WO1993004393A1

WO1993004393A1 - Improved lighting technique for color displays

Info

Publication number: WO1993004393A1
Application number: PCT/US1992/006934
Authority: WO
Inventors: John S. Wyler
Original assignee: Smiths Industries, Inc.
Priority date: 1991-08-19
Filing date: 1992-08-18
Publication date: 1993-03-04

Abstract

A color display (10) is disclosed which operates in cooperation with an ultraviolet source. The display comprises a liquid crystal solution contained within a light valve. A pair of electrodes (12, 14) is positioned on either side of the light valve to generate an electric field in the light valve. A pair of transparent sheets (18, 20) is positioned on either side of the electrodes (12, 14) and one of the transparent sheets (18, 20) is positioned adjacent the ultraviolet source. A phosphor layer (16) which comprises a plurality of discrete phosphor elements of different colors is advantageously positioned between this transparent sheet (18) and the electrode (12) located nearer the ultraviolet source to increase light transmission efficiency in the display.

Description

IMPROVED LIGHTING TECHNIQUE FOR COLOR DISPLAYS

Cross-Reference to Related Application

This application is copending with application

S.N. of inventors John S. yler and

Frederick E. Hankins for ENLARGED AREA ADDRESSABLE MATRIX filed concurrently herewith, which is hereby incorporated by reference.

Field of The Invention

This invention relates to displays and more particularly to color liquid crystal displays.

Background of The Invention

In the prior art, liquid crystal displays (commonly known as LCD's), have been used to display alphanumeric information in calculator readouts, digital watches, and the like. Typically, these LCD's have been monochromic displays.

More sophisticated LCD's have been comprised of a plurality of discrete monochromic picture elements (or pixels) arranged in a matrix format. These LCD's are capable of displaying more complicated images than simple alphanumeric characters. For example, LCD's of the above-described type have heretofore been utilized to generate displays for portable computers.

Fig. 1 shows a schematic diagram of a typical prior art LCD display cell, generally designated 1, for use in a matrix display. The cell includes an LCD element 2, a ground plane 3, a transistor 4, row lines 5, 6 and column lines 7, 8. LCD element 2 is a passive transmitter of light generated from an independent light source (not shown) . The transmission of light through LCD element 2 depends upon whether transistor 4 has placed a charge on

SUBSTITUTESHEET element 2. Ground plane 3 is the reference with respect to which each LCD display element is charged. Each row line 5, 6 is also called a scan line since its function is to turn on the gates of all LCD cells in a row of the display. Each column line 7, 8 is also called a data line since, if scan line 5 allows, it places or removes a charge on element 2. In the conventional physical realization of the device of Fig. 1, a large number of parallel scan lines and orthogonal parallel data lines are disposed in a planar array with a transistor 4, and an LCD element 2 in the area between each intersecting pair of scan lines and pair of data lines.

Scan lines 5, 6 and data lines 7, 8 of Fig. 1 form part of the display's control matrix. Each scan line may be connected to its driver on either the left or right side of the matrix, and each data line may be connected to its driver on either the top or bottom of the matrix. In conventional matrix addressable displays, tens or hundreds of thousands of identical cells are arranged in rectilinear arrays and used to generate a display by selective activation of the individual cells under control of numerous scan and data lines. Transistor 4 advantageously comprises a field effect transistor (FET) as illustrated, preferably being of the amorphous silicon type. FET 4 includes a gate electrode G connected to scan line 5, a source electrode S connected to data line 7, and a drain electrode D connected to LCD element 2. As will be appreciated by those skilled in the art, LCD element 2 is electrically connected via FET 4 to data line 7 whenever gate G of FET 4 is provided via scan line 5 with an appropriate electrical gating signal to render FET 4 conductive between its source S and drain D

SUBSTITUTE SHEET electrodes. While only a single FET has been illustrated within LCD display cell 1, an additional FET or FETs may be included in the cell to provide redundancy in the event that one or more of the FETs is defective.

Color capability is also possible in the present LCD technology. To generate a color display, color display LCD elements are arranged in a predetermined matrix pattern. These elements are controlled electronically to produce a plurality of different colors. By selectively turning the color elements on or off in a predetermined pattern, a variety of different colors can be produced with only three primary color elements: red, blue and green. Typical LCD color matrix arrangements include triad, quad and stripe patterns.

