal cell and display may be embodied in a projection type display. In such a case images produced by the display, for example, can be projected onto a screen, a wall, etc., for viewing.

The display 23 also may include a quarter wave plate 67 positioned optically upstream/downstream, i.e. , in front of the face of the liquid crystal cell 110 to intersect light incident on and received from the face. The slow axis of the quarter wave plate is oriented relative to the rub direction or primary alignment direction of the homogeneously aligned liquid crystal material 113a proximate the substrate 11. The quarter wave plate therefore tends to convert the circularly polarized light from polarizer 31 to linearly polarized light having a plane of polarization such that optical phase retardation of such plane polarized light transmitted through the liquid crystal cell in a sense effects a rotation of that plane of polarization or otherwise alters the state of polarization of light transmitted through the liquid crystal 113. The quarter wave plate 67 ordinarily would provide quarter wave retardation for only one wavelength of light; and the amount of retardation provided other wavelengths may vary. If the angular relationship between the slow axis of the quarter wave plate and the primary rub direction of the homogeneously aligned liquid crystal material 113 is at an angle different from 90 degrees relation, chromaticity characteristic of the display 23 tends to increase; the greater the difference from that 90 degrees relationship, the greater the chromaticity and the smaller the difference the smaller the chromaticity characteristic and the less critical chromaticity considerations become. Thus, the quarter wave plate permits the display system 11 including the cholesteric reflector 22 to produce a dark field condition. The quarter wave plate 67 also converts plane polarized light received from the liquid crystal cell to elliptically polarized light the characteristics of which are a function of the plane of polarization of the plane polarized light received from the liquid crystal cell 110.

It will be appreciated that the reflective or folded light path characteristic of the liquid crystal cell 110 traveled by light 16 in Fig. 11, for example, complements or dovetails with the folded light path of the illumination system 20 of the display system 11. Also, the cholesteric liquid crystal reflector 22, as is described in the above referenced concurrently filed patent application, complements with the folded light paths of both the illumination system and the liquid crystal cell 110 to help direct light to the cell at an angle that is reasonably close to normal to provide for accurately formed high quality images produced by the display 23.

Operation of Display System 11 with a Display 23 using Liquid Crystal Cell 110 THnminated hv the -Illumination System 20. In Fig. 14 the polarization characteristics of a light ray as it travels from the light source 30 to the eye of a viewer is shown. Light symbolized by light ray 90 emitted by the light emitting diode 30 is unpolarized. The polarizer 31 placed in front of the light emitting diode 30 converts the light ray 90 to right circularly polarized light 91. The light ray 91 is refracted by the lens 24 and is reflected by the cholesteric liquid crystal reflector 22. The reflected light ray 91 remains right circularly polarized, as is depicted at light ray 91a.

The light ray 91a may be converted to plane polarized light by the quarter wave plate 67 and enters the front face of the display 23 at 92 and then is designated light ray 93. In the display 23 the polarization characteristic of the light ray 93 may or may not be changed. Such a change would be a function of the characteristics of the display 23, e.g., whether or not it retards light transmitted therethrough.

The polarized light ray 93 is reflected by the reflector associated with the liquid crystal cell 110, e.g., by substrate 112 and passes back through the liquid crystal material 113. Such reflection alters the polarization state or condition of light 93; and further optical phase retardation may occur as the light ray travels back through the liquid crystal material 113. The quarter wave plate 67, if used, changes or alters the phase of the light 93 received from the liquid crystal cell 110, for example, altering phase of elliptically polarized light, or, if appropriate alignment, wavelength, and polarization conditions exist for the light incident on the wave plate 67, such wave plate can change incident linearly polarized light (or possibly some of such light) to elliptically polarized light. Such elliptically polarized light may actually be circular or plane depending on the orientation of the plane of polarization, for example, and wavelength(s) of light 93 relative to the slow axis of the quarter wave plate 67. The reflection mentioned just above preferably is specular reflection by reflector 134; such specular reflection affects circularly polarized light by reversing the direction of polarization; the specular reflection does not change the direction of plane polarized light. For plane polarized light there is a phase change at the surface of the reflector 134, but the change is the same for both components of plane polarized light so that the direction of polarization does not change. However, the circularly polarized light changes direction of rotation, e.g., from right circularly polarized light to left or vice versa. For elliptically polarized light the reflector 134 changes the phase of the light incident on it and reflected by it. The elliptically polarized light 94 in effect is comprised of left and right circularly polarized components, which may have different amplitudes and are out of phase. The lens 24 refracts the light 94, and the cholesteric liquid crystal reflector 22 transmits the left circularly polarized component 95 and reflects the right hand circularly polarized component. The light 95 is refracted by lens 76 and is subsequently viewed directly by an eye or by projection onto a screen, etc.

