WO1991007851A1

WO1991007851A1 - Matrix addressed liquid crystal light valve

Info

Publication number: WO1991007851A1
Application number: PCT/US1990/006447
Authority: WO
Inventors: Duane A. Haven; Michael E. Stefanov; Matthew F. Bone; Frederic J. Kahn; David Slobodin
Original assignee: Greyhawk Systems, Inc.
Priority date: 1989-11-14
Filing date: 1990-11-09
Publication date: 1991-05-30

Abstract

This light valve comprises a light modulating and amplification device consisting of a photoconductor activated liquid-crystal light valve (50) together with an underlying structure. The underlying structure provides spatial, temporal and amplitude modulation to the liquid crystal light valve by means of a x, y matrix of photo-emissive, photo-refractive, or photo-transmissive elements (60) such as an active matrix liquid crystal (60b) or an AC thin film electroluminescent panel (60a).

Description

MATRIX ADDRESSED LIQUID CRYSTAL LIGHT VALVE

Related Application This application is a continuation-in-part of U.S. patent application Serial No. 436,477 filed November 14, 1989.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to photoaddressed liquid crystal light valves, and more particularly to photoaddressed liquid crystal light valves that are addressed by an imaging subsystem that includes a fixed matrix of addressing light elements. Such a device eliminates convergence errors in full color video projectors and provides brighter images due to improved aperture ratio.

Summary of the Prior Art

Video projectors are devices that project video images onto a screen or other media such as photographic film. Video projectors differ from direct view video displays such as a cathode ray tube (CRT) or thin film transistor addressed liquid crystal light valve in that the image to be viewed is projected rather than viewed directly. Video projectors are used extensively in applications that range from sophisticated military command and control to industrial training and presenta¬ tions to low end consumer large screen television sets. The sources of information signal are equally varied from conventional broadcast TV to computer generated images. If the projector is designed to project full color images, the color image signal is usually separated into three primary color signals. Each of the primary color signals drives a separate imaging device such as CRT. Finally, the three primary color images are co-projected onto the screen to form a composite full color image. This step is generally known as image convergence.

Prior art video projectors are of three types: CRT, CRT driven light valve and thin film transistor driven liquid crystal light valve.

The first and most common video projector in use today is a CRT projector where the image formed by an electron beam exciting a phosphor is projected usually through a lens system onto a screen. In such projection devices there are three CRT's, one for each primary color, which are designed to be as bright as possible. Usually the CRT's are from 2 to 7 inches diagonally, have one continuous coating of one primary color phosphor and draw relatively large cathode currents. The problems with CRT projectors are two fold: first, they are not bright and second, they converge poorly.

The limit of control of electron beams also sets a fundamental limit on resolution of any CRT system. It is not possible to make phosphors emit enough light to form a bright image many feet from the projector and many times larger than the CRT. While it is possible to increase the brightness of CRT's to some degree, the cost is great for even a small degree of improvement. Furthermore, increasing brightness of CRT's requires a large increase in beam power, which lowers resolution.

The second problem of CRT based projectors is color convergence. Color convergence refers to the over¬ lapping of the separate primary color images at the projection plane. Video images are made up of picture elements commonly called pixels. In the case of a three CRT projector, each pixel in the viewed image consists of three separate pixels—one for each primary color. .

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To provide a continuous tone full color pixel in a video projector, for example as described in U.S. Patent No. 4,715,684, the correct pixel from each of the three primary colors must spatially overlap at the projection plane.

And therein lies the second problem, because the precise spatial location of the addressing electron beam as a function of time varies differently for each CRT. This is well known in the art and is due to the near impossibility of matching the parameters of the elec¬ trical components of CRT's and the differential changes in these parameters with time and usage. As a result, the location of a particular pixel on the CRT screen due to a given input signal will vary from CRT to CRT and with the same CRT over time. This effect is not notice¬ able to the human eye on a monochrome CRT. However, it is noticeable in a color system. It causes color fringes and reduces resolution. This problem is well known. Some solutions have been proposed in the prior art, for example in U.S. Pat. No. 4,715,684 an autocon- vergence scheme is described wherein the addressing beams must be continuously monitored and adjusted. However, this approach, while technically feasible, is expensive and has never been commercialized. An unsatisfactory solution to the brightness problem has been found with the CRT addressed light valves projectors. Such devices use a low level addres¬ sing light signal from a CRT to cause changes in the electrical impedance of a photoconductor in series with an electro-optic material such as liquid crystal. These changes cause the electric field across the liquid crys¬ tal to vary as a function of the location and amplitude of the addressing light. This change in electric field results in molecular reorientation of the electro-optic material. Light passing through the electro-optic material is changed in character in accordance with the addressing light. A high power projection lamp reflects the image from the light valve onto the screen. The problem with such devices is that while they are bright, they are very expensive and still have all of the convergence problems of CRT's. Also, because the best quality images result from focussing the addressing light directly at the photoconductor plane, a system of relay optics is needed to relay the light from the CRT to the photoconductor.

