Efficient optical system for a high resolution projection display employing reflection light valves

Abstract: An optical system is described consisting of reflection birefringent light valves, polarizing beam splitter, color image combining prisms, illumination system, projection lens, filters for color and contrast control, and screen placed in a configuration offering advantages for a high resolution color display. In particular, the present invention includes a color splitting prism assembly having a plurality of output faces through which a separate color component is directed to be reflected by a respective light valve. Color absorbing filters are disposed at each of the output faces to absorb the color components not corresponding to the color directed through each output face.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to optical systems for projection displays and more particularly to a high resolution improved optical system employing reflection light valves. Discussion of the Prior Art The development of liquid crystal flat panel displays has led to interest in using such panels as transmission light valves for a color projection display for data or video applications. A number of such systems are commercially available. However, in order to prepare light valves with a higher number of pixels as required for high resolution applications, the liquid crystal panel becomes large. It is difficult to produce panels with very small pixels since the electronic circuit structure needed for the operation of the panel obscures the light passage through the pixels to an intolerable degree. Instead, there is interest in light valves consisting of an array of pixels designed to operate in reflection instead of transmission. In reflection mode, it becomes possible to fabricate the mirror structure directly above the electronic circuitry. This mode allows smaller pixel areas without the obstruction of light passage caused by the circuitry, thus allowing maximum light throughput. This solves the pixel size problem, but introduces complexity into the optical system required to project an image of one or several cells onto a screen. A major class of liquid crystal light valves operates in a birefringent mode. Liquid crystal light valves are one known type of a spatial light modulator. The operating principle is based on rotation of polarization of the incoming light for each pixel. An image is formed in a transmission liquid crystal cell when the cell is placed between two polarizers. In a reflection mode of operation, the challenge is to direct linearly polarized light onto the light valves and to form an image only using the light reflected with rotated polarization. The reflective light valve mirrors will reflect both polarizations of light. Light of the original polarization must be selectively removed from the reflected light beam and not be allowed to reach the screen. U.S. Patent 4,687,301 to Ledebuhr discloses a liquid crystal light valve projection system that includes a beam polarizer, a color separating assembly that comprises two color separators enclosed in a liquid filled enclosure, and liquid crystal reflection light valves for reflecting the separated colored light back into the separating assembly to be combined and directed into a projection lens. One of the drawbacks of this system is the lack of polarization control throughout the system, as well as poor contrast. Thus there is a need for an efficient optical system for achieving a full color high resolution display using several reflective birefringent light valves. US 5,379,135 discloses an optical system which is for a display apparatus having a light source for emitting read light and a reflection type spatial light modulator for modulating the read light incoming thereto into in accordance with image written thereon. The modulated light is projected onto a screen by a projection lens to display the image. A field lens is provided between the modulator and screen at such a position that both the read and modulated light pass therethrough. The filed lens focuses the modulated light on near an entrance pupil of the projection lens so that the intensity of the modulated light is increased. SUMMARY OF THE INVENTION The object of this invention is to provide an efficient compact optical system for a high resolution data or video projection display, having full color and utilizing reflection-type birefringent spatial light modulators such as liquid crystal light valves that eliminates the problems of prior art systems. The present invention accomplishes the above objects by providing color absorbing means adjacent each spacial light modulator for absorbing color not corresponding to the color of the light reflected by the modulator for color band selection, color balance and purity for improving efficiency. The liquid crystal display (LCD) projector of the present invention comprises an illumination system having a light source such as a metal-halide arc lamp, an ultraviolet and infrared filter or filters positioned in the optical path from the light source for filtering out the infrared and ultraviolet light emitted from the light source and a relay lens system for magnifying the illumination system output plane and imaging said plane onto the liquid crystal light valves. Between the relay lens system and the light valves, the LCD projector also includes a polarizing beam splitter cube for selecting one of the two polarizations of the light and directing said polarization along the optical path, and an optical assembly comprised of a plurality of prisms for separating the light into a plurality of color components. In the preferred embodiment, the prism assembly is comprised of three prisms for sequentially separating the red (R), blue (B) and green (G) light components from the visible light and directing each light element onto a reflective LCD light valve. A color absorbing means is disposed at each output face of the prism assembly to absorb the color component not being directed through that output face. The three light valves are positioned with their reflective surfaces perpendicular to the optical path and serve to spacially modulate the light beam by means of rotation of the polarization of the light, and to reflect each of the R, G, B light elements back into the optical path such that each of the three elements will retrace its original path through the color prism assembly which serves to recombine the three R, G, B light elements into one light beam. The color absorbers provide color balance and purity to enhance the image when the components are recombined. The light beam will then reemerge from the color prism assembly, pass through the polarizing beam splitter cube which will separate the modulated light from the unmodulated light and direct the modulated light beam into the projection lens which serves to combine the images of the three light valves and project the composite image onto a rear projection screen comprising, for example, a fresnellens and a diffusing screen. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of an optical projection display system for the present invention; Figure 2 is a schematic cross-sectional view of light paths through a color separating assembly of the system of Figure 1. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, Figure 1 shows a schematic of the optical arrangement for an LCD projector 10 of the present invention. The projector 10 includes a lamp source 12, an infrared filter 14, an ultraviolet filter 16, and an illumination system 18 including an intensity uniformization means such as a light tunnel 56. The projector 10 also includes a polarizing beam splitter 22, a prism assembly 24 for sequentially separating the light into the desired plurality of colors, such as red, green and blue light components, and directing each of the three color components onto one of three birefringent reflective light valves 26, 27 and 28. The prism assembly 24 is comprised of three prisms 30, 31 and 32 arranged at angles to provide two color separating surfaces 34 and 36 having dichroic coatings thereon that provide the desired color separation. The prism assembly 24 serves the additional function of recombining the light reflected from the light valves. The dichroic coatings are designed to preserve performance for both polarizations. The projector 10 further includes a projection lens 38 and a screen 39 upon which the desired image is formed. The optical image forming module comprising the polarizing beam splitter cube 22, the color splitting/ combining prism assembly 24 used in a double-pass mode, and the reflective light valves 26, 27, 28 form the color images that may be projected onto a screen. It should be noted that the relative position of the illumination system 18 with the light source 12 and filters 14 and 16 as shown in Figure 1 may be exchanged with the projection lens 38 and screen system 39. In accordance with the present invention, the projector system 10 includes color enhancement means. These additions include color and polarization controlling filters placed at key locations for the enhancement and control of color and contrast. These filters include color filters 43, 44 and 45 placed between the light valves and the color prism assembly 24. In addition, a polarizing film 40 is placed in the illumination path between the lamp 12 and the polarizing beam splitter cube 22 and/or a polarizing film 42 placed between the polarizing cube 22 and the projection lens 38 for additional polarization control and for contrast enhancement. Moreover, a color filter 46 can be placed within the illumination path between the lamp 12 and the polarizing beam splitter cube 22 for color and stray light control. A further efficiency means is provided by a screen element consisting of a quarter wave film 50 and a polarizing film 52 disposed on screen 39 to reduce the effects of room light reflection. The polarizing film 52 is placed on the viewing side of the screen 39 and the quarter wave filter 50 is placed between the film 52 and screen 39. The function of the major components within the display system can be understood by following the path of light rays from the light source 12 through the system to the screen 39. The light source 12 is a metal-halide arc lamp with a parabolic reflector. Immediately following the lamp reflector 12 are filters 14, 16 for removing the infrared and ultraviolet radiation from the light source. The visible light is then directed into the illumination optics 18 which functions to provide uniform light intensity and to form a largely parallel beam that is to be used to bring light upon the light valves at a largely normal incidence. The illumination optics includes a lens 54 to focus the emitted visible light into a light tunnel 56 comprised of either a set of mirrors to form a hollow tunnel or a slab of glass to form a solid light tunnel. A relay lens system shown for example by lens 58 and 60 magnifies the uniform light tunnel output plane and images the plane onto the spatial light modulators through the beam splitting cube 22 and prism assembly 24. The light incident upon the light valves must be linearly polarized, but light reflected having the same polarization is to be excluded from the image forming beam. Application of a voltage to the liquid crystal light valves 26, 27, 28 causes a rotation of polarization, therefore light of polarization rotated relative to the incident beam is selected for forming the image upon the screen. This is achieved by use of the polarization beam splitter cube 22 designed for use over a wide range of wavelengths of the visible light spectrum and over a suitable range of angular divergence of the beam, typically a number of degrees. The light path through the image-forming module is shown in detail in Figure 2. Light beam 62 enters the beam splitter 22 and the so-called p polarization component propagates directly towards the light valves whereas the s component is reflected by the prism as is well known for a polarizing beam splitter. Light of p polarization from the polarizing beam splitter enters prism assembly 24 which is similar to that used to divide light into primary color bands in a video camera. This is achieved by dichroic coatings upon the prism faces as is well known for the camera application. Surface 34 reflects the red component and transmits blue and green components. Surface 36 reflects the blue component and transmits the remaining green component. The