The lighting pattern in the illumination system 20 used in an exemplary display system 11 is shown schematically in Fig 14. The cholesteric liquid crystal reflector 22 is spaced from the display 23 by a distance equal to about half the diagonal of the display 23. In the illustrated example the display 23 is approximately square in viewable area. The shape of the lens 24 and the emission pattern of the light emitting diode light source 30 are adjusted so that each of the illustrated two light emitting diodes 30 would illuminate about half the face 67 of the liquid crystal cell 110 of the display 23. In this way the angle of incidence of the light rays on the lens 24 and hence the cholesteric liquid crystal reflector 22 will not be too oblique. The less oblique, the larger the viewing angle of the display 23 without degradation of the viewed image by allowing the wavelength of the light emitted by the diodes 30 to remain within the reflective band of the cholesteric liquid crystal material.

The lens 24 may be part of a lens system of a virtual reality display or other heads-up display or head mounted display in order to place the image created by the display system 11 at a convenient viewing distance from the eye. The illustrated plano¬ convex lens 24 on the display side of the cholesteric liquid crystal reflector 22 as part of the illumination system also may serve as part of the optics required to adjust the image viewing system. A second plano-convex lens 76 and/or other optical components generally designated 75 may be placed on the other side of the cholesteric liquid crystal reflector 22 to complete the optics in a very compact and convenient form. In one example of the illumination system 20 the light source provides light of more than one, e.g., two, wavelength. Although the cholesteric reflector 22 may be able to reflect light of one of those wavelengths, it is not able to reflect light of the other wavelength. However, a second cholesteric liquid crystal reflector is able to reflect light of the other wavelength. Therefore, the two reflectors would be able to reflect both wavelengths produced by the light source.

The light source may produce more than two wavelengths of light, for example, red, green and blue wavelengths. In such case, three cholesteric liquid crystal reflectors may be used respectively to reflect a particular wavelength of light in the manner described above. The respective cholesteric liquid crystal reflector which reflects a particular wavelength is transparent to other wavelength.

Thus, it will be appreciated that the illumination system 20 may include a white light source 30, which includes red, green and blue lights. The several cholesteric liquid crystal reflectors are operative to reflect the various wavelengths of such white light to the liquid crystal cell 110. The display system 11 using a white light illumination system 20 is able to produce output images that are black and white. This output is distinguished from the color of the output images produced by the display systems 23 described above wherein the color of the output light is a function of the wavelength produced by the light source of the respective illumination system. The variable birefringence liquid crystal cell 110 is useful with white light sources because it can be operated to provide optical phase retardation approximately in or overlapping the zero order range. Such zero order operation helps to avoid color dispersion of light passing the liquid crystal cell which improves clarity, accuracy, resolution and overall quality of the images produced.

In a full color display the illumination system 20 includes several sources of light, each having a different wavelength. For example, three separate light sources provide red, green and blue wavelength light, respectively, or light that is in respective wavelength bands or ranges that include red, green and blue, respectively. The light sources may be respective light emitting diodes or they may be other sources of red, green and blue light or other respective wavelengths of light, as may be desired. The cholesteric liquid crystal reflectors are able to reflect red, green and blue light respectively. Such reflection occurs, as was described above, when the circular polarization characteristic of the light is the same direction as the twist direction of the cholesteric liquid crystal material in the respective reflector. The cholesteric reflectors are transparent to the other polarizations of incident light and to the other wavelengths of incident light.

The illumination system 20 sequentially illuminates the display 23 (or respective portions of the display) with respective wavelengths of light. For example, for a period of time the display (or portion thereof) is illuminated with red light; subsequently illumination is by either green or blue light; and still subsequently illumination is by the other of green or blue light. Such sequential illumination may be carried out sufficiently rapidly so that respective red, green and blue images created by the display when illuminated by the respective colors of light are output from the display system and are integrated by the human eye. As a result, the human eye effectively sees a multicolor image. Other examples of frame sequential switching to provide multicolor and/or full color outputs are known in the art. Various advantages inure to a frame sequential multicolor display, including the ability to provide high resolution with approximately one-third the number of picture elements required for a full color r, g, b display system in which respective pixels are red, green or blue.