An unsatisfactory solution to the convergence problem has been found with the thin film transistor liquid crystal light valve. This device works by reflecting light off a liquid crystal layer that is addressed by a matrix of photolithographically defined thin film transistors (TFT) . These active matrix light valves are well known in the art. For example, such a device is described in U.S. Patent No. 4,470,667 by Okubo et al. This device has the advantage of precise convergence. This can be done since the individual pixels are defined by a photolithographic process during manufacture and never change. The major disadvantages of this approach are the low level of optical efficiency due to the space needed for the active surfaces of the TFT and the busbars to drive the TFT. That is, the percentage of area of the device through which light may pass is relatively small. Therefore the viewed display will be dim.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a full color video projector that is bright. It is another object of this invention to provide a video projector that projects a well converged image.

It is another object of this invention to provide a video projector that is inexpensive.

It is another object of this invention to provide a matrix addressed liquid crystal light valve.

It is another object of this invention to provide a method of manufacturing a matrix addressed liquid crystal light valve that minimizes cost through sepa¬ rately manufacturing the addressing subsystem and the light valve subsystem and joining the two in the final step. It is another object of this invention to provide a method of manufacturing a matrix addressed liquid crystal light valve that minimizes cost by manufacturing the entire system by thin film techniques except for inserting the liquid crystal material. These and other objects of the invention are achieved in accord with the aspects of the present invention. In the first aspect, a light valve that includes an addressing matrix consisting of an array of light providing pixels and a photoaddressed liquid crystal light valve which is optically coupled to the addressing subsystem. In a first embodiment the addressing subsystem is matrix of light emitting pixels such as an AC thin film electroluminescent (ACTFEL) panel. In a second embodiment the addressing subsystem is a matrix of light transmitting pixels such as a TFT addressed liquid crystal cell. Coupling between the photoaddressed liquid crystal light valve and the addressing matrix can be achieved by means of either a fiber optic plate or a thin film layer, or the light valve can be directly bonded to the light valve without coupling. In a second aspect of the present invention, a projection system is provided with a plurality of photoaddressed light valves each having an addressing matrix. In a third aspect of the present invention, an addressing matrix provides a plurality of divergent light beams to the photoaddressed light valve. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of a multi-color video projector made in accordance with the present invention. Figure 2 is a section view of the preferred photoaddressed cell, a reflective liquid crystal cell which operates in the polarization mode. Figure 3 is a section view of the first embodiment of the addressing matrix, the array of light emitting pixels. The particular addressing matrix shown is an electroluminescent panel. Figure 4 is a section view of an second embodiment of the addressing matrix, the array of light transmit¬ ting pixels. The particular addressing matrix shown is a thin-film-transistor driven liquid crystal light panel. Figure 5 is a section view of the second embodiment in which the addressing matrix is coupled to the photo¬ addressed cell by a fiber optic plate.

Figure 6 is a section view of the second embodiment in which the addressing matrix is coupled to the photo- addressed cell by a thin film layer.

Figure 7 is a section view of the first embodiment in which the photoaddressed cell is coupled to the addressing matrix.

Figure 8 is a schematic of the preferred mode of the second embodiment of the invention in which an improved aperture is achieved.

Figure 9 is a comparison of the improved aperture of the present invention against the prior art.

Figure 10 is a simplified section view of an optical system used in the preferred mode.

Figure 11 is a schematic of the preferred mode of the first embodiment of the invention in which an improved aperture ratio is achieved.

DETAILED DESCRIPTION OF THE INVENTION The Projection System

Referring to Figure 1, the multi-color projection system 10 of the present invention is presented in layout form. Light 20 from an illumination system 25, such as a xenon arc lamp, enters the system of lenses 28 which relays the light 20 into the rest of the system 10. Lenses 28 could also serves as a means for colli- mating the light 20. The light enters polarizing beam splitter 30, and one polarization state is reflected as beam 33. Light beam 33 is split with dichroic mirrors 35,36 into three beams, each beam containing one of the primary colors. The beams are reflected off three light valves 40 each of which is designed in accordance with the present invention. The three light valves 40 are substantially identical, but may differ in minor details such as layer thickness in order to maximize efficiency for the wavelength of light that is being reflected by that particular light valve. Each light valve 40 contains polarization altering images. The reflected beams are recombined by the dichroic mirrors 35,36 and returned through the polarizing beam splitter 30. The portion of the beam that has been polarization modulated by the light valve 40 is transmitted through polarizing beam splitter 30 and into projection optics 45. Projec¬ tion optics 45 relay the combined image formed by the light valves onto a screen 48. Each light valve 40 includes a photoaddressed cell 50 and an addressing matrix 60. Addressing matrix 60 provides a plurality of individually controlled light beams to activate cell 50. Some means 70 for optically connecting the matrix 60 to the cell 50 may be necessary.