Generally, a color liquid crystal display of the prior art utilizes a white backlight source to illuminate the display. The light from the source first passes through a diffuser so that it has uniform intensity over the entire surface of the display. The light is then polarized by transmitting it through a first polarizer (i.e., only that portion of light which is oriented in correct linear alignment with the polarizer is passed by the polarizer) . The light then passes through a liquid crystal solution.

As in the case of a monochromatic LCD, the color LCD comprises a matrix of display elements or pixels each of which may be controlled electronically. In particular, when an appropriate voltage is applied to a display element, the liquid crystal solution twists the linear orientation of the light passing through that element. As a result, the direction of polarization of light from that element is aligned with the direction of polarization of a second

SUBSTITUTESHEET polarizer on the viewing side of the display and accordingly can pass through the second polarizer.

In prior art color displays, the light that has passed through the liquid crystal solution is first incident on a plurality of red, blue and green color filters which generate a plurality of color pixels corresponding to the filter locations and colors. The light is then incident on the second polarizer which transmits only those pixels whose polarization has been rotated into proper alignment with the second polarizer. As a result, the pattern of transmitted and non-transmitted light provides a visual display.

Unfortunately, the above-described color liquid crystal display has substantial light transmission losses. Typically, the diffuser cooperating with the white backlight source loses approximately 15 percent of the total light emanating from the source; even when the pair of polarizers are aligned for transmission, the pair of polarizers lose approximately 75 percent of the light that they receive; the color filters lose approximately 50 percent of the light that they receive; and the electronic elements which control the display elements block approximately 50 percent of the light incident on the display elements. Thus, the total light transmission emerging from the liquid crystal display is approximately 5 percent of the light which initially enters. These substantial losses result in a display which is inadequate in terms of brightness. Alternatively, a lighting power source which is able to compensate for the transmission losses and provide adequate display brightness can be utilized to generate an acceptable display. However, such a display inevitably has higher operating temperatures and possibly greater cost. Moreover, if the power

SUBSTITUTE SHEET source is a battery as in the case of portable displays used, for example, in lap-top computers, the user must be prepared to accept shorter battery life instead of unacceptable increases in the size and weight of the battery.

To decrease the large percentage of transmission losses, some color liquid crystal displays utilize red, blue and green color phosphors in cooperation with, or simply in lieu of, the color filters. This alternative either reduces or eliminates the 50 percent transmission loss to the LCD from the color filters. However, the addition of phosphor in prior art displays has not provided an acceptable light transmission for displays with small pixel size. Thus, a need exists for a more efficient color high- resolution liquid crystal display.

Summary of The Invention

. According to the invention, an improved color liquid crystal display is disclosed.

The liquid crystal display (LCD) of the present invention is a layered planar structure which encloses a liquid crystal solution between an array of electrodes. A source of light is located on one side of the structure (the source side) and the display is viewed from the other side (the display side) . On one side of the liquid crystal solution (illustratively, the source side) is a single transparent electrode which extends across the entire area of the display and is maintained at a constant potential, typically ground. On the other side of the liquid crystal solution is a matrix of tiny transparent electrodes each one of which can be individually addressed to apply a voltage thereto. Each tiny electrode controls one picture element (or pixel) in the liquid crystal

SUBSTITUTE SHEET display. In particular, by applying a voltage to the control electrode, an electric field is established between the control electrode and the ground electrode which rotates the polarization of the light passing through that portion of the liquid crystal solution between the two electrodes such that the light exits from the display.

The LCD further includes a pair of transparent cover sheets comprising a source side transparent sheet and a display side transparent sheet. A color phosphor layer comprising red, blue and green phosphor elements is positioned in the display between the source side transparent sheet and the source side electrode with each phosphor element aligned with one of the tiny control electrodes and hence one pixel of the display. An ultraviolet light source is positioned adjacent the LCD element. It activates the individual phosphor elements of the phosphor layer with ultraviolet radiation to induce the phosphor elements to emit visible light.

Because the phosphor layer is positioned on the source side of the display, the phosphor layer will absorb the UV radiation from the source without exposing other layers of the display which are farther from the source of ultraviolet radiation. Thus, the layers of the display which are positioned closer to the display side than the phosphor layer need not and do not transmit ultraviolet radiation therethrough. This increases the transmission efficiency through the display while decreasing the cost of the individual elements of the display.