The sequential delivering of red, green and blue light to the display 23 is coordinated by the control system, such as the circuit 13, with the driving of the display 23. Therefore, when a red image or a portion of a red image is to be produced, it is done when red light is incident on the display 23; and the similar type of operation occurs with respect to green and blue images. The variable birefringence liquid crystal cell 110 is able to operate sufficiently fast to provide field sequential switching to develop respective red, green and blue (or other color) images that can be integrated by the eye to obtain a full multicolor image. A dark condition is produced by a pixel 151a, 151b, etc., or by a substantial portion of the display 23 is produced by the display, as follows. Say right circularly polarized is directed from the light source 71 and the cholesteric reflector 22 reflects right circularly polarized light and transmits left circularly polarized light. Therefore, reflector 22 reflects light from the source 71 is reflected toward the display 23. The quarter wave plate 67 converts the light to plane polarized light. The liquid crystal cell is fully energized and ideally provides zero optical phase retardation to the light transmitted therethrough, including on the incident pass to the reflector 134 and the reflected pass from the reflector. The plane polarized light is converted by the quarter wave plate 67 to right circularly polarized light again, which is blocked by the cholesteric reflector 22 then serving as an analyzer for the display system 11. However, if the liquid crystal cell 110 is operative to provide optical phase retardation, then the wave plate 67 converts the light received from the cell to elliptically polarized light, which is comprised of two circularly polarized components, one right and the other left, which are proportional to the ellipticity of the light received from the cell. The left circularly polarized component will be transmitted by the analyzer 22 and the right circularly polarized component will be blocked. Turning briefly to Fig. 15 a schematic elevation view of a reflective variable birefringence liquid crystal cell 210 using homeotropically aligned liquid crystal material 213 that has negative dielectric anisotropy is shown. In Fig. 15 parts corresponding to parts described above with respect to Figs. 11-14 are identified by corresponding reference numerals increased by the value two hundred. The liquid crystal cell 210 is a reflective variable birefringence liquid crystal cell having an active matrix substrate on at least one side. The liquid crystal cell 110 includes a pair of substrates 211, 212 and liquid crystal material 213 in the space 214 between the substrates. The liquid crystal material has negative dielectric anisotropy and is birefringent. A seal (not shown) is provided about the perimeter of the liquid crystal cell to retain the liquid crystal material therein preventing leakage.

Of the liquid crystal material 213 that liquid crystal 213a, 213b relatively proximate the substrates 211, 212 has generally homeotropic alignment. The bulk 244 of the liquid crystal material also has homeotropic alignment. During operation of the liquid crystal cell 210 optical phase retardation of light 216 (represented by a light ray line shown) traveling through the liquid crystal material can be controlled as a function of the applied electric field across the liquid crystal material. As the field is increased, at least some of the liquid crystal material 113 tends to assume homogeneous alignment, thus causing optical phase retardation of light traveling through the cell. Surface treatment, e.g., as was described above of the substrates 211, 212 may be used to cause a preferred homeotropic alignment with a small tilt angle in order to induce a preferred homogeneous alignment for controlled optical phase retardation upon application of an electric field.

The liquid crystal cell 210 ordinarily would not require any pre-alignment conditioning for the liquid crystal material. Therefore, such cell could be operated in a field on or field off condition to obtain the desired minimum or maximum optical phase retardation. If desired, however, there may be a pre-aligned conditioning of the cell 210, e.g., by electric field input, by mechanical or magnetic means, etc.

The liquid crystal cell 210 may be used in the display 23. In one operative mode, e.g., electric field on or field off, of the cell 210, for example, the cell would provide minimal optical phase retardation to obtain one light transmissive condition, e.g. , transmission or blocking of light; and the other condition mode would be provided in the other operative mode.

Fig. 16 is a schematic illustration of an alternate form of liquid crystal cell 310 and display 323 using a plane polarizer. In Fig. 16 the various parts which are the same as those used in the liquid crystal cell 10 and display 23 are labeled with the same reference numerals as in Figs. 1 and 2, but increased by the value 300. The display 323 alternatively may use the cell 110 of Fig. 15. In the display 323 there is a plane polarizer 400 at the face 379 of the cell 310. The transmission axis of the plane polarizer is aligned at an angle of 45 degrees with the primary alignment direction of the homogeneously aligned liquid crystal material 13a. The polarizer 400 may be separate from or may be adhered to the face 379 of the cell 310. If desired, the polarizer may be an evaporated coating or material, for example, on the face 379.

In the display 323 the plane polarizer 400 transmits light into the liquid crystal cell 310. If the liquid crystal cell is at the minimum retardation state of operation, the plane polarized light will transmit to the reflector 334, will be reflected back to the plane polarizer and will be further transmitted for further use in the display 323. Such further use may include conversion to circularly polarized light for analysis or selective transmission by the analyzer 372 or by some other device. Alternatively, the polarized light from the polarizer 400 may be transmitted for viewing as the output of the display 323. If the liquid crystal cell is at a condition that it provides retardation, the plane polarized light will transmit to the reflector 334, will be reflected back to the plane polarizer, but the plane of polarization will have been rotated and, therefore, the amount of light that will exit the plane polarizer 400 will be decreased. Referring to Fig. 17, the illustrated display system 501 includes a light source

502, liquid crystal display 503, optics 504 for projection or viewing of the images created by the liquid crystal display 503, a computer control 505, and an image signal source 506. A photodetector 507 also may be included in the system 501. The light source and display 502, 503, the controller 505, as well as the display system 501, may be the same or similar as those described above or they may be of some other type. The light source 502 may be one or more light emitting diodes, incandescent light source, fluorescent light source, light received via fiber optics or other means, a metal halide lamp, etc.