red and blue components are internally reflected by their respective prisms 30 and 31 and exit through output faces 64 and 66. The green component exits directly through output face 68. It should be noted that the sequence of separation of the R, G, B components may be changed as desired. The individual red, green and blue color components impinge upon light valves 26, 27, 28 situated at the output faces 64, 66, 68 of the prism assembly 24. In camera applications of these color prisms, CCD detectors would be placed at the location of the light valves. In the present invention, the light is reflected back from each valve so as to traverse the prism assembly 24 a second time, so reconstituting a "white" light beam again from the R, G and B color components. The red component is again reflected from surface 34. The blue component is reflected at point 70 of surface 35 of prism 31 but is transmitted at point 72. The angle of incidence at point 70 is steeper than the so-called critical angle so that total internal reflection occurs. The blue component is transmitted at point 72 because the angle of incidence is below the critical angle. The combined R, G, B reflected beam 74 traverses the polarizing beam splitter 22, but this time the original p component is transmitted through the prism and discarded while the rotated s polarization component is reflected by the prism as desired, to the projection lens. It is to be noted that the projection lens must be of a special design to accommodate the large glass working distance and the telecentric illumination. Such a lens is described as a retrofocus telecentric lens. The projection lens forms an image of all three light valves upon a screen which may be viewed either from the front or rear. The images of the light valves are brought into coincidence upon the screen by mechanical adjustment of the light valves relative to the prism output faces. An alternative optical arrangement is also possible in which the s polarization is used for illumination and the p polarization is used in the imaging path. This arrangement can be simply achieved by interchanging the illumination optics and the projection lens in Figure 1. As stated above, the efficiency of the optical system is improved by quarter wave plate 48 incorporating a thin birefringent layer placed between the projection lens 38 and the polarizing beam splitter 22 for the reduction of the possibility of light reflected back into the optical system by the surface of the lens elements being reflected back again to the screen. Light going through the quarter wave retardation plate or film 48 toward the lens 38 and then reflected back through the film a second time will have its polarization rotated by 90° and thus will be removed from the optical system by the polarizing cube 22 or by the polarizing film 42. The optical system of the present invention is also directed to a screen element consisting of a quarter wave plate or film 50 and a polarizing film 52. These elements are placed on the screen 39 with the polarizing film 52 on the outside and the quarter wave film 50 toward the lens 38. These screen elements serve to reduce room light reflections from the screen while transmitting essentially all the light from the projection lens. By converting the circularly polarizedlight from the lens back to linear polarization, all the linearly polarized light will be transmitted through the polarizing film. The first polarization of room light will be absorbed in the polarizing film 52 as the light enters the film. The second polarization will pass through the quarter wave film 50, reflect off the screen and go back through the quarter wave film 50 a second time. This double pass converts the second polarization to the first polarization which will be absorbed by the polarizing film 52. Several additional features are disclosed to further improve efficiency. It is preferred that all dichroic coatings be optimized for their function within the system. That is, although polarizing beam splitters and color combining prisms are known in the art, they are poorly suited to the present application unless care is taken to design for the optimization of the incident p polarization and emergent s polarized beam. Additional system design elements of the present invention include the incorporation of a beam intensity homogenizer such as a light tunnel. In a preferred embodiment, the light tunnel 56 has a cross-sectional shape substantially identical to the cross-sectional shape of the light valves 26, 27, 28 in order to capture substantially all of the light emerging from the light tunnel in the system. When not so matched, significant portions of the edges of the image will be lost or significant portions of the illumination light will be lost. The use of a lamp of small arc size matched by optical design for efficient illumination of the light valves is also preferred. In addition, the use of color filters 43, 44, 45 and 46 provide selection of the color bands, color balance and color purity for improving efficiency. Each spatial light modulator has its own filter or filters. The filters absorb band edges of each color since the color splitting/combining prism assembly 24 is not efficient at band edges, because band edges are particularly sensitive to polarized light. The filter operates in both directions, i.e., light going to the modulator and light reflected by the modulator, and absorbs light of the other two colors. Filter 46 removes stray light in the assembly. Pre-polarizer 40 is an absorbing film that absorbs one polarization of light and transmits the other. Film 40 pre-selects the polarization used in the optics in order to enhance their performance, as well as provide improved contrast. Post-polarizer or clean-up polarizer 42 is an absorbing polarizing film. Film 42 is aligned to transmit the modulated light, absorbs any residual unmodulated light, and provides enhanced performance and better contrast. The disclosed optical system has application to desk top data monitors having compact form and very high resolution compared with CRT monitors. It also has application to conference room displays, for video and for HDTV usage. It may be implemented as either a front or rear screen projection display.