Further, the liquid crystal solution is protected from exposure to the ultraviolet radiation and therefore is not adversely affected by that radiation which can cause degradation in the liquid crystal,

SUBSTITUTE SHEET thus weakening the crystal and eventually destroying the display.

In preferred embodiments, the liquid crystal solution of the light valve comprises a twisted nematic active matrix. Thus, the LCD can be powered by a relatively low voltage as compared to the voltage required to power other liquid crystal solutions.

A pair of polarizing layers are included in the LCD arrangement. These layers are positioned on opposite sides of the light valve either within or outside of the transparent cover sheets. The polarization layers are advantageously positioned toward the display side of the phosphor layer. This eliminates the need for ultraviolet polarizers which are costly and have a lower transmission efficiency than visible light polarizers. Further, the polarizer on the display side of the light valve is typically positioned farther from the light valve than the display side transparent sheet. In this way, that polarizer does not add to the transmission distance of the visible light and consequently does not decrease the transmission efficiency.

The liquid crystal display can also include additional elements. Conventional alignment layers can be provided to orient the liquid crystal molecules in a uniform direction. Planarization layers can be provided to ensure smooth contact surfaces between elements. Various filters can be provided to improve the resolution of the displayed image or to increase the light transmission efficiency through the display. The advantages of the invention include achieving a better light transmission efficiency at relatively low manufacturing and operating costs. Additionally, the LCD of the invention produces a display having high resolution and good purity of color. These

SUBSTITUTE SHEg-T advantages are achieved by proper positioning of the elements in the LCD. Specifically, positioning the phosphor layer within the two transparent cover sheets reduces the required transmission distance of the visible light and therefore increases the transmission efficiency of the display. Further, because of the reduced transmission distance, the color pixels in the display are maintained separate and distinct from one another upon reaching the display side. Thus, there is a reduced mixing of colors resulting in an increased color purity in the display.

Additionally, the specific positioning of the phosphor layer between the source side transparent sheet and the source side electrode reduces the number of elements that must transmit ultraviolet light therethrough. Since the remaining elements need not transmit ultraviolet radiation, they can be lower in cost and more efficient in light transmission. Further, because the phosphor layer is positioned closer to the source, the ultraviolet radiation initially impinges on the discrete phosphor elements, stimulating them to their maximum intensity. Again, this increases light transmission efficiency in the display.

Brief Description of The Drawings

These and other objects, features and advantages of the present invention will be more readily apparent from the following Detailed Description of Preferred Embodiments in which:

Fig. l is a schematic circuit diagram of a single cell of an LCD matrix of the prior art.

Fig. 2 is a diagrammatic cross-sectional view of a portion of an embodiment of a display of the invention.

SUBSTITUTESHEET Fig. 3 is a diagrammatic cross-sectional view of a portion of another embodiment of a display of the invention.

Detailed Description of Preferred Embodiments

Fig. 2 illustrates a display of the present invention and the relative positioning of elements contained therein. The display, generally designated 10 comprises a series of layers surrounding a confined structure 11 which is any one of a general class of devices which may be referred to as light valves. All of these devices are responsive to applied electric fields to selectively pass light. An example of such a device is a liquid crystal solution with a polarizer and analyzer on opposite sides of the solution. A set of transparent electrodes is positioned around either side of the light valve, comprising a plane electrode 12 on one side of the display (illustratively the source side) and a matrix of discrete electrodes 14 on the other side of the display. The plane electrode extends over the whole area of the display which may be approximately 70 mm on a side. The discrete electrodes have a shape and position corresponding to pixels in the display. A plastic matrix element 15, typically a polyamide matrix, maintains the discrete electrodes 14 in their proper position and orientation. Signal lines and transistors that control the electrodes are advantageously located in the area covered by plastic matrix element 15. Illustratively, the discrete electrodes are matrix electrodes approximately 0.1 mm on a side and the signal lines are a multitude of scan lines and a multitude of orthogonal data lines such as those illustrated in Fig. 1 and in the copending application Serial No. , referenced above.

SUBSTITUTESHEET Alternatively, electrodes of other shapes and dimensions, such as strip electrodes, and other types of electrode control arrangements can be used.