The display 503 may be a twisted nematic liquid crystal cell, a variable birefringence liquid crystal cell, a supertwist liquid crystal cell, or some other type or liquid crystal cell able to modulate light. The liquid crystal display 503 may include polarizers, wave plates, such as quarter wave plates or other wave plates, means for compensating for residual birefringence or for problems encountered during off axis viewing, etc. The optics 504 may be one or more lenses separate from and/or included as part of the liquid crystal display for the purpose of providing an output image for viewing or for projection. If for viewing, such optics 504 may be one or more lenses which focus an image for close, e.g., as in a head mounted display of the heads up display type, virtual reality type or multimedia type, or far viewing, e.g., as in a slide viewer or a television. If for projection, such optics 504 may include projection optics which project an image formed by the display 503 onto a screen for transmissive viewing or reflective viewing.

The image signal source 506 may be a source of computer graphics signals, NTSC type television (video) signals, or other signals intended to produce an image on the display 503. Such signals are decoded in conventional manner by the computer control 505, for example, as is the case in many display systems, and in response to such decoding or deciphering, the computer control 505 (or some other appropriate control, circuit, etc.) operates the display 503 to produce desired images. If desired, the computer control 505 can operate the display 503 in sequential manner to produce multiple images in sequence while the display is being illuminated by only a single light source or color of light, e.g., a monochromatic type of operation. Other exemplary types of operation of the computer control 505 include those employed in conventional liquid crystal display televisions of the hand-held or larger type and/or liquid crystal type computer monitors. Alternatively, the computer control can operate the display 503 in a field sequential or frame sequential manner whereby a particular image is formed in several parts; while one part is formed, the display is illuminated by light of one color; while another part is formed, the display is illuminated by light of a different color; and so on. Using this field sequential type operation, multicolor images can be produced by the display system apparatus 501.

In a typical input signal to a television or liquid crystal television, there is information indicating brightness of the light to be transmitted (or reflected) at a particular pixel. The computer control 505 is operative to compute the brightness information of a particular image or scene and in response to such computation to control the intensity or brightness of the light source 502. While intensity or brightness of the light source is controlled in this manner, the computer control 505 operates the liquid crystal display 503 to modulate light without having to reduce the number of pixels used to transmit light. Therefore, the full number or a relatively large number of pixels can be used to form the image or scene even if the brightness of the scene as controlled by the controlled light source is relatively dark.

Information coming through from the image signal source 506 may indicate various levels of illumination. There usually is a blanking pulse and a data line pulse. The computer control 505 can take the integral of the data line electrically or an integral of the whole set of data (from all of the data lines of the scene) or all of the pixels while electrically skipping the blanking. Based on that integral, the brightness of the light incident on the display 503 is controlled by the computer control 505. It will be appreciated that a person having ordinary skill in the art would be able to prepare an appropriate computer program to provide the integral functions and to use the results of such integration to provide brightness control for the light source 502. From the foregoing, then, it will be appreciated that the apparatus 501, including the computer control 505, is operative to control or to adjust the brightness of a scene without degrading the contrast ratio. Thus, the same contrast ratio can be maintained while brightness of a scene or image is adjusted. For example, the same contrast ratio or substantially the same contrast ratio can be maintained by the apparatus 501, whether depicting a scene of a bright sunlit environment or of the inside of a dark cave. Therefore, the scene will have the appearance of illumination under natural illumination conditions.

The features of the invention can be used in virtually any passive display system.

Power requirements of the apparatus 501 can be reduced over prior display systems because the intensity of light produced by the source 502 is controlled to create dark images. In prior systems, though, the intensity of the light produced by the source was maintained substantially constant while the amount of light permitted to be transmitted through the passive display would be reduced to create a dark scene image.