Claims: An apparatus for projecting an image onto a screen comprising: a light source (12) for providing light; a polarizing beam splitting means (22) for splitting the light into first and second polarized lights having first and second polarizations respectively; a color splitting prism assembly (24) having a plurality of output faces, said color splitting prism assembly splitting the first polarized light into a plurality of color components and directing each color component through a corresponding one of said output faces; color absorbing means (43, 44, 45) at each of said output faces for absorbing each of said plurality of color components not corresponding to the color components of light directed through each of said output faces; a spatial light modulator (26, 27, 28) at each of said output faces for reflecting said color components passed through each of said output faces as a reflected color component of the image passed back into said prism assembly; said color splitting prism assembly (24) recombining the reflected color components of the image to form a combined reflected image, and said polarizing beam splitting means (22) splitting the combined reflected image into first and second polarized images and directing said second polarized image to a projection lens (38) projecting said second polarized image as the image onto the screen (39). The apparatus of claim 1 further including: means (54, 56, 58, 60) for directing the light from said source into said polarizing beam splitting means. The apparatus of claim 2 wherein said means for directing the light from said light source to said polarizing beam splitting means (22) includes an illumination assembly (18) having means for filtering out ultraviolet (16) and infrared (14) light and means (54, 56, 58, 60) for providing uniform light intensity to the light directed to the polarizing beam splitting means (22). The apparatus according to claim 2 further including pre-polarizing filter means (40) disposed within a light path between the light source and said polarizing beam splitting means (22) for absorbing one of said first or second polarizations. The apparatus according to claim 2 further including a post-polarizing filter means disposed within a light path between said polarizing beam splitting means (22) and said projection lens (38) for absorbing one of said first or second polarizations. The apparatus according to claim 1 to 5 further including a color absorbing film (46) disposed between said light source (12) and said polarizing beam splitting means (22). The apparatus of claim 1 wherein said spatial light modulator includes a liquid crystal light valve assembly. The apparatus of claim 1 wherein said prism assembly is comprised of three solid glass prisms (31, 32, 33) forming two color separating surfaces (34, 36) located internally of said assembly. The apparatus of claim 1 wherein said polarizing beam splitting means (22) is comprised of two solid glass prisms forming a polarization separating surface internally of said beam splitting means. The apparatus according to claim 1 wherein said color absorbing means includes means for absorbing band edges of each of said plurality of color components. The apparatus of claims 1, 3 wherein said illumination assembly (18) having a lens (54, 56, 58, 60) and means for filtering out ultraviolet (16) and infrared light (14), said means for filtering out ultraviolet and infrared light being located between said lens and said light source (12).

Inventors: CHIU GEORGE LIANG-TAI, CIPOLLA THOMAS MARIO, DOANY FUAD ELIAS, DOVE DEREK BRIAN, ROSENBLUTH ALAN EDWARD, SINGH RAMA NAND, WILCZYNSKI JANUSZ STANISLAW, CHIU, GEORGE LIANG-TAI, CIPOLLA, THOMAS MARIO, DOANY, FUAD ELIAS, DOVE, DEREK BRIAN, ROSENBLUTH, ALAN EDWARD, SINGH, RAMA NAND, WILCZYNSKI, JANUSZ STANISLAW
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES
Application year: 1996
Main classification: H04N9
IPC codes: G02F1, H04N9, H04N5