A phosphor layer 16 is positioned adjacent to the plane electrode 12 and comprises a plurality of discrete color phosphor elements surrounded by a polyamide matrix 17 which maintains the phosphor elements in proper position. The phosphor elements comprise the primary colors of red, blue and green, which can combine in various ways to generate many more colors in the output display. A source side transparent sheet 18 and a display side transparent sheet 20 are positioned around the exterior surfaces of the LCD to protect and contain it. Sealing members (not shown) seal the edges of the LCD to eliminate contamination and to provide structural support.

In operation, an ultraviolet light source (not shown) is positioned adjacent the source side of the LCD.- Ultraviolet radiation is transmitted through the source side transparent sheet and absorbed by the color phosphor elements 16. The phosphor elements, activated by the ultraviolet radiation, emit visible colored light to the display. The colored light is then transmitted through the light valve 11. The light valve selectively transmits light through those volumes of the valve where an electric field has been established by applying a voltage to a discrete electrode 14. For example, if the light valve comprises a polarizer, a liquid crystal solution and an analyzer, the light from the phosphor layer is polarized by the polarizer prior to entering the light valve. In each volume of the liquid crystal solution where an electric field has been established by applying a voltage to a discrete electrode 14, the electric field then rotates the polarization of the

SUBSTITUTE SHEET colored light passing through that volume so that the direction of polarization is aligned with that of the analyzer. This light then passes through the analyzer. Where there is no electric field, the direction of polarization is not rotated and the light is blocked by the analyzer. As a result, each discrete electrode 14 controls one picture element (or pixel) to create a color display which is observable through the display side transparent sheet 20. Fig. 3 illustrates a specific structure for a preferred embodiment of a liquid crystal display, generally designated 30. The display includes a transparent quartz layer 32 on the source side which allows ultraviolet radiation to pass therethrough. A glass sheet 34 is provided on the display side of the display. Typically, glass does not allow ultraviolet radiation to pass therethrough. A light valve 36 is provided in the display comprising a liquid crystal solution 38, a pair of alignment layers 40, 42 which are positioned on either side of the liquid crystal solution, and a planarization layer 44 positioned on the display side of alignment layer 40. The purpose of layers 40, 42, 44 will be discussed below.

A plane transparent electrode 46 is positioned adjacent to the source side of the light valve 36, and a matrix of discrete transparent electrodes 48 is positioned adjacent to the display side of the light valve 36. Again, this arrangement enables each discrete electrode to control a picture element (or pixel) in the display generated by the device of Fig. 3. Different embodiments of electrodes for liquid crystal displays are well known in the art. A polyamide matrix 50 is formed around the discrete electrodes 48 to maintain their position and to provide support. The alignment layers 40, 42 maintain

SUBSTITU E SKE-rr the proper alignment of the liquid crystal molecules within the picture element controlled by each discrete electrode. The planarization layer 44 is formed to mate with the discrete electrodes 48 so that a smooth contact surface is created between the planarization layer 44 and the alignment layer 40. Thus, no gaps are created in the display that can affect the transmission of light therethrough.

A polarizer 52 is provided to polarize the light transmitted through the display; and an analyzer 54 is used to block any of the polarized light whose direction of polarization is not aligned with that of the analyzer.

A phosphor layer 56 is positioned on the source side of the light valve 36, and comprises a plurality of discrete color phosphor elements. These elements comprise the primary colors of red, blue and green and, when activated, generate a variety of different colors in the display. A polyamide matrix 58 is formed around the phosphor elements to maintain the elements in their proper position and also to provide structural support to the phosphor elements.

A planarization layer 60 is positioned adjacent to the phosphor layer on the source side, and is formed to mate therewith so that a smooth contact surface is provided between the planarization layer 60 and the polarizer 52. This layer eliminates any gaps which might have formed between the phosphor layer 56 and the polarizer 52 which could adversely affect the transmission of light in the display.

An interference filter 62 is positioned between the quartz layer 32 and the phosphor layer 56. The interference filter allows ultraviolet light to be transmitted to the phosphor layer 56 while it reflects all visible light generated by the phosphor elements

SUBSTITUTE SHEET toward the display side. Thus, the interference filter 62 improves visible light transmission efficiency in the display.

The liquid crystal display 30 operates as follows. An ultraviolet light source such as a helium/mercury gas lamp (not shown) is positioned adjacent to the quartz layer 32 and transmits ultraviolet radiation at a wavelength of 251 nm therethrough to the phosphor layer 56. The colored phosphor elements are activated by the radiation and emit colored visible light. Different combinations of the colored phosphor elements generate visible light in a plurality of different colors in the display. The phosphor elements can be arranged in a variety of color matrices, but are typically arranged in standard stripe, triad or quad matrices.

