WO2001026384A1

WO2001026384A1 - Color separating prism assembly and image projection display system

Info

Publication number: WO2001026384A1
Application number: PCT/US2000/017589
Authority: WO
Inventors: Brett J. Bryars; Michael R. Greenberg; Paul G. Hannan
Original assignee: Optical Coating Laboratory, Inc.
Priority date: 1999-10-06
Filing date: 2000-06-27
Publication date: 2001-04-12

Abstract

A color separation device is provided in the form of a prism assembly that has three joined prisms for use in an image projection display system. The prism assembly is configured such that each of the prisms has a total internal reflection surface with a phase control optical coating thereon. The phase control optical coatings utilized in the prism assembly provide enhanced image contrast and quality in the display system.

Description

COLOR SEPARATING PRISM ASSEMBLY AND IMAGE PRO ECTION DISPLAY SYSTEM

BACKGROUND OF THE INVENTION 1. The Field of the Invention

The present invention is related to a color separation device for use in an image projection display system. More particularly, the present invention is related to a color separating prism assembly v/hich provides improved brightness and contrast in color image projection display systems. 2. The Relevant Technology

In image projection display systems, a high contrast ratio is desirable in order to produce visually pleasing images. When such projection display systems employ a polarizing beam splitter cube in combination with a Philips prism as part of the optical system, polarization state changes in a converging light cone occur that degrade image contrast. Image degradation occurs in conventional image projection systems due to rotations and ellipticity in the polarization that occur as light traverses in double-pass mode through the system. Rotation and ellipticity in the polarization of light have the net result that the portions of the image which are supposed to be dark or black are instead light or gray, which reduces the image contrast and quality. A display screen should project a bright image relative to the ambient lighting conditions. High brightness of the "on" pixels in an imager enhances the contrast ratio and allows the display system to be used in a broader range of ambient lighting conditions, i.e., a darkened room is not required.

A prior method used to reduce the effects of unwanted polarization rotation and ellipticity is described in U.S. Patent No. 5,327,270 to Miyatake. There, a liquid crystal imaging system is disclosed in which a quarter wave plate, i.e., a phase compensating plate with retardance value = 0.25, is used to partially compensate for the rotations and ellipticity that occur in the polarization vectors as they traverse through the imaging system. This system requires precise alignment of the phase compensating plate with respect to the other optical components, which can be a difficult and time consuming procedure. Additionally, in most systems, the optimum compensating value of retardance is not 0.25, but is some other number depending on the parameters of the individual system, as disclosed for example in co-pending application entitled "Apparatus and Methods for Enhancing Contrast in a Projection System Using Reflective Imager," the disclosure of which is incorporated herein by reference. For optimum imaging, it is therefore necessary to find a method to compensate as completely as possible for polarization shifts in a particular system.

Yet another method used to compensate for differences in polarization states is described in U.S. Patent No. 5,594,591 to Yamamoto et al., which employs a multilayer dielectric coating such as antireflection coating on the two total internal reflection (TIR) surfaces of a standard Philips prism in lieu of quarter wave compensating plates. The antireflection coating on the TIR surface is designed with a phase control function which can be used to compensate for the phase differences which occur due to polarization rotations and ellipticity which are incurred throughout the imaging system. While this method assists in increasing image contrast, the benefits are limited to the first two color channels of the Philips prism since it is only those two channels in a conventional Philips prism that include total internal reflection surfaces where dielectric phase control coatings can be used. In U.S. Patent No. 5,644,432 to Doany, a color separator for liquid crystal display imagers and similar projection systems is formed from three joined prisms. The light path through the first prism has a total internal reflection, while the light paths of the second and third prisms have no total internal reflections. As in the Philips prism, dichroic coatings are used on the surfaces between the first and second prisms and also between the second and third prisms; however, the coatings are only used for color separation and are not specially designed for phase control.

Accordingly, there is a need for improved color separation devices for use in color projection display systems which overcome the above difficulties, while being easy to manufacture and providing maximum contrast ratio. SUMMARY

In accordance with the invention as embodied and broadly described herein, a color separation device for an image projection display system is provided and includes an optical body adapted to split an entering light beam into a first color component, a second color component, and a third color component, with the optical body configured to cause each color component to exit the optical body after a total internal reflection therein. The color separation device is preferably in the form of a prism assembly that has three joined prisms, with the prism assembly configured such that each of the prisms has a total internal reflection (TIR) surface with a phase control optical coating thereon. The phase control optical coatings provide enhanced image contrast and quality in the display system. In more detail, a color separating prism assembly for use in an image projection display system includes a first prism adapted to provide a first color channel by splitting an incident light beam into a first color component that exits from the first prism after at least one total internal reflection along a first optical path. A second prism is in optical communication with the first prism at a first interface, with the second prism adapted to provide a second color channel by splitting the light beam arriving from the first prism into a second color component, which exits from the second prism after at least one total internal reflection along a second optical path. A third prism is in optical communication with the second prism at a second interface, with the third prism adapted to provide a third color channel for the third or remaining color component of the incident light beam which exits from the third prism after at least one total internal reflection along a third optical path. Each of the first, second and third prisms can be configured to have a TIR surface with a phase control optical coating thereon.