When projection system 10 is in operation, an image signal 74 from an external source enters the computing means 76. The signal 74 could be analog or digital. Computing means 76 serves a wide variety of functions, one of which is separation of the image signal 74 into three primary color signals. The three separate signals are sent to the addressing matrix 60 of each light valve 40. The addressing matrix 60 causes a spatial change in the optical properties of the light valve 40. In the prior art, a CRT was used to address each cell 50. A manual control system was used to monitor the conver- gence of the images from the light valve due to varia¬ bility of the CRT pixel location and alter the signals sent to the CRT so as to ensure convergence. In the present invention, because each light valve 40 is matrix addressed, the projection system 10 does not need a feedback system to converge the images. Unlike CRT systems, in which the position of a pixel might vary, with a solid state matrix addressing device, the pixels have a fixed, unchanging position. In a projection system with multiple light valves, after an initial convergence of the images by appropriate manipulation of each light valve 40, the convergence will be maintained indefinitely.

The Photoaddressed Cell

The light valve 40 contains a photoaddressed cell 50. In its broadest concept, photoaddressed cell 50 can be any compact light modulating system which accepts an optical input. Electrochromic, scattering, absorbing, polarizing, transmissive, and reflective devices are all possible. However, it is preferred that the photoad¬ dressed cell be a reflective liquid crystal cell. It is also preferred that the liquid crystal cell be of the type which modulates polarization. It is further pre¬ ferred that the photoaddressed cell be of the refresh- type. By refresh-type is meant a light valve in which the image in the liquid crystal cell automatically blanks after the addressing light is removed. Referring to Figure 2, the preferred aspect of the photoaddressed cell 50 is shown. Write light from the addressing matrix 60 travels through the substrate 80 and transparent electrode 82 to the photo-conductor 85. The transparent electrode 80 is typically indium-tin- oxide (ITO) . The photoconductor layer is preferably composed of hydrogenated amorphous silicon in a layer approximately 5 microns thick. Transparent electrode 88 of composition similar to transparent electrode 80 is disposed on a substrate 90. A fraction of a bias vol- tage applied by voltage generator 95 across transparent electrode 82 and transparent electrode 88 will be dropped across the photoconductor 85, dielectric mirror 102, light blocker layer 105, and alignment layers 107, 108, and a fraction of the applied voltage will also drop across the liquid crystal layer 110. Photoconduc¬ tor 85 is protected from projection light by light blocking layer 105 and dielectric mirror 102. Light blocking layer 105 is preferably amorphous germanium alloy as described in pending US Patent Application Serial Number 436,400 (incorporated herein by reference) filed November 14, 1989 and assigned to the same assignee as the present invention. Dielectric mirror 102 is formed from sequentially evaporated layers of high and low dielectric materials; such methods have been well described in the literature. When light is relayed onto the photoconductor it causes the electrical impedance of the photoconductor to become lower in proportion to the intensity of the relayed light. The magnitude of the fraction of the voltage across the liquid crystal layer 110 under conditions of illumina¬ tion of the photoconductor 85 will therefore be propor- tional to the intensity of light from the addressing matrix 60. The electric field generated by this voltage is inversely proportional to the thickness of liquid crystal layer 110.

Under these conditions of applied electric field, for materials with a positive dielectric anisotropy, the molecular axis of the LC molecules will become aligned in the direction of the field. The extent of this alignment will be a function of the magnitude of the applied field and of the alignment layers 107, 108. Alignment layers may be polyamide, polyamide, or evapo¬ rated SiO as is well known in the art.

When linearly polarized read light is transmitted through the liquid crystal layer 110 in which the molecular alignment has been altered by application of electric field, the polarization of the light will be changed such that an appropriately aligned polarization analyzer (that is beam splitter 30) will block the fraction of the light modulated by the liquid crystal. A description of the photo-addressed LCLV and its mode of operation is described in a pending U.S. Patent Application Serial No. 436,400 filed November 14, 1989 and assigned to the same assignee as the present appli¬ cation. High intensity projection light from light source 25 transmitted through LC layer 110 and reflected off dielectric mirror 102 will therefore be modulated according to the modulation of low intensity addressing light from addressing matrix 60.

As an alternate embodiment, in a full-color projec¬ tion system, a single light valve with color filters is coupled to an addressing matrix. When red, blue, and green color filters 115 are positioned in the path of the read light that is reflected from the dielectric mirror 102, (for example, between the transparent elec¬ trode 88 and the alignment layer 88, or alternately on the outer surface of substrate 90) individual pixels can be arranged in a pattern corresponding to the orienta- tion of the addressing matrix 60. This allows a full color, single light valve projection system to be realized, although a loss of resolution results. The Licrht Emitting Matrix

As previously stated, in a first embodiment of the invention the addressing matrix 60 is an array of light emitting pixels. Possible arrays of light emitting pixels are AC thin film electroluminescent (ACTFEL) panels, LED arrays, and vacuum-florescent arrays. As an example of the first embodiment, the version of the invention in which the addressing subsystem is a ACTFEL panel will be described.

Referring to Figure 3, the addressing matrix 60a is a laminar, planar structure which will be placed in con¬ tact with the photoaddressed cell 50. Addressing matrix 60a is formed on a glass substrate 120. Adjacent to substrate 120 and typically formed thereon is data elec¬ trode array 123. Electrode array 123 has a preferred spacing of 1 mil wide lines with 0.5 mil spaces. Alter¬ native spacing configurations to give higher aperture ratios are possible, but may not be required, as will be detailed later. The data electrode 123 is constructed of a material of high electrical conductivity such as aluminum to minimize electrical voltage drop along the line.