In addition to controlling intensity of the light source 502 as a function of brightness of a scene, the computer control 505 also may be responsive to measurement or detection of the ambient environment in which the apparatus 501 is located. The brightness of such ambient environment may be detected by the photodetector 507. The photodetector 507 may be place in a room or elsewhere where the image created by the display 503 is to be viewed; and the brightness of the source 502 can be adjusted appropriately. For example, if the room is dark, it usually is desirable to reduce brightness of the source; and if the room is bright or the apparatus is being used in sunlight, the brightness of the source may be increased. Turning to Fig. 18, (primed reference numerals refer to elements which are similar to those referred to above designated by the same unprimed reference numerals) a light reflective display system according to the invention is illustrated at 501'. The display system 501' includes a light source 502', liquid crystal display 503', optics 504' for projection or viewing of the images created by the liquid crystal display 503', a computer control 505', and an image signal source 506. A photodetector 507 also may be included in the system 501. The various parts of the display 503' and optics 504' may be the same or similar to those disclosed in the concurrently filed, commonly owned U.S. patent application referred to above. The light source 502' and display 503' may be of the type disclosed above.

For example, the light source 502' may include a source of circularly polarized light 502a' and a cholesteric liquid crystal reflector 508 (also designated above by reference numeral 22). The liquid crystal display 503' may be a reflective variable birefringence liquid crystal display device. Full Color Frame Sequential Illumination System and Display.

Turning to Fig. 19, which is similar to Fig 11, a full color display subsystem 11 including illumination system 320 is shown. However, in the display subsystem 11 the illumination system 320 includes several sources of light, each having a different wavelength. For example, three separate light sources 502r, 502g, 502b provide red, green and blue wavelength light, respectively, or light that is in respective wavelength bands or ranges that include red, green and blue, respectively. The light sources may be respective light emitting diodes or they may be other sources of red, green and blue light or other respective wavelengths of light, as may be desired for use in the display subsystem 11. The cholesteric liquid crystal reflector 508 is able to reflect green light; the reflector 508a is able to reflect red light; the reflector 508b is able to reflect blue light. Such reflection occurs when the circular polarization characteristic of the light is the same direction as the twist direction of the cholesteric liquid crystal material in the respective reflector. The reflectors 508, 508a, 508b are transparent to the other polarizations of incident light and to the other wavelengths of incident light.

The illumination system 320 is intended sequentially to illuminate the display 503', which may include a wave plate, such as a quarter wave plate 509, (or respective portions of the display) with respective wavelengths of light. For example, for a period of time the display 503' (or portion thereof) is illuminated with red light; subsequently illumination is by either green or blue light; and still subsequently illumination is by the other of green or blue light. Such sequential illumination may be carried out sufficiently rapidly so that respective red, green and blue images created by the display 503' when illuminated by the respective colors of light are output from the display subsystem 11 and are integrated by the human eye. As a result, the human eye effectively sees a multicolor image. Other examples of frame sequential switching to provide multicolor and/or full color outputs are known in the art. Various advantages inure to a frame sequential multicolor display, including the ability to provide high resolution with approximately one-third the number of picture elements required for a full color r, g, b display system in which respective pixels are red, green or blue.

The sequential delivering of red, green and blue light to the display 503' is coordinated by the control system 505 with the driving of the display 503'. Therefore, when a red image or a portion of a red image is to be produced by the display 503', it is done when red light is incident on the display 503'; and the similar type of operation occurs with respect to green and blue images.

If the respective light sources 502r, 502g, 502b are light emitting diodes, they may be sequentially operated or energized to provide light in coordination with the operation of the display 503' under direct control and/or energization by the control system 505. Alternatively, the control system 5 may be coordinated with whatever other means are used to provide the respective red, green and blue color lights of the light source. Referring to Fig. 1 briefly, a head mounted display 10 includes a pair of display systems 11, 12 and a control system 13 (like the system 505 in Figs 17-19, for example) for creating images intended to be viewed by the eyes 64, 65 of a person. The control system 13 (505) in conjunction with the display systems 11, 12 are intended to create images for viewing by the eyes. Those images may be monochromatic. The images may be multicolor. The images may be two-dimensional or they may provide a three dimensional, stereoscopic effect. Stereoscopic effect viewing is obtained when the control system 13 (505) operates the display systems 11,

12 to provide, respectively, right eye and left eye images that are sufficiently distinct to provide depth perception. Right eye, left eye imaging and depth perception are techniques used in some stereoscopic imaging and viewing systems which are commercially available.