In various embodiments of the prism assembly, at least two of the prisms are substantially alike. For example, the second and third prisms can have a triangular profile with substantially the same dimensions, or can also be right angle triangular prisms. In addition, the first prism can also be a right angle triangular prism. The second and third prisms can also be configured such that the second and third color components exit from the second and third prisms in substantially opposite directions. In a further alternative embodiment, the third prism has a phase retardation optical coating embedded therein instead of having a coated TIR surface such that a third color component of the light beam is directed through the optical coating prior to exiting from the third pris

In a method for modifying incident light in an image projection display system according to the present invention, a light beam is directed from a light source to a polarizing device which separates a first polarized component of the light beam from a second polarized component of the light beam. The first polarized component of the light beam is directed toward a color separating device which splits the first polarized component into a first color component, a second color component, and a third color component. The first, second and third color components are directed to a corresponding reflective imaging device after at least one total internal reflection in the color separating device. Each of the first, second and third color components undergoes a phase modification by either reflection from or transmission through a multilayer optical coating of the color separating device. The first, second and third color components reflected from the imaging devices are recombined in the color separating device to form a single light beam with a modulated image which is directed back to the polarizing device. The single light beam with the modulated image is projected to a display screen after transmission through the polarizing device.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the manner in which the above-recited and other advantages of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1 is a schematic depiction of a prior projection display imaging system that employs a conventional Philips prism;

Figure 2 is a schematic depiction of one embodiment of a prism assembly in accordance with the present invention; Figure 3 is a schematic depiction of an image projection display system that employs the prism assembly of Figure 2;

Figure 4 is a schematic depiction of another embodiment of a prism assembly in accordance with the present invention;

Figure 5 is a schematic depiction of another embodiment of a prism assembly in accordance with the present invention;

Figure 6 is a schematic depiction of a further embodiment of a prism assembly in accordance with the present invention; and

Figure 7 is a schematic depiction of a prism assembly according to an alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an improved color separation device for splitting incident light into three different colors and an image projection display system that incorporates the color separation device. The color separation device is an optical body provided in the form of a prism assembly having total internal reflection (TIR) surfaces with phase control optical coatings thereon, which eliminate the need for external waveplate compensators that are costly and difficult to align. The prism assembly of the invention provides a simple and less expensive way to enhance the contrast and image quality in a color image projection display system.

Referring to the drawings, wherein like structures are provided with like reference designations, a conventional projection display imaging system 10 is shown in Figure 1. A light source 12 provides the illumination in imaging system 10 necessary to form an image. The light source 12 emits unpolarized light 14 which passes through an optical filter 15, such as a color tuning filter or a notch filter, which tunes the wavelength range of the light required for imaging system 10. The light from optical filter 15 is incident on a polarizing device 16, such as a polarizing beam splitter which can be in cubic form, and polarized light 18 of a selected first polarization state, such as

light, is reflected into prism assembly 20, while polarized light of a second polarization state, such as p-po\aήzed light, is transmitted through polarizing device 16. The ^-polarized light refers to light which has its polarization vector perpendicular to the plane of

light refers to light which has its polarization vector lying in the plane of incidence.

The prism assembly 20, such as a Philips prism, includes a first triangular prism 22 and a second triangular prism 24, with a third prism 26 having at least four sides. The triangular prisms 22 and 24 are positioned with respect to each other to provide an air gap 21 at their interface. The second triangular prism 24 and third prism 26 are optically cemented at the interface of these prisms with no air gap. The first triangular prism 22 of prism assembly 20 receives incident light at an incident surface 22a thereof. The prism 22 has a dichroic coating (not shown) on a first color splitting surface 22b which is opposite incident surface 22a. The dichroic coating defines the wavelength range for a first color channel in imaging system 10, and is generally a dichroic multilayer thin film designed to reflect and transmit discrete wavelength ranges. Light reflected by this dichroic coating is totally internally reflected at surface 22a toward an exit surface 22c and is transmitted through exit surface 22c. Light of a first color such as red is thereby selected and directed to a first reflective imaging device 30 such as a liquid crystal light valve (LCLV), although the dichroic coating can be alternatively designed to select either blue or green light.

The second triangular prism 24 is attached at an incident surface 24a to surface 22b of prism 22 so as to form a precise air gap therebetween. The prism 24 has a dichroic coating (not shown) on a second color splitting surface 24b opposite the air gap. This dichroic coating defines the wavelength range for a second color channel in imaging system 10. Light reflected at surface 24b exits prism 24 at an exit surface 24c after total internal reflection at surface 24a. Light of a second color such as blue (or alternatively green or red) is thereby selected and directed to a second reflective imaging device 32.

The third prism 26 has an incident surface 26a which is optically bonded or cemented to surface 24b of prism 24 and is in contact with the dichroic coating on surface 24b. A third color channel is defined by the remaining wavelengths of light which pass into prism 26 from prism 24 that have not been subtracted from the incident beam by the preceding dichroic coatings in the physical path of the light. Light of a third color such as green (or alternatively red or blue) exits prism 26 at an exit surface 26b and is directed to a third reflective imaging device 34.