A planar insulating layer 128 consisting of high dielectric constant material such as Y₂0₃ on the order of 0.2 microns thick is deposited on the aluminum data lines 123. An electroluminescent (EL) layer of ZnS:Mn 130 approximately 0.5 to 0.7 microns thick is disposed between the previously described insulating layer 128 and an additional insulating layer 132 of similar compo- sition to layer 128. A thin, transparent, conductive layer 135 (60 to 100 angstroms of gold is preferred) is deposited on insulator 132 and patterned with a linear array of 1 mil wide lines and 0.5 mil spaces. This linear array is oriented orthogonal to the data elec- trodes 123. On this electrode array 135 may be depo¬ sited a transparent insulator 138 typically of high dielectric material approximately 2000 angstroms thick.

The operation of this general type of device (ACTFEL) has been described in the literature, "Feasibi- lity of a 40 x 40 cm² TFEL Display with 2000 x 2000 Lines Operating at 50 W", Kuwata et al., SID 88 Digest, p.297- 300 for example, but in the interests of operational clarity will be described briefly again. When video data is applied to electrode array 123, together with appropriately timed strobe pulses applied to layer 135 and when the magnitude and timing of the data and strobe pulses are such that the voltage difference between the two signals at the intersection or intersections of the orthogonal electrodes causes a critical voltage thresh- old to be exceeded, the electroluminescent phosphor 130 (ZnS:Mn) will emit light at the particular intersection or intersections. The Liσht Transmitting Matrix

As previously stated, in a second embodiment of the invention, the addressing matrix 60 is an array of light transmitting pixels. The previously described first '5 embodiment used an addressing mode whereby the light to address the photoconductor was derived from a light emitter in immediate proximity to the photoconductor. It is also viable to use a matrix addressed light modu¬ lator to transmit and modulate low-level light to 0 activate the photoconductor in the photoaddressed cell 50. Examples of arrays of light modulators include TFT addressed liquid crystal cells, plasma addressed cell, and multiplex addressed cell. Figure 4 shows a second embodiment of the invention which uses an active matrix 5 liquid crystal display 60b instead of an ACTFEL panel as the addressing matrix.

Referring to Figure 4, low-level light of appropri¬ ate wavelength from light source 142 is transmitted through polarizer 140 and glass substrate 145, to the 0 surface supporting the TFT matrix. TFT matrix techno¬ logy is widely known in the art with configurations similar to that shown in Figure 4. Light is transmitted through drain electrode 150 constructed of transmissive- conductive material, typically ITO. Due to the inherent 5 photoconductivity of the a-Si active element 152 of the TFT causing degraded performance in the presence of light, a light blocking layer 154 (a-Ge alloys are preferred, as previously described) is disposed between layer 152 and the glass substrate 145. Separating the 0 a-Si layer 152 from the gate 156 and the drain 150 from the source 158 is an insulating layer 160.

Video signals appropriately applied to drain 150, gate 156, and source 158 electrodes of the TFT cause an electric field to appear across liquid crystal layer 165 5 that is sandwiched between alignment layers 167 and 168 in the region of electrode 150 between electrode 150 and transparent-conductive layer 170. As was previously -13-

described, liquid crystal molecules in this region will align to the applied field; the degree of alignment is a function of the magnitude of the field. Analyzer 175 is disposed, usually on the outer surface of substrate 172, 5 sot that when voltage is applied to the liquid crystal 165, the polarization is altered and thereby blocked by the analyzer 175 to create a dark spot.

As an alternative to a liquid crystal cell which operates in the polarization mode, an absorptive cell could be used. When dye molecules with properties such that they align with the liquid-crystal molecules, (high degree of ordering) are dissolved in the liquid crystal host, they will orient to the applied field along with the LC molecules. Therefore, dyes chosen for maximum light absorbance at the emitted wavelengths will absorb or extinguish the addressing light in all regions of LC layer 165 where video signals have caused an electric field to exist. This alternative has the advantage that analyzer 175 is not required. Optical Coupling

Figures 5, 6 and 7 indicate various ways in which the addressing matrix can be optically coupled to the photoaddressed cell. Figure 5 shows a light valve 40 in which the ACTFEL panel 60a has been joined to the liquid crystal cell 50. In typical operation, video informa¬ tion signals would be applied to the addressing x,y electrodes 123, 135. The resultant spatially varying light is emitted from the phosphor 130 at the intersec¬ tion of the x,y electrodes. Light from the phosphor is then coupled by means of a silicon oil index matching fluid 180 to the entrance aperture of a fiber optic plate 183. In addition to matching the refractive index of the glass, this fluid serves to carry away electrode material that oxidizes due to shorts. Note that trans- parent insulator 138 is not required in this construc¬ tion. The fiber optic plate 183 then relays the light without substantial loss of intensity or resolution into the light valve subsystem 50, through the transparent conductor 82 and onto photoconductor 85.