The interference filter 62 passes the ultraviolet radiation emanating from the ultraviolet source, but reflects all visible light toward the display side. In this way, the colored visible light generated by the phosphor elements is reflected, thus enhancing the light transmission efficiency of the display, and consequently increasing the brightness of the ultimate display. The visible light emitted by the phosphor elements is transmitted through the source side polarizer 52 which transmits that portion of the light whose direction of polarization is aligned with the direction of polarization of the polarizer. Typical polarizers of the art can be utilized such as a stretched vinyl alcohol sheet containing a dye. However, it is beneficial to minimize the thickness of the LCD in order to maximize the light transmission efficiency. Therefore, preferred source side polarizers 52 are relatively thin: for example

^SUB^STITUTESHEE_T conducting grid polarizers. This type of polarizer comprises a plurality of equally spaced parallel wires or metal films. Additionally, Beilby-layer polarizers manufactured by Polacoat are thinner than a standard polarizer.

The polarized light is then transmitted through the light valve 36. In each volume of the light valve where an electric field has been established by applying a voltage to a discrete electrode 48, the electric field rotates the polarization of the colored light passing through that volume so that the direction of polarization is aligned with that of the analyzer 54. Where there is no electric field, the direction of polarization is not rotated and the light is blocked by the analyzer. As a result, the light that emerges from the display side analyzer constitutes the ultimate display.

The above-described liquid crystal display has an improved light transmission efficiency. The ultraviolet light source in cooperation with the phosphor elements give three times the light transmission efficiency as a conventional white backlight source. Phosphor elements are twice as efficient in transmitting light as color filters. Further, the specific positioning of the elements relative to one another in the liquid crystal display substantially increases the light transmission efficiency. These individual inventive improvements comprise a major net improvement in light transmission in the LCD of the present invention.

While it is apparent that the invention herein disclosed fulfills the objects above stated, it will be appreciated that numerous embodiments and modifications may be devised by those skilled in the art, and it is intended that the appended claims cover

SUBSTITUTESHEET all such modification and embodiments as fall within the true spirit and scope of the present invention.

SUBSTITUTE SHEET

Claims

What is claimed is:

1. A color display comprising: a light valve which selectively passes light in response to an electric field applied thereto; an array of transparent first electrodes positioned on one side of said light valve and at least one transparent second electrode positioned on the other side of said light valve for applying an electric field to selected portions of said light valve; transparent sheets positioned on a side of said first and second electrodes remote from the light valve, wherein the first transparent sheet is adapted to transmit ultraviolet radiation from a source, and the second transparent sheet is adapted to transmit visible light; and a phosphor layer comprising a plurality of discrete color phosphor elements adapted to receive ultraviolet radiation from said first transparent sheet and emit visible light of different colors, said phosphor layer being positioned between said light valve and said first transparent sheet.

2. The display of claim 1 wherein said second electrode is a single electrode extending over the entire display.

3. The display of claim 1 wherein the electrodes in the array of first electrodes are individually addressable.

4. The display of claim 1 wherein said phosphor layer comprises red, blue and green phosphor elements.

5. The display of claim 1 further comprising at least one polarizing layer.

6. The display of claim 5 wherein said polarizing layer comprises a stretched vinyl alcohol sheet.

SUBSTITUTESHE?

7. The display of claim 5 wherein said polarizing layer comprises a grid polarizing sheet.

8. The display of claim 5 comprising two polarizing layers.

9. The display of claim 8 wherein a first of said polarizing layers is positioned between said first electrode and said phosphor layer, and a second of said polarizing layers is positioned adjacent said second transparent sheet.

10. The display of claim 1 wherein said first transparent sheet comprises quartz.

11. The display of claim 1 wherein said second transparent sheet comprises glass.

12. The display of claim 1 comprising at least one alignment layer.

13. The display of claim 1 further comprising at least one planarization layer.

14. The display of claim 1 further comprising an interference filter.

15. The display of claim 1 wherein the light valve is made of a liquid crystal solution.

16. The display of claim 15 wherein said liquid crystal solution comprises a twisted nematic active liquid crystal.

SUBSTITUTESHEET