The light which has been separated into the three primary colors (red, green, blue) by prism assembly 20 and directed toward the reflective imaging devices is reflected therefrom back through each prism. When individual pixels of the reflective imaging devices are activated to an "on" state, they reverse the polarization of incident light upon reflection. The light which strikes pixels in the "off" state is not reversed in polarization upon reflection. The light reflected from the imaging devices is recombined within prism assembly 20 and redirected to polarizing device 16. The polarizing device 16 then transmits the light which has undergone a reversal of polarization state into image projection optics 36 and an image is projected on a display screen 38 that is observable by a viewer. The light not reversed in polarization by the imaging devices is reflected away from the image projection optics, and the final image is thus formed from the selected activated pixels of the three imaging devices.

As discussed above, a high contrast ratio is desirable in order to produce visually pleasing images in an image projection display system. Prior display systems have suffered from degradation of image contrast due to rotations and ellipticity in the polarization that occur as light traverses in double-pass mode through the system. Accordingly, the present invention provides a novel color separation device that enhances the contrast and image quality in an image projection display system. Figure 2 is a schematic depiction of a color separation device in the form of a prism assembly 40 according to one embodiment of the invention. The prism assembly 40 includes a first triangular prism 42, a second triangular prism 44, and a third triangular prism 46. The first and second triangular prisms 42 and 44 are optically attached in order to provide an air gap 41 at their interface. The second and third triangular prisms 44 and 46 are optically attached at their interface with no air gap. The second and third triangular prisms 44 and 46 are preferably formed such that they are substantially alike, having substantially the same dimensions. Each of the prisms 42, 44, and 46 is also preferably composed of solid glass.

The first triangular prism 42 of prism assembly 40 has an incident surface 42a, a color splitting surface 42b, and an exit surface 42c. The incident surface 42a acts as a total internal reflection (TIR) surface for light reflected within prism 42. The prism 42 has a dichroic coating 43 on color splitting surface 42b which can be formed thereon by conventional deposition processes. The dichroic coating 43 is formed so as to reflect a first color component of light (e.g., red light (R)), while transmitting the second and third color components of light (e.g., blue light (B) and green light (G)). The prism 42 also has a phase control optical coating 45 on incident surface 42a which prevents incident rays entering prism 42 from reflecting while controlling the phase characteristics of light reflected within prism 42. The optical coating 45 thus has both antireflection and phase control functions, and is typically a multilayer film such as a multilayer dielectric thin film that is formed by conventional deposition processes. The multilayer film can be formed from alternating layers of at least two different dielectric materials which include a low refractive index material such as SiO₂ and a high refractive index material such as TiO₂. The phase characteristics of light can be controlled as desired by changing the thickness, the number, or the components of the layers in optical coating 45. Examples of suitable phase control multilayer films that can be utilized in the present invention are described in further detail in U.S. Patent No. 5,594,591 to

Yamamoto et al., the disclosure of which is incorporated herein by reference. The optical coating 45 generally has a nonzero phase retardance that is optimized, i.e., dΔ/dλ ~ 0 for the wavelength, λ, of light encountering coating 45.

The prism 42 receives incident light L through surface 42a thereof which is directed toward dichroic coating 43. A first color component of the light (e.g. , red light) is reflected at an angle by dichroic coating 43 along a first optical path and is totally internally reflected at surface 22a after traveling through phase control optical coating 45. The first color component is then transmitted through exit surface 42c to reflective imaging device 30. The second triangular prism 44 of prism assembly 40 has an incident surface 44a, a color splitting surface 44b, and an exit surface 44c. The incident surface 44a acts as a TIR surface for light reflected within prism 44. The prism 44 has a dichroic coating 47 on color splitting surface 44b. The dichroic coating 47 is formed so as to reflect a second color component of light (e.g., blue light), while transmitting the third color component of light (e.g., green light (G)). The prism 44 also has a phase control optical coating 48 on incident surface 44a that controls the phase characteristics of light reflected within prism 44. The optical coating 47 is a multilayer film similar to optical coating 45 discussed above.

The prism 44 receives the second and third color components of light transmitted through prism 42 and are directed toward dichroic coating 47. The second color component (e.g., blue light) is reflected at an angle by dichroic coating 47 along a second optical path and is totally internally reflected at surface 44a after traveling through phase control optical coating 48. The second color component is then transmitted through exit surface 44c to reflective imaging device 32.

The third triangular prism 46 of prism assembly 40 has an incident surface 46a, a TIR surface 46b, and an exit surface 46c. The prism 46 also has a phase control optical coating 49 on TIR surface 46b that controls the phase characteristics of light reflected within prism 46. The optical coating 49 is a multilayer film similar to optical coating 45 discussed above.

The prism 46 receives the third color component of light transmitted through prism 44 which is directed toward TIR surface 46b. The third color component (e.g. , green light) is reflected at an angle at TIR surface 46b after traveling through phase control optical coating 49 along a third optical path. The third color component is then transmitted through exit surface 46c to reflective imaging device 34.