This causes the impedance of the photoconductor 85 to vary according to the intensity of the addressing light. The fraction of the bias voltage applied to the liquid crystal layer 110 causes molecular realignment corresponding to changes in the applied voltage. Appro¬ priately polarized read light reflected from dielectric mirror 102 may then be transmitted to screen 48 and viewed directly as in the case when the video signal contains television information or may be compared to a reference signal and used for optical processing. Prior art matrix addressed light valves used reflective electrodes which were fabricated with litho- graphic processes. These reflective electrodes have low optical efficiency and low yield. Since there is no lithography involved in the fabrication of the liquid- crystal light modulator or dielectric mirror, the optical efficiency and yield of this invention is much greater than prior art matrix light valves.

Additionally, when the conductor array 123,135 is fabricated from a transparent conductive material such as ITO or gold, a supplementary light image may be focused through the transparent insulator layers 123, 132, the transparent phosphor 130, and onto the photo¬ conductor layer 85, so as to provide a high resolution image (such as a photographic slide) which may then be annotated, processed, or viewed when projected onto a screen. Figure 6 shows another method of coupling the addressing subsystem 60a to the photoaddressed cell 50 of the first embodiment so that the light valve 40 operates in essentially the same manner as the prior method of coupling. Disposed on transparent insulating layer 138 is transparent isolation plane 190 of ITO with sheet resistance approximately 20 ohms/square. The isolation layer 190 is operated at the average potential of the strobe electrode array 135 and serves both to isolate the photoaddressed cell 50 from the electric fields generated in the normal operation of the electro¬ luminescent panel 60a and as a common electrode 82 to drive the liquid crystal 110. Note that there is no specific means for optically coupling because the device is fabricated as a unit.

Figure 7 shows the second embodiment of light valve 40 in which the addressing matrix 60 is optically coup- led to the light valve 50. The addressing matrix 60 is an array of light transmitting pixels 60b. More speci¬ fically, the addressing matrix 60 as shown in Figure 7 is TFT addressed liquid crystal cell 60b. Coupling is achieved in a manner similar to the first embodiment. A substrate 195 is interposed between transparent conduc¬ tor 82 and analyzer 175. Substrate 195 serves as both substrate 80 and substrate 172. Substrate 195 is shown as a fiber optic plate, but it could also be glass. In the configuration shown in Figure 7, the liquid crystal layer 165 contains a light absorbing dye as previously described. Alternately, liquid crystal layer 110 could act to polarize light, and substrate 195 would be com¬ posed of two separate pieces joined by an index matching fluid and analyzer 175. In typical operation, electronic signals are applied to elements 150,156,158,170 of the TFT matrix to create an electric field across the liquid crystal layer 165. Each pixel can be individually addressed to be switched into the bright or dark state. When a particu- lar pixel is in the bright state, light is transmitted through analyzer 175 and substrate 195 and onto photo¬ conductor layer 85 in order to activate liquid crystal layer 110. Construction The first embodiment can be fabricated in three parts, as shown in Figure 5. The first part 200 is constructed by successive thin film depositions, -16-

starting with the conductive layer 128 on substrate 120 and ending with strobe electrode 134 upon insulating layer 132. Part 200 is equivalent to addressing matrix 60a. The second part 203 is formed by successive depositions starting with transparent conductive layer 82 on substrate 183 and ending with first alignment layer 107 on dielectric mirror 102. The third part 205 consists of the second alignment layer 108, transparent electrode 88 and substrate 90. Then the liquid crystal 110 is disposed between the alignment layers 107,108 so that the second and third parts form the photoaddressed cell 50. Finally the two subsystems 60a and 50 are placed in contact, separated by a index matching fluid 180. Fiber optic plate 183 and index matching fluid 180 essentially serve as the means for optically coupling

70. A benefit of this fabrication procedure is that if the fabrication yield of either the photoaddressed cell or the addressing matrix is low, the defective parts can be excluded. When only functional parts are combined, the completed system has a higher yield.

The method of fabrication of this invention described above uses the concept of separate fabrication of the liquid crystal light valve section and the matrix addressed section, and then assembling the two sections with optical coupling fluid between (As described in the previous embodiment) . This concept has advantages when the fabrication yields for one section or another are low. However, the light coupling element (fiber optic plate) is quite expensive and adds to the overall cost. The coupling element can be eliminated when fabrication yields are high, as will now be described. The first embodiment can also be fabricated in just two parts as shown in Figure 6. The first part 210 is formed by the subsequent depositions, beginning with conductor layer 123 onto substrate 120 and ending with first alignment layer 107 onto dielectric mirror 120. The second part 213 is equivalent to the third part 204 described in relation to Figure 4. The liquid crystal 110 is then deposited between the alignment 107,108 to form the complete device 40. The benefit of this fabrication procedure is that if fabrication yields are high, the cost of an expensive fiber optic plate is avoided.

Another benefit of this fabrication process is that the light valve 50 is directly integrated with the addres¬ sing matrix 60a to form a solid state device.