The display systems 11, 12 may be identical. The control system 13 (505) provides control and/or power input to the display systems 11, 12 to create images for display to the eyes 14, 15. The display 10 may be a head mounted display, such as a heads-up display, a virtual reality display, or a multimedia display. The control system

13 (505) may be generally a control system of the type used in known head mounted displays to create such images. Such a control system may provide for control of color, light intensity, image generating, gamma, etc. The display systems 11, 12 may include focusing optics so as to focus the image created by the display systems for comfortable viewing, for example from a few inches up to a few feet in front of the eyes, say, from about 20 inches to about several feet in front of the eyes. Turning to Figs. 20-25, operation of the apparatus is described. In Fig. 20 a plan view of a dot matrix liquid crystal display is shown. The shade of grey measured at several pixels is indicated. According to the bottom graph in Fig. 20, the actual hade is shown; according to the dot matrix image at the side and top of Fig. 20, the actual shade of the pixel is shown. Thus, at location 1 on the graph at the bottom of Fig. 6, there is a shade 2. At location 2, there is a shade 1. At location 3 there is a shade 0, and so on. In pixel 1 marked in the top of Fig. 20, the pixel is a shade gray of 2; and at the adjacent pixel the pixel is a shade gray of 1, and so on. This is conventional. This would indicate the signals coming in to the computer control 505 (13). In Fig. 21, an example of a bright image scene produced by back light at a medium (normal) illumination level is illustrated at the top; the shades of gray are shown at the middle left; and the lamp light level is constant at the bottom left. The viewer sees a bright low contrast image of a person as seen at the top right of the drawing. A side view of the display representing respective pixels and the tray levels thereof is shown at the bottom right of the figure.

Fig. 22 is similar to Fig. 21 again with average constant lamp light level. The average light level is produced; the average brightness output from the display is to be produced; and the viewer sees an average brightness high contrast image because all conditions are optimized. Fig. 23 is similar to Fig. 21 again with average constant lamp light level and a dark transmission provided by the liquid crystal cell; the viewer sees a dim low contrast image.

Figs. 21-23 represent operation of a standard display apparatus. Figs. 24 and 25 represent applying the principles of the present invention to develop high contrast images. In Fig. 24 it is seen that there is the intent to produce a wide range of gray levels; and this is possible by using a high intensity lamp level; the result is a bright high contrast image. In Fig. 25 it is intended that the viewer see a dim image; the same range of grey shades are provided as is depicted in the middle left graph of the drawing; but the lamp level is low. Therefore, there is a good contrast ratio provide to the viewer; from 0 to about 7 at the brightness level shown in the graph at the upper left of the drawing.

Claims

CLAIMSI claim,

1. An illumination system for a display, comprising a source of polarized light, an analyzer for reflecting light to the display or transmitting light as a function of the wavelength and/or polarization characteristic of incident light, and light from the display being directed to said analyzer for reflection or transmission thereby.

2. The system of claim 1, said source comprising a multicolor source.

3. The system of claim 2, said source comprising a source of substantially white light.

4. The system of claim 2, said analyzer comprising a plurality of analyzers which transmit or reflect light as a function of polarization characteristic of the light incident thereon and the respective color of the light.

5. The system of claim 1, said source comprising a plurality of separately operated different color light sources, for sequentially providing to the analyzer light of different respective colors.

6. The system of claim 5, said analyzer comprising a plurality of analyzers which respectively transmit or reflect light as a function of polarization characteristic of the light incident thereon and the respective color of the light.

7. The system of claim 1, said analyzer comprising a cholesteric reflector which selectively reflects or transmits light as a function of the circular polarization direction and the wavelength of the light incident thereon.

8. The system of claim 7, said analyzer comprising a plurality of cholesteric reflectors in optical series, each being operable with respect to a different wavelength range of light.

9. The system of claim 1, said source comprising a source of circularly polarized light.

10. A method for illuminating a display, comprising supplying light having a polarization to an analyzer, reflecting or transmitting light by the analyzer as a function of the wavelength and polarization characteristic of incident light, said reflecting comprising reflecting light to a display for illuminating the display.

11. The method of claim 10, said supplying comprising supplying elliptically polarized light.

12. The method of claim 11, said supplying comprising supplying circularly polarized light.

13. The method of any of claims 10-12, said reflecting comprising using a cholesteric reflector to reflect light of a wavelength range and polarization characteristic and to transmit light which is outside such wavelength range or has a different polarization characteristic.

14. The method of any of claims 10-13, further comprising reflecting light from a display for transmission of at least some of such light as a light output.

15. The method of claim 14, further comprising providing such light output for projection display or for direct viewing.

16. A display system, comprising the illumination system of any of claims 1-9, and further comprising a display receiving light from said analyzer, said display being operable selectively to change a polarization characteristic of incident light, and light from said display being directed to said analyzer for reflection or transmission thereby.

17. A display system, comprising a source of polarized light, an analyzer for reflecting or transmitting light as a function of the wavelength and polarization characteristic of incident light, and a display receiving light from said analyzer, said display being operable selectively to change a polarization characteristic of incident light, and light from said display being directed to said analyzer for reflection or transmission thereby.

18. A projector comprising the display of claims 16 or 17 and further comprising a projection lens.

19. A display as set forth in claims 16 or 17, and further comprising means for mounting the display on the head of a person.