Figure 3 is a schematic depiction of a color image projection display system 100 that employs prism assembly 40 of Figure 2. The projection display system 100 includes components similar to those discussed above for imaging system 10. Accordingly, projection display system 100 has a light source 12, which typically produces white light such as from a xenon, metal halide, or tungsten lamp. An optical filter 15 is also utilized, such as a conventional color tuning filter or notch filter, which is in optical communication with light source 12.

The projection display system 100 also includes means for polarizing a light beam from light source 12 and separating a first polarized component of the light beam from a second polarized component of the light beam. A suitable polarizing means is shown in Figure 3 as polarizing device 16 such as a polarizing cubic beam splitter. Other polarizing means known to those skilled in the art can also be utilized, such as a slab beam splitter, wire grid polarizer, frustrated total internal reflection polarizer, and the like. The polarizing device 16 transmits light of one type of polarization, usually

light, while reflecting light of the other type of polarization (.y-polarized in this case) generally at a direction of 90° with respect to the direction of incidence.

The projection display system 100 also includes means for modulating a first polarization state of the first, second and third color components exiting the prism assembly 40 and reflecting the first, second and third color components in a second polarization state back through prism assembly 40. A suitable modulating means is shown in Figure 3 as the three reflective imaging devices 30, 32, and 34, which can be liquid crystal light valves. Other modulating means known to those skilled in the art can also be utilized, such as various liquid crystal displays (LCD's) including liquid crystal on silicon (LCOS) devices and ferroelectric LCD's, as well as digital icromirror devices (DMD), and the like. The projection display system 100 further includes suitable image projection optics 36 such as a conventional projection lens and a display screen 38.

Other conventional optical coatings and filters may also be employed in projection display system 100 including on prism assembly 40, such as infrared and UN blocking coatings, antireflection coatings, reflective mirrors, and the like. Preferably, the exit surfaces of prism assembly 40 have antireflection coatings formed thereon.

During operation of projection display system 100, unpolarized light 14 is emitted by light source 12 and passes through optical filter 15 in collimated or convergent form to polarizing device 16, which delivers polarized light 18, such as s -polarized light, to prism assembly 40 preferably at normal incidence. The prism assembly 40 separates the polarized component of light into three primary color components as described above, such that 5-polarized red light, ^-polarized blue light, and s-polarized green light each exit prism assembly 40 toward the respective reflective imaging devices 30, 32 and 34. Each of the reflective imaging devices modulates the polarization state of each of the three color components of light and reflects the modulated light back into prism assembly 40 in accord with a desired image such that the light is "double-passed" through prism assembly 40. Accordingly, if a polarized component of light, such as s-polarized light, initially enters prism assembly 40, the light is returned to prism assembly 40 in the "on" state as three color components that are

and which are recombined. The prism assembly 40 then outputs a divergent cone of (-polarized light that passes through polarizing device 16 to image projection optics 36, which projects the image on display screen 38.

Each of the color components of light incident upon the respective reflective imaging device in projection display system 100 is reflected such as to contain the optical signal information that will form the final image for viewing. Typical liquid crystal light valves or reflecting type of liquid crystal panels comprise an array of pixels. When each pixel is activated, typically when a voltage or signal is applied to an individual pixel, the array of pixels reflect incident light while simultaneously rotating the polarization vector of the light by 90°. Thus, the signal or image information is contained in the light which is of a particular polarization. If particular pixels in the liquid crystal imager are not activated, then those pixels are in the "off" state, and the light reflected therefrom will have no rotation of the polarization state. The signals from these "off" pixels should correspond to dark spots in the final image.

One aspect of the quality of an image in projection display systems is measured through a parameter known as the contrast ratio, which is defined as the amount of light transmitted through the system in the "on" state divided by the amount of light transmitted in the "off" state. The higher the contrast ratio, the better the overall quality of the image. Changes in polarization state are often introduced during transmission due to geometrical and thin film coating effects. When a portion of the light has polarization properties different than the desired polarization state, the contrast ratio of the imaging system is decreased. The contrast ratio is thus a measure of the purity of the polarization state of the transmitted light.

The polarization in a three panel color projection display system deploying a color splitting assembly generally cannot be corrected simply with a quarter wave plate as the polarization is not simply rotated. The polarization vectors can be shifted or rotated off axis each time the light goes through the multiple layers of the dichroic coatings. As a result, the light suffers from phase lags between the color components of the beam as they progress through the optical media, which yields a degree of off axis components in the polarization. Accordingly, light may be transmitted with a residual ellipticity and off-axis rotations of the polarization planes which yield unacceptable image contrast. The present invention utilizes the phase control optical coatings on the three total internal reflection surfaces of the prism assembly to compensate for the residual ellipticity and rotation of the polarization planes. The reflected light from the pixels of each reflective imager in the "off" state traverses back through the prism assembly with the phase control optical coatings thereon, which are designed to effectively compensate for any rotations and ellipticity in the polarization vector. Accordingly, the contrast ratio in the projection display system of the invention is enhanced by reducing the leakage of light in the "off" state into the "on" state of the image.