The second embodiment can fabricated in three parts. The first part 220 is formed by deposition of the polarizer 140 on one side of the substrate 145 and by deposition of the TFT matrix and alignment layer 167 on the other side of the substrate 145. The second part 223 is formed by successive depositions of electrode 170 and alignment layer 168 on one side of substrate 195 and transparent electrode 85, photoconductor 85, light blocking layer 105, dielectric mirror 102, and alignment layer 107 on the other side of substrate 195. The third part 225 is equivalent to the third part 204 described in relation to Figure 4. Liquid crystal is deposited between alignment layers 107 and 108. Liquid crystal containing light absorbing dyes is disposed and between alignment layers 167 and 168 to form the completed device 40. In this case, substrate 195 acts as the optical coupling means 70. Alternately, the device could be fabricated in four parts by splitting substrate 195 into two pieces, depositing the appropriated layers on those pieces, and then linking the two pieces by an index matching fluid and an analyzer. The common feature of all of these embodiments is that the matrix addressing subsystem 60 and the LCLV subsystem 50 are closely coupled into a single unit. This produces a compact device which is far less cumbersome than CRT or laser addressed LCLV's and consequently far easier to use in a projection system. Furthermore, no system of autoconvergence is needed as in CRT or CRT addressed LCLV devices. Finally, the use -18-

of thin film manufacturing technology can provide a considerable decrease in cost. Improved Aperture Ratio

One of the inherent disadvantages of the matrix liquid crystal light modulator is that the percentage of light transmitted through the modulator is low and consequently the viewed image is dim. A thin film transistor driven LCLV consists of an active area (the region where the liquid crystal can modulate the input light) and inactive area (the region composed of elec¬ trode data lines, busbars, electric devices, and the areas separating adjacent pixel elements) . The ratio of active area to total area is defined as the aperture ratio. As the number of pixels is increased to achieve higher resolution, the number of components in the inactive area is also increased. The lithography process used to define the components making up the inactive area is constrained as to minimum feature size. Consequently, the inactive area will increase and the aperture ratio, picture brightness, and contrast will accordingly be reduced. In general, brightness is directly proportional to aperture ratio. Typical aper¬ ture ratios of transmissive projection light valves are less than 50% for 240 line devices and less than 20% for 1000 line devices.

The preferred mode of the present invention is a reflective light valve with an improved effective aperture ratio, preferably an effective aperture ratio of 100%. By effective aperture ratio is meant the portion of the liquid crystal which can be switched. In accordance with an aspect of the present invention, this is accomplished by having the addressing matrix pixels produce light with a particular angular spread, and by having the optical thickness between the addressing matrix and the photoconductor layer controlled. By pro¬ per manipulation of the angular spread and the optical thickness (a function of both physical thickness and the refractive index of the materials) between the photocon¬ ductor and the matrix, it is possible to arrange for the light from the pixel areas to spread out and cover larger regions, and thus illuminate a higher percentage of the photoconductor than would otherwise be possible. It is even possible to make the regions congruent or overlapping. Because the entire photoconductor can receive light, all of the liquid crystal material can be switched, and thus a higher brightness and contrast ratio can be achieved. This preferred mode can be achieved by both embodiments (addressing matrix 60a and matrix 60b) of the invention.

Referring to Figure 8, a schematic of a simple apparatus for implementing the improved aperture ratio in the embodiment using the matrix of light transmitting pixels is shown. For this example, the addressing matrix 60b (a TFT matrix addressed liquid crystal cell) is coupled to the photoaddressed cell 50. For simpli¬ city, some of the layers of the device are not shown. The matrix has inactive regions 230 that are formed by the addressing electrodes, busbars, and TFT regions, and active regions 232 that will transmit light. As shown in Figure 8, the addressing TFT matrix 60b and the photoaddressed light valve 50 can be fabricated sepa- rately and then optically connected. Divergent light enters the TFT matrix 60b. This process essentially creates a magnified image of the pixel 132 on the photoconductor 85. The image of the active pixel area is projected through the spacer region, ITO (not shown) , substrate 172, polarizer 238, and onto substrate 195. The substrate could be fiber optic plates, glass substrate with silicon index matching oil, or any other appropriate combination. As the light propagates through these layers it undergoes lateral spreading. Some further spreading occurs within the photoconductor layer, but because this layer is only about 5um thick, this effect is minimal. As can be seen in Figure 8, the -20-

image of each active area pixel 132 will be magnified and projected onto the photoconductor layer 85 to activate region 142 of the liquid crystal layer 110. If standard collimated light were used, region 244 of the liquid crystal layer 110 would be activated. Alternate¬ ly, a fiber optic plate could be interposed in place of the glass substrate to reduce the effective optic thick¬ ness of the space between the matrix and the photocon¬ ductor. No lateral spreading occurs within the fiber optic plate, so the size of the pixel when it impinges the fiber optic plate is equal to the size of the pixel on the photoconductor. In the preferred mode, the angular spread of the light is just enough to make the activated regions 246 contiguous or slightly overlap- ping. Of course, if a particular pixel 250 is the dark state, then light does not pass through polarizer 238, and thus the liquid crystal material 110 behind that pixel is not switched.