20 A display system comprising a plurality of displays as set forth in claims 16-18 for simultaneously providing plural images.

21. A method of display, comprising providing a source of polarized light, analyzing polarized light by selectively reflecting or transmitting light as a function of the wavelength and polarization characteristic of incident light, and creating a desired light pattern from light received from said analyzing step by selectively changing a polarization characteristic of incident light, and selectively transmitting light according to such pattern.

22. A liquid crystal cell, comprising a birefringent liquid crystal material, a pair of surfaces containing the birefringent liquid crystal material therebetween, the birefringent liquid crystal material in proximity to one surface providing relatively minimal optical phase retardation, the birefringent liquid crystal material being preconditioned to switch between two retardations to provide relatively minimal optical phase retardation and increased optical phase retardation.

23. The liquid crystal cell of claim 22 , said switching being over a range that is between such relatively minimal increased optical phase retardation.

24. The liquid crystal cell of claim 22, said switching being between approximately zero and 1/4 lambda.

25. A liquid crystal cell, comprising a birefringent liquid crystal material, a pair of surfaces containing the birefringent liquid crystal material therebetween, the birefringent liquid crystal material in proximity to one surface providing relatively minimal optical phase retardation, the birefringent liquid crystal material being preconditioned to switch between two retardations to provide relatively minimal optical phase retardation and increased optical phase retardation, and a plane polarizer selectively to block or to transmit light to or from one of said surfaces as a function of the direction of the plane of polarization of such light.

26. A birefringent liquid crystal apparatus, comprising, a pair of surfaces; liquid crystal material between said surfaces, the liquid crystal material in relative proximity to one surface exhibiting generally homeotropic alignment, the liquid crystal material in relative proximity to the other surface exhibiting generally homogeneous alignment, means to alter alignment of at least one of said alignments to change the optical phase retardation characteristic exhibited by the liquid crystal material in the apparatus, and reflector means for reflecting light transmitted in said liquid crystal material back through said liquid crystal material.

27. The apparatus of claim 26, one of said surfaces comprising a semiconductor substrate.

28. The apparatus of claim 27, said reflector means being at said semiconductor substrate.

29. The apparatus of claim 27, said one surface comprising a surface associated with a semiconductor substrate.

30. The apparatus of claim 29, said semiconductor being substantially optically non-transparent.

31. A reflective liquid crystal apparatus, comprising, an optically reflective semiconductor substrate having electrical components, a further substrate, liquid crystal between said substrates, electrical means cooperative with electrical components of said semiconductor substrate to apply electric field to liquid crystal material to alter the optical phase retardation characteristics of the liquid crystal, the further substrate being able to transmit light therethrough for transmission through the liquid crystal and reflection by said optically reflective semiconductor substrate.

32. A liquid crystal apparatus, comprising first and second substrates, one substrate having a relatively unsmooth surface and the other having a relatively smooth surface, birefringent material between the substrates, said birefringent material comprising liquid crystal material, said liquid crystal material in relative proximity to said relatively unsmooth substrate having generally homeotropic alignment, and said liquid crystal material in relative proximity to said relatively smooth surface having generally homogenous alignment.

33. A display apparatus, comprising, a source of polarized light, a liquid crystal display for selectively modulating light, said liquid crystal display comprising a semiconductor substrate and a further substrate with birefringent liquid crystal material between said substrates, said source being operative to provide light to pass into said liquid crystal material through said further substrate toward said semiconductor substrate, and means to reflect light to pass through said liquid crystal material and subsequently through said further substrate, the liquid crystal material being operative in response to an input to change the optical phase retardation characteristics of the liquid crystal material.

34. The display apparatus of claim 33, said liquid crystal material having generally homeotropic alignment relatively proximate said semiconductor substrate, and generally homogeneous alignment relatively proximate said further substrate.

35. The display of claim 34, said liquid crystal material having positive dielectric anisotropy.

36. The display apparatus of claim 33, said liquid crystal material having generally homeotropic alignment relatively proximate each of said substrates, said liquid crystal material having negative dielectric anisotropy.

37. A birefringent liquid crystal optical apparatus, comprising, a pair of substrates and a birefringent liquid crystal material between the substrates, means for applying electric field between said substrates to alter liquid crystal alignment and therefore optical phase retardation characteristics of liquid crystal subject to such field, one of said substrates including means for causing generally homeotropic alignment of liquid crystal material that is relatively proximate to said substrate, said means for applying electric field comprising mean to buttress such homeotropic alignment.

38. The apparatus of claim 37, the liquid crystal material relatively proximate the other of said substrates having generally homogeneous alignment.