While prior image projection display systems have utilized phase control coatings on the first two color channels of a Philips prism, such coatings have not been used on the third color channel because of the lack of a TIR surface. The present invention solves this problem by utilizing novel prism assembly designs that provide a TIR surface in the third prism providing the third color channel. Alternative prism designs of the invention similar to prism assembly 40 discussed above, and which can be utilized in projection display systems, will be described as follows. Figure 4 is a schematic depiction of a color separation device in the form of a prism assembly 50 according to another embodiment of the invention. The prism assembly 50 includes a first triangular prism 52, a second triangular prism 54, and a third triangular prism 56. The first and second triangular prisms 52 and 54 are optically attached in order to provide an air gap 51 at their interface. The second and third triangular prisms 54 and 56 are optically attached at their interface with no air gap. The second and third triangular prisms 54 and 56 are formed such that they are substantially alike, having a triangular profile with substantially the same dimensions and being in the form of right angle triangular prisms (i.e., 30°-60°-90° prisms). As shown in Figure 4, prisms 54 and 56 are joined such that the right angle corners of the prisms are abutting each other. Each of the prisms 52, 54, and 56 is also preferably composed of solid glass.

The first triangular prism 52 of prism assembly 50 has a dichroic coating 53 on a color splitting surface thereof. The prism 52 also has a phase control optical coating

55 on an incident surface that controls the phase characteristics of light reflected within prism 52. The optical coating 55 is a multilayer film similar to optical coating 45 discussed above. The prism 52 receives incident light L through the incident surface thereof which is directed toward dichroic coating 53. A first color component of the light (e.g, red light) is reflected by dichroic coating 53 along a first optical path and is totally internally reflected after traveling through phase control optical coating 55. The first color component is then transmitted to reflective imaging device 30.

The second triangular prism 54 of prism assembly 50 has a dichroic coating 57 on a color splitting surface thereof. The prism 54 also has a phase control optical coating 58 on an incident surface thereof that is similar to optical coating 55. The prism 54 receives the second and third color components of light transmitted through prism 52 which are directed toward dichroic coating 57. The second color component (e.g., blue light) is reflected by dichroic coating 57 along a second optical path and is totally internally reflected after traveling through phase control optical coating 58. The second color component is then transmitted to a reflective imaging device 130 having an imaging region 132 for receiving the second color component. The third triangular prism 56 of prism assembly 50 has a phase control optical coating 59 on a TIR surface that controls the phase characteristics of light reflected within prism 56. The prism 56 receives the third color component of light transmitted through prism 54 which is directed toward the TIR surface. The third color component (e.g., green light) is totally internally reflected after traveling through phase control optical coating 59 along a third optical path. The third color component is then transmitted to an imaging region 134 of reflective imaging device 130.

Alternatively, separate imaging devices can be utilized in place of reflecting imaging device 130 for prism assembly 50, such as shown in the embodiment of Figure 2. Figure 5 is a schematic depiction of a color separation device in the form of a prism assembly 60 according to a further embodiment of the invention. The prism assembly 60 includes a first triangular prism 62, a second triangular prism 64, and a third triangular prism 66, which are joined together as described in the previous prism embodiments, with first and second triangular prisms 62 and 64 optically attached in order to provide an air gap 61 at their interface. The second and third triangular prisms

64 and 66 are also formed such that they are substantially alike, having a triangular profile with substantially the same dimensions and being in the form of right angle triangular prisms. In addition, prism 62 is also a right angle triangular prism, although prism 62 does have larger dimensions than the other prisms. The first triangular prism 62 has a dichroic coating 63 on a color splitting surface thereof. The prism 62 also has a phase control optical coating 65 on an incident surface thereof. The prism 62 receives incident light L which is directed toward dichroic coating 63. A first color component of the light (R) is reflected by dichroic coating 63 and is totally internally reflected after traveling through phase control optical coating 65. The first color component is then transmitted to reflective imaging device 30.

The second triangular prism 64 has a dichroic coating 67 on a color splitting surface thereof. The prism 64 also has a phase control optical coating 68 on an incident surface thereof that is similar to optical coating 65. The prism 64 receives the second and third color components of light transmitted through prism 62 which are directed toward dichroic coating 67. The second color component (B) is reflected by dichroic coating 67 and is totally internally reflected after traveling through phase control optical coating 68. The second color component is then transmitted to reflective imaging device 32.

The third triangular prism 66 has a phase control optical coating 69 on a TIR surface. The prism 66 receives the third color component of light transmitted through prism 64 which is directed toward the TIR surface. The third color component (G) is totally internally reflected after traveling through phase control optical coating 69. The third color component is then transmitted to reflective imaging device 34.