Referring to Figure 9, a schematic of the improve- ment of aperture ratio is shown. In the previous devices, the active area of a pixel would be region 244. In the preferred mode of the present device, region 246 is used. Because the regions 246 are adjacent, the entire photoconductor surface can receive light. Conse- quently, all of the liquid crystal material can be switched, and thus a higher brightness and contrast ratio can be achieved. Figure 9 shows a device in which the pixels are square. In many constructed TFT matrices the pixels and spacings between them are not squares. In such cases, it is not possible to achieve the full 100 percent aperture ratio without substantial overlap of pixels that would result in blurring. However, the concept of improvement of the aperture ratio still applies, and a substantial increase in -brightness can be achieved.

As previously noted, divergent light is provided to the light transmitting matrix 60b. One way to provide this divergent light is to send a beam of substantially collimated light, having an angular spread of about 1.5 degrees to the matrix 60b. The difficulty in maintain¬ ing a collimated beam may make this method impractical. A better way to provide the divergent light to the matrix 60b is shown in Figure 10. Referring to Figure 10, a simplified schematic of the preferred apparatus for implementing a controllable improved aperture ratio is shown. Light 260 from a lamp 142 is condensed by a condenser lens 270 to form an image in a relay lens 275, like many other illumination systems. As shown, the light 260 at the array of light transmitting pixels 60b has an angular spread and produces practically adjacent regions 246 on the photoconductor 85. The extent of this angular spread can be controlled by adjusting the numerical aperture of the relay lens 270. If the numerical aperture of the relay lens 270 is reduced to that of relay lens 280, the degree of divergence of the light can be reduced. The controllable nature of the angular spread allows greater freedom in the optical spacing between the photoconductor and the matrix.

A light valve 40 was constructed to use the illumination system as shown in Figure 10. The relay lens 270 had a variable f/# between 1.4 and 8. The TFT array 60b had active areas of 4.3 mil vertical by 4.6 mil horizontal, and a spacing of about 6 mil vertical and 7 mil horizontal. The physical spacing between the photoconductor 85 and the matrix 20 was 3.2 mm in total, of which 3.0 mm was glass and 0.2 mm was polarizer, both with n=1.5. The transparent conductor layers are too thin to make an appreciable difference. The spacing between the TFT array 60 and the relay lens 160 was about 15 cm. The resultant spots projected from reflec¬ tive liquid crystal display 40 onto projection screen 48 could be made to overlap.

Although the above description related to the second embodiment of the invention using an addressing matrix with light transmitting pixels, the preferred mode also applies to the first embodiment of the inven¬ tion using an addressing matrix with a light emitting pixels. It is possible to achieve high aperture ratio with an electroluminescent panel as shown in Figure 11. For simplicity, some of the layers have been left out of Figure 11. Referring to Figure 11, each activated pixel 290 of electro-luminescent material 130 radiates light 293 in all directions, and therefore produces divergent rays. Light passes through insulating layer 138 and substrate 183. If substrate 183 is glass, then additional lateral spreading will occur. If substrate 183 is a fiber optic plate, then the light produced by pixel 290 will be relayed without optical spreading to photoconductor 85. When the light from an individual pixel 290 impinges the photoconductor layer 85, there is an intensity distribution 296 at that surface. The shape and height of intensity distribution 296 can be affected by the optical thickness between the photo- conductor layer 85 and the electroluminescent material

130 and by the intensity of light from the pixel 290. The photoconductor layer 85 usually has a critical illumination value I_on below which the adjacent liquid crystal material 110 does not switch. By proper manipulation of optical thickness and intensity, the size of region 300 of the liquid crystal 110 in which the light intensity is greater than I_on can be controlled. This allows intensity distribution 298 to be achieved, with the result that adjacent activated pixels 302 will have activated regions 305 which are contiguous or slightly overlapping. If contiguous regions 305 are not possible, a larger portion of the photoconductor will be illuminated than would be possible if the light emitting pixels were directly adjacent to the photoconductor.

Claims

WHAT IS CLAIMED:

1. A matrix addressed light valve apparatus, comprising: two-dimensional matrix addressed light source means defined by creating pixels in a fixed position for producing a spatial light pattern; and refresh type photo addressed liquid crystal light valve means for receiving said light pattern from said light source means and for modulating a read light in accordance with said light pattern.

2. The apparatus of Claim 1 wherein said photo addressed liquid crystal light valve means modulates the polarization state of the read light.

3. The apparatus of Claim 2 wherein said photo addressed liquid crystal light valve means includes a first transparent insulator layer, a first transparent conductor layer positioned adjacent said first transparent insulator layer, a photo conductor layer positioned adjacent said first transparent insulator layer, a light blocker layer positioned adjacent said photoconductor layer, a dielectric mirror layer positioned adjacent said light blocker layer, a first alignment layer positioned adjacent said dielectric mirror layer, a liquid crystal layer positioned adjacent said dielectric mirror layer, second alignment layer positioned adjacent said liquid crystal layer, and a second transparent conductor layer adjacent layer positioned adjacent said second alignment layer.