39. The apparatus of claim 38, said means for applying comprising means to apply electric field to said liquid crystal material primarily to align liquid crystal material in the area between said homeotropically aligned liquid crystal material and the other of said substrates in a direction generally parallel with said homeotropically aligned liquid crystal thereby to alter the optical phase retardation characteristics of the liquid crystal material.

40. The apparatus of claim 39, said liquid crystal material having approximately 1/4 wave optical phase retardation characteristics in one operative condition thereof in response to application of a relatively low voltage electric field by the means for applying, and approximately zero wave optical phase retardation characteristics in a different operative condition thereof in response to application of a relatively larger voltage electric field by the means for applying.

41. A method of variably phase retarding light, comprising preconditioning the orientation of liquid crystal material in a liquid crystal cell to have substantially homogeneous alignment at one surface of the cell and substantially homeotropic alignment at a different surface of the cell, and applying an input to the cell to change the orientation of at least some of the liquid crystal material thereby to alter the optical phase retardation of light transmitted in the liquid crystal material.

42. Apparatus for displaying an image, comprising a light modulating passive display, and control means for controlling the intensity of light supplied to the light modulating passive display.

43. The apparatus of claim 42, said control means comprising means for detecting the brightoess of at least a portion of an image to be produced by the apparatus and for controlling intensity of such light supplied to the light modulating passive display as a function of such detecting.

44. The apparatus of claim 43, said control means comprising a computer.

45. The apparatus of claim 43, said control means comprising means for detecting the average brightness of the entire scene produced by the apparatus and for controlling intensity as a function of such average brightness.

46. The apparatus of claim 42, further comprising means for detecting ambient brightness in the local environment where such image is to be displayed.

47. The apparatus of claim 46, said control means comprising means for controlling intensity of light supplied to the light modulating passive display as a function of the ambient brightness.

48. The apparatus of claim 42, the control means comprising means for controlling the light modulating passive display to produce respective images.

49. The apparatus of claim 48, the control means comprising means for controlling the intensity of a plurality of light sources.

50. The apparatus of claim 49, the control means comprising means for controlling the passive display to produce frame sequential images and means for controlling respective light sources as a function of which image is produced at a particular time.

51. The apparatus of claim 42 , further comprising a plurality of light sources of different respective colors, and said means for controlling comprising means for operating said display in a field sequential mode while controlling the brightness of light emitted from respective light sources, whereby multicolor images are developed.

52. A display comprising a light modulator, a source of light, and a control for controlling the intensity of light from the source supplied to the light modulator.

53. The display of claim 52, said light modulator comprising a liquid crystal display.

54. The display of claim 53 , said liquid crystal display comprising a variable birefringence liquid crystal apparatus.

55. The display of claim 53, said liquid crystal display comprising a reflective liquid crystal device.

56. The display of claim 52, said control comprising means for determining the brightoess of at least a portion of an image to be produced by the light modulator and for controlling intensity of such light supplied to the light modulator as a function of such detecting.

57. The display of claim 52, said control being operative to control intensity while operating the light modulator in a field sequential mode.

58. A method of displaying an image using a passive light modulating display apparatus, comprising controlling the intensity of light illuminating the display apparatus as a function of a brightoess characteristic of the image.

59. The method of claim 58, comprising using a computer to determine the average brightoess of the image.

60. The method of claim 58, comprising producing respective images using the display apparatus in a field sequential mode, and as each image is produced controlling intensity of the light illuminating the display apparatus.

61. The method of claim 58, comprising producing respective images using the display apparatus in a field sequential mode, and as each image is produced controlling intensity of different respective color light --Uuminating the display apparatus.

62. A method of correcting a display to alter the brightness of the display, comprising detecting the brightoess of at least part of an image produced by the display, and adjusting the brightoess of the display as a function of such detecting.

63. The method of claim 62, said detecting comprising detecting effective brightness as a function of signals intended to be sent to a display to produce a light output.

64. The method of claim 62, comprising computing the brightness information of a particular image or scene and in response to such computation to control the intensity or brightness of the output light.

65. The method of claim 64, further comprising supplying -tight from a source to a display, and said controlling of intensity or brightness comprising t controlling intensity or brightoess of the light source.

66. The method of claim 65, wherein an output image is produced by plural pixels included in a display, and said controlling comprises modulating light without having to reduce the number of pixels used to transmit light.

67. The method of claim 62, further comprising supplying -light from a source to a display, and wherein an output image is produced by plural pixels included in a display, and said adjusting comprises adjusting brightoess with a full number or a relatively large number of pixels forming an image or scene even if the brightness of the scene as controlled by the controlled light source is relatively dark.

68. The method of claim 62, further comprising supplying information from an image signal source to indicate various levels of illumination, integrate signals representing image data.

69. The method of claim 68, further comprising controlling, as a function of such integration, brightoess of light incident on a display to form an image output.