Alternatively, a single reflective imaging device with separate imaging regions may be used in place of imaging devices 32 and 34 for prism assembly 60, as described above for the embodiment of Figure 4. Figure 6 is a schematic depiction of a color separation device in the form of a prism assembly 70 according to an additional embodiment of the invention. The prism assembly 70 includes a first triangular prism 72, a second triangular prism 74, and a third triangular prism 76, which are joined together as described in the previous prism embodiments, with first and second triangular prisms 72 and 74 optically attached in order to provide an air gap 71 at their interface. The second and third triangular prisms 74 and 76 are joined in a configuration such that the second color component and the third color component of light exit from prisms 74 and 76 in substantially opposite directions. The first triangular prism 72 has a dichroic coating 73 on a color splitting surface thereof, and a phase control optical coating 75 on an incident surface thereof. The prism 72 receives incident light L which is directed toward dichroic coating 73. A first color component of the light (R) is reflected by dichroic coating 73 and is totally internally reflected after traveling through phase control optical coating 75. The first color component is then transmitted to reflective imaging device 30.

The second triangular prism 74 has a dichroic coating 77 on a color splitting surface thereof. The prism 74 also has a phase control optical coating 78 on an incident surface thereof. The prism 74 receives the second and third color components of light transmitted through prism 72 which are directed toward dichroic coating 77. The second color component (B) is reflected by dichroic coating 77 and is totally internally reflected after traveling through phase control optical coating 78. The second color component is then transmitted to reflective imaging device 32.

The third triangular prism 76 has a phase control optical coating 79 on a TIR surface. The prism 76 receives the third color component of light transmitted through prism 74 which is directed toward the TIR surface. The third color component (G) is totally internally reflected after traveling through phase control optical coating 79. The third color component is then transmitted to reflective imaging device 34.

Figure 7 is a schematic depiction of a color separation device in the form of a prism assembly 80 according to an alternative embodiment of the invention. The prism assembly 80 includes a first triangular prism 82, a second triangular prism 84, and a third quadrangular prism 86, which are joined together as described in the previous prism embodiments, with first and second triangular prisms 82 and 84 optically attached in order to provide an air gap 81 at their interface.

The first triangular prism 82 has a dichroic coating 83 on a color splitting surface thereof, and a phase control optical coating 85 on an incident surface thereof. The prism 82 receives incident light L which is directed toward dichroic coating 83. A first color component of the light (R) is reflected by dichroic coating 83 along a first optical path and is totally internally reflected after traveling through phase control optical coating 85. The first color component is then transmitted to reflective imaging device 30. The second triangular prism 84 has a dichroic coating 87 on a color splitting surface thereof. The prism 84 also has a phase control optical coating 88 on an incident surface thereof. The prism 84 receives the second and third color components of light transmitted through prism 82, with the second color component (B) being reflected by dichroic coating 87 along a second optical path. After being totally internally reflected at phase control optical coating 88, the second color component is transmitted to reflective imaging device 32.

The third quadrangular prism 86 has a phase control optical coating 89 embedded therein such as a phase retardation multilayer dielectric film. The optical coating 89 is positioned at a critical angle with respect to the incident light that allows transmission of the third color component (G) therethrough while providing the required phase retardation to give the desired phase control. The embedded optical coating 89 thus performs a phase control function similar to the phase control coatings on the TIR surfaces of the pris The third color component of light passes through optical coating 89 prior to exiting prism 86 along a third optical path toward reflective imaging device 34.

The prism 86 is formed from a pair of prism components 91 and 92 that are optically interconnected, with optical coating 89 interposed between prism components 91 and 92 at their interface. The optical coating 89 can be formed on either of the interfacing surfaces of prism components 91 and 92 by conventional thin film coating processes. The optical coating 89 is preferably a multilayer dielectric phase retarder designed with specific phase shifts on transmission to compensate for the transmitted phase shifts at the coatings 83, 87, and 88. The optical coating 89 can be optimized by refinement to a desired phase retardance by using conventional thin film design techniques available in commercial software, such as "TFCALC" which is available from Software Spectra, Inc., of Portland, Oregon. Examples of such design techniques are illustrated on pages PR-1 and PR-2 of the TFCALC Manual (1993), the disclosure of which is incorporated herein by reference.

Additionally, the particular configuration of optical coating 89 can aid in the correction of geometrical effects of a polarizing device, such as a polarizing beam splitter. The geometrical effects of a polarizing beam splitter have been described in U.S. Patent No. 5,459,593 to Ootaki, the disclosure of which is incorporated by reference herein. The Ootaki patent describes an approach which corrects for the geometrical effects of a polarizing beam splitter by utilizing dielectric thin films on a tilted surface. Such an approach for optimizing contrast is applicable to designing the embedded optical coating 89 of prism assembly 80.

The prism assembly of the present invention provides many advantages and benefits over prior color separating devices. The prism assembly provides high contrast color images in projection display systems, while eliminating the requirement of external phase compensating retarder plates such as used in prior display systems, thereby reducing system complexity and cost. The prism assembly also has the additional benefit of being simpler to manufacture than a conventional Philips prism in that two out of the three prisms have the same shape in the embodiments with a TIR surface in the third channel, thereby reducing manufacturing costs.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. What is claimed is:

Claims

1. A color separating device for an image projection display system, the color separating device comprising an optical body adapted to split an entering light beam into a first color component, a second color component, and a third color component, the optical body configured to cause each color component to exit the optical body after a total internal reflection therein.