4. The apparatus of Claim 1 wherein said matrix addressed light source means is an array of light emitting pixels.

5. The system of Claim 4 wherein said matrix addressed light source means is an electroluminescent panel.

6. The system of Claim 5 wherein said electro¬ luminescent panel includes a substrate, first data electrode means positioned adjacent said substrate, a first insulator layer positioned adjacent said first data electrode means, light emitting means positioned adjacent to said first data electrode means, a second insulator layer positioned adjacent to said light emitting means, second data electrode means positioned adjacent to said second insulator means such that said electrode means is substantially perpendicular to said first data electrode means, said first and second data electrode means combining to form a two dimensional matrix for addressing said light emitting means, and a transparent insulator positioned adjacent to said second data electrode means.

7. The system of Claim 1 wherein said light source means includes an illumination source and an array of light transmitting pixels positioned to receive light from said illumination source.

8. The system of Claim 7 wherein said array of light transmitting pixels is a thin film transistor matrix addressed liquid crystal light valve.

9. The system of Claim 8 wherein said thin film transistor matrix addressed liquid crystal light valve includes a substrate, a plurality of thin film transistors means posi¬ tioned adjacent said substrate, first alignment means positioned adjacent said thin film transistors means, liquid crystal means positioned adjacent said first alignment layer means, second alignment means positioned adjacent said liquid crystal means, transparent-conductive layer means.

10. The system of Claim 9 wherein said liquid crystal means contains light absorbing means.

11. The system of Claim 1 wherein said liquid crystal light valve means includes a plurality of color filter means.

12. The system of Claim 7 wherein said light valve further comprises a polarizer means.

13. The system of Claim 1 further comprising a means for optically coupling the light source means to the light valve means.

14. The system of Claim 13 wherein said optical coupling means includes refractive index matching fluid and fiber optic light transmitting means.

15. The system of Claim 13 wherein said optical coupling means includes optically transmissive insulator means for preventing electrical contact with said light source means and optically transmissive electrical isolation means for preventing electrical fields from said light providing means from interacting with said photo addressed liquid crystal light valve means.

16. The system of Claim 1 wherein said light source means is directly bonded to said photoaddressed liquid crystal light valve means without optical coupling mean.

17. In a system for projecting an image onto a given projection plane, said system including means for providing first and second beams of light, first and second photoaddressed liquid crystal light valves respectively optically coupled to said first and second beams for independently modulating said beams depending upon the way in which the light valves are photoaddressed, and means for projecting the modulated beams onto said projection plane in registration with one another, the improvement comprising: first and second matrix addressed light providing means respectively optically coupled to said first and second light valves in a way which insures registration of said beams onto said projection plane.

18. The improvement according to Claim 17 wherein each of said matrix addressed light providing means includes means for producing an array of individually controlled addressing light beams, and wherein each of said light valves and its associated light providing means are positioned relative to one another such that corresponding addressing light beams from said first and second light providing means impinge on light receiving photoaddressing surfaces of said first and second light valves at substantially identical points on said surfaces.

19. The improvement according to Claim 17 including means for optically coupling together each of said light valves and its associated light providing means such that each of said addressing light beams diverges in a controlled manner so as to impinge over a larger area on its associated photoaddressing surface than it would otherwise impinge if it did not diverge.

20. The improvement according to Claim 17 wherein each of said light providing means includes an array of light emitting pixels.

21. The improvement according to Claim 17 wherein each of said light providing means includes an array of light transmitting pixels.

22. A liquid crystal light valve assembly com¬ prising: a photoaddressed liquid crystal light valve adapted to modulate a given light beam depending upon the way in which the light valve is photoaddressed at its photoaddressing surface; and a matrix addressed light providing device for producing a matrix array of individual addressing light beams, said light providing device being optically coupled to said light valve such that each of said addressing light beams impinges on said photoaddressing surface in a region and is caused €b diverge in a controlled manner as it does so.

23. The assembly according to Claim 22 wherein the regions where addressing light beams impinges on said photoaddressing surface are contiguous or slightly overlapping.

24. The assembly according to Claim 22 further comprising means for optically coupling the matrix addressed light providing device to the photoaddressed liquid crystal light valve.

25. The assembly according to Claim 22 wherein the light providing device includes an array of light emitting pixels.

26. The assembly according to Claim 22 wherein the light providing device includes an array of light trans¬ mitting pixels.

27. A light valve assembly, comprising: a light source which provides incoming light having a controlled degree of divergence; an active matrix liquid crystal light valve positioned in the path of the incoming light, the active matrix light valve having a plurality of controllable light transmitting pixels through which the incoming light passes to produce a plurality of outgoing light beams, each outgoing light beam having a degree of divergence; and a liquid crystal cell including a photoconductor layer, the cell optically coupled to the active matrix so that the photoconductor layer receives the plurality of outgoing light beams at a plurality of illuminated regions; the distance between the photoconductor layer and the active matrix liquid crystal light valve and the degree of divergence being selected so as to provide illuminated regions of a predetermined size.

28. A light valve device, comprising: a matrix addressed electroluminescent panel which emits write light; and a photoaddressed liquid crystal cell bonded directly to the electroluminescent panel, the cell receiving write light from the matrix addressed electroluminescent panel and modulating read light in accordance with the intensity of the write light.