2. The color separating device of claim 1, wherein the optical body comprises a first prism, a second prism in optical communication with the first prism, and a third prism in optical communication with the second prism.

3. The color separating device of claim 2, wherein the second prism is attached to the first prism so as to form an air gap therebetween.

4. The color separating device of claim 2, wherein at least two of the prisms are substantially alike.

5. The color separating device of claim 2, wherein the second and third prisms have a triangular profile with substantially the same dimensions.

6. The color separating device of claim 2, wherein the second and third prisms are right angle triangular prisms.

7. The color separating device of claim 6, wherein the second and third prisms have substantially the same dimensions, and the first prism is a right angle triangular prism.

8. The color separating device of claim 2, wherein the second and third prisms are triangular prisms that are configured such that the second color component and the third color component exit from the second and third prisms in substantially opposite directions.

9. The color separating device of claim 2, wherein a first dichroic coating is disposed on an exit surface of the first prism, and a second dichroic coating is disposed on an exit surface of the second prism.

10. The color separating device of claim 2, wherein the third prism is configured to have a total internal reflection surface with a phase control optical coating thereon.

11. The color separating device of claim 2, wherein each of the prisms is configured to have a total internal reflection surface with a phase control optical coating thereon.

12. A color separating prism assembly for an image projection display system, the prism assembly comprising: a. a first prism adapted to provide a first color channel by splitting an incident light beam into a first color component that exits from the first prism after at least one total internal reflection along a first optical path; b. a second prism in optical communication with the first prism at a first interface, the second prism adapted to provide a second color channel by splitting the light beam from the first prism into a second color component which exits from the second prism after at least one total internal reflection along a second optical path; and c. a third prism in optical communication with the second prism at a second interface, the third prism adapted to provide a third color channel for a third color component of the light beam which exits from the third prism after at least one total internal reflection along a third optical path; wherein the third prism is configured to have a total internal reflection surface with a phase control optical coating thereon.

13. The prism assembly of claim 12, wherein the first prism and/or the second prism is configured to have a total internal reflection surface with a phase control optical coating thereon.

14. The prism assembly of claim 12, wherein the second prism is attached to the first prism so as to form an air gap therebetween.

15. The prism assembly of claim 12, wherein at least two of the prisms are substantially alike.

16. The prism assembly of claim 12, wherein the second and third prisms have a triangular profile with substantially the same dimensions.

17. The prism assembly of claim 12, wherein the second and third prisms are right angle triangular prisms.

18. The prism assembly of claim 17, wherein the second and third prisms have substantially the same dimensions, and the first prism is a right angle triangular prism.

19. The prism assembly of claim 12, wherein the second and third prisms are triangular prisms that are configured such that the second color component and the third color component exit from the second and third prisms in substantially opposite directions.

20. The prism assembly of claim 12, wherein a first dichroic coating is disposed on an exit surface of the first prism, and a second dichroic coating is disposed on an exit surface of the second prism.

21. The prism assembly of claim 12, wherein the phase control optical coating is a multilayer dielectric film.

22. A color separating prism assembly for an image projection display system, the prism assembly comprising: a. a first prism adapted to cause an incident light beam to split into a first color component that exits from the first prism after a total internal reflection along a first optical path; b. a second prism in optical communication with the first prism at a first interface, the second prism adapted to cause a second color component of the light beam to exit from the second prism after a total internal reflection along a second optical path; and c. a third prism in optical communication with the second prism at a second interface, the third prism having a phase retardation optical coating embedded therein such that a third color component of the light beam is directed through the optical coating prior to exiting from the third prism along a third optical path.

23. The prism assembly of claim 22, wherein the second prism is attached to the first prism so as to form an air gap therebetween.

24. The prism assembly of claim 22, wherein the first and second prisms have a triangular profile, and the third prism has a quadrangular profile.

25. The prism assembly of claim 22, wherein a first dichroic coating is disposed between the first and second prism, and a second dichroic coating is disposed between the second and third prism.

26. The prism assembly of claim 22, wherein the first and second prisms are configured to have a total internal reflection surface with a phase control optical coating thereon.

27. An image projection display system, comprising: a. a light source; b. means for polarizing a light beam from the light source and separating a first polarized component of the light beam from a second polarized component of the light beam; c. a color separating device comprising an optical body adapted to split the first polarized component of the light beam into a first color component, a second color component, and a third color component, the optical body configured to cause each color component to exit the optical body along a different optical path after a total internal reflection therein; and d. means for modulating a first polarization state of the first, second and third color components exiting the optical body and reflecting the first, second and third color components in a second polarization state back through the color separating device.

28. The system of claim 27, wherein the polarizing means comprises a polarizing beam splitter.

29. The system of claim 28, wherein the polarizing beam splitter is a polarizing cubic beam splitter.

30. The system of claim 27, furthering comprising a notch filter configured to tune the light beam such that light entering the color separating device has a preselected wavelength range.

31. The system of claim 27, wherein the optical body comprises a first prism, a second prism in optical communication with the first prism, and a third prism in optical communication with the second prism.

32. The system of claim 31, wherein the second and third prisms have a triangular profile with substantially the same dimensions.

33. The system of claim 31, wherein the second and third prisms are right angle triangular prisms.

34. The system of claim 33, wherein the second and third prisms have substantially the same dimensions, and the first prism is a right angle triangular prism.

35. The system of claim 31 , wherein the second and third prisms are triangular prisms that are configured such that the second color component and the third color component exit from the second and third prisms in substantially opposite directions.

36. The system of claim 31, wherein a first dichroic coating is disposed on an exit surface of the first prism, and a second dichroic coating is disposed on an exit surface of the second prism.

37. The system of claim 31 , wherein the third prism is configured to have a total internal reflection surface with a phase control optical coating thereon.

38. The system of claim 31 , wherein each of the prisms is configured to have a total internal reflection surface with a phase control optical coating thereon.

39. The system of claim 27, wherein the modulating means comprises three reflective imaging devices, each of which is disposed in the optical path of one of the color components of the light beam that exit from the optical body.

40. The system of claim 39, wherein the reflective imaging devices are liquid crystal light valves.

41. An image projection display system, comprising: a. a light source adapted to generate a light beam; b. a polarizing beam splitter capable of separating a first polarized component of the light beam from a second polarized component of the light beam; c. a color separating prism assembly comprising: i. a first prism adapted to provide a first color channel by splitting the first polarized component of the light beam into a first color component that exits from the first prism after at least one total internal reflection along a first optical path; ii. a second prism in optical communication with the first prism at a first interface, the second prism adapted to provide a second color channel by splitting the light beam from the first prism into a second color component which exits from the second prism after at least one total internal reflection along a second optical path; and iii. a third prism in optical communication with the second prism at a second interface, the third prism adapted to provide a third color channel for a third color component of the light beam which exits from the third prism after at least one total internal reflection along a third optical path, wherein the third prism is configured to have a total internal reflection surface with a phase control optical coating thereon; and d. a plurality of reflective imaging devices in optical communication with the prism assembly and capable of modulating a first polarization state of the first, second and third color components exiting the prism assembly, and reflecting the first, second and third color components in a second polarization state back through the prism assembly.

42. The prism assembly of claim 41 , wherein the first prism and/or the second prism is configured to have a total internal reflection surface with a phase control optical coating thereon.

43. The system of claim 41, wherein the polarizing beam splitter is a polarizing cubic beam splitter.

44. The system of claim 41, wherein the second and third prisms have a triangular profile with substantially the same dimensions.

45. The system of claim 41, wherein the second and third prisms are right angle triangular prisms .

46. The system of claim 45, wherein the second and third prisms have substantially the same dimensions, and the first prism is a right angle triangular prism.

47. The system of claim 41 , wherein the second and third prisms are triangular prisms that are configured such that the second color component and the third color component exit from the second and third prisms in substantially opposite directions.

48. The system of claim 41 , wherein the reflective imaging devices are liquid crystal light valves.

49. An image projection display system, comprising: a. a light source adapted to generate a light beam; b. a polarizing beam splitter capable of separating a first polarized component of the light beam from a second polarized component of the light beam; c. a color separating prism assembly comprising: i. a first prism adapted to cause an incident light beam to split into a first color component that exits from the first prism after a total internal reflection along a first optical path; ii. a second prism in optical communication with the first prism at a first interface, the second prism adapted to cause a second color component of the light beam to exit from the second prism after a total internal reflection along a second optical path; and iii. a third prism in optical communication with the second prism at a second interface, the third prism having a phase retardation optical coating embedded therein such that a third color component of the light beam is directed through the optical coating prior to exiting from the third prism along a third optical path; and d. a plurality of reflective imaging devices in optical communication with the prism assembly and capable of modulating a first polarization state of the first, second and third color components exiting the prism assembly, and reflecting the first, second and third color components in a second polarization state back through the prism assembly.

50. The system of claim 49, wherein the polarizing beam splitter is a polarizing cubic beam splitter.

51. The system of claim 49, wherein the first and second prisms have a triangular profile, and the third prism has a quadrangular profile.

52. The system of claim 49, wherein the reflective imaging devices are liquid crystal light valves.

53. The system of claim 49, wherein the phase retardation optical coating is a multilayer dielectric film.

54. A method for modifying incident light in an image projection display system, comprising the steps of:

(a) providing a light source;

(b) directing a light beam from the light source to a polarizing device which separates a first polarized component of the light beam from a second polarized component of the light beam;

(c) directing the first polarized component of the light beam toward a color separating device;

(d) splitting the first polarized component of the light beam incident to the color separating device into a first color component, a second color component, and a third color component;

(e) directing the first, second and third color components to a corresponding reflective imaging device after at least one total internal reflection in the color separating device, wherein each of the first, second and third color components undergoes a phase modification by either reflection from or transmission through a multilayer optical coating of the color separating device;

(f) recombining the first, second and third color components reflected from the reflective imaging devices in the color separating device to form a single light beam with a modulated image which is directed back to the polarizing device; and (g) projecting the single light beam with the modulated image after transmission through the polarizing device.