Abstract:
An image projection system having a reflective imaging device and a projection device, characterized in that a quarter wave plate is provided between the reflective imaging device and projection lens in such manner as to suppress reflections from the projection lens from reaching the reflective imaging device while minimizing reflections from its own surface reaching the reflective imaging device. Preferably, the quarter wave plate is laminated to a polarizing beamsplitter exit face or to an optional linear polarizer sheet, to eliminate an air interface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of projection imaging systems, and more particularly to a system and method for reducing stray light in a projection system in order to improve projected image contrast. 
     BACKGROUND OF THE INVENTION 
     In Reflective LCD projection displays, also known as a liquid crystal on silicon (LCOS) display, a polarizing beamsplitter (PBS) is often used to separate the illumination, off light, and projected image beams. Great effort goes to maximizing system contrast ratio of white to dark. Most of the effort is directed towards the PBS, the RLCD panel, and other polarization optics near the panel. The effort is toward creating a good dark state when tie entire panel is driven to black. 
     In addition to full white to full black (sometimes called sequential) contrast, intrascene or checkerboard contrast significantly impacts picture quality. The projection lens is a key element for checkerboard contrast. As such the lens alone is often designed and tested for checkerboard contrast, or ghost images. In a reflective LCD system this is not enough. The reflections from the lens elements are reflected back towards the panel and can again be reflected forward. The result is a loss of checkerboard contrast and in some cases results in fairly strong ghost images. 
     In order to reduce the ambient reflection, one of a conventional techniques is by use of a circular polarizer, which reduces the reflection of the ambient light because the reverse of the handedness of polarization in reflection. The reduction of ambient reflection is significant. But for a high quality color display system, particularly for the high performance system utilizing the on-axis or near on-axis virtual image techniques, the reduction is still not adequate. 
     Ziegler discloses in U.S. Pat. No. 4,657,348 entitled “Arrangement to Remove Reflection from Liquid Crystal Displays (LCDs)” (issued on Apr. 14, 1987), expressly incorporated herein by reference, an optical arrangement to remove reflection from LCD display by employing a cover disk in front of the LCD, which is disposed obliquely to and apart from the LCD. A quarter wave retarding foil is disposed on the liquid crystal cell. The cover disk includes a polarizer, The quarter wave foil cooperates with the polanier to substantially remove the reflection from the light passing through the cover disk. Application of the polarizer together with the quarter wave (λ/4) plate to remove the ambient reflection is thus well known in the art. However, such technique imposes several limitations for modem display devices, particularly for the virtual image color display systems. First of all, for a virtual image display, the blocking of the ambient reflection by this conventional method also reduces the brightness of the image display. It is caused by the reduction of the image reflection by the poplarizer and the λ/4 retarding foil. 
     U.S. Pat. No. 5,786,934, (issued on Jul. 28, 1998, Chiu et al.), and U.S. Pat. No. 5,621,486 (issued Apr. 15, 1997, Doany et al.), each entitled “Efficient Optical System for a High Resolution Projection Display Employing Reflection Light Valves” (issued on Jul. 28, 1998, Chiu et al.), expressly incorporated herein by reference, provides an image projection system including a quarter wave plate positioned to suppress stay reflection from the projection lens. 
     U.S. Pat. No. 5,831,712, entitled “Optical Apparatus Having Ocular Optical System” (issued Nov. 3, 1998, Tabata et al.), expressly incorporated herein by reference, discloses a liquid crystal display device including a beam splitter prism, having quarter wave plates at its surfaces to suppress ghost images from ambient light. 
     U.S. Pat. No. 6,088,067, provides a “Liquid Crystal Display Projection System Using Multilayer Optical Film Polarizers”. 
     U.S. Pat. No. 5,278,532, entitled “Automotive Instrument virtual Image Display” (issued on Jan. 11, 1994, Hegg et al.) discloses a virtual image automotive instrument display system. 
     EP 0 991 281 A2 (20000504) relates to a projection-type display device having a beamsplitter prism, having a polarization filter as an analyzer at its output to block transmission of undesired light of a particular polarization axis. 
     JP-11015074 A (19990122) relates to a projected type color image display device for a liquid crystal projector having a selective reflector in the form of a sheet as an analyzer, which transmits only linearly polarized light components irrespective of incidence angle and reflects orthogonally crossing polarized light components. 
     JP-2000098322 A (20000314) relates to a liquid crystal projector having three liquid crystal panels to form an image of each color, with a dichroic prism including a quarter wave plate disposed after a lens on its output side. 
     JP-2000075246 A (20000314) relates to a projection-type display device, having a polarized plate arranged between a polarized beamsplitter and projection lens as an analyzer, so that reflected polarized light incident on the beamsplitter is absorbed. 
     JP-11352478 A (19991224) relates to a color light separator assembly in a liquid crystal projector for color image formation, having a polarizing plate to reflect blue light toward a prism having a quarter wavelength sheet on various surfaces, including the output surface facing toward the projection lens. 
     Image projection systems often provide a liquid crystal spatial light modulator (SLM), which requires illumination with a high intensity polarized light source. It is desirable to maintain a high contrast ratio, and internal reflections from the high power light source typically lead to stray light, which results in loss of contrast. When the internal reflections or stray light form an image in a focal plane of the system, ghost images often result. Although the lens elements of the projection lens are provided with anti-reflective coatings, and thus reflect but a very small percentage of light impinging thereon, even this small percentage of reflected light from the high brightness source is sufficient to be reflected by the OFF state of the SLM back onto the projection screen. This causes ghosts, which are, for example, unwanted weaker replications, which may be transformed, of the modulated image pattern, and are most noticeable in black areas of the screen when only a portion of the screen is white. The unwanted ghost is that portion of light retroreflecting from the air glass interface of a lens element, which then forms a focused image on the SLM surface. This image is reflected by the liquid crystal light valve through the lens and continues onto the screen. The problem is related to the lens element surface of all lenses that form images back onto the SLM, which then are imaged by the projection lens onto the screen. Even those surfaces that do not image may still result in stray light leading to loss of image contrast. 
     Accordingly, it is an object of the present invention to provide an image projection system reflective liquid crystal light valve projection system which avoids or minimizes above mentioned problems. 
     In is known to reduce internal reflections in image projection systems by changing the polarization state of such light so as to prevent it from being re-transmitted back through the polarizing beam splitter of the system. More specifically, a quarter wave plate may be positioned between the polarizing beam splitter and the projection lens, whereby light transmitted from the polarizing beam splitter to the lens obtains a circular polarization by the quarter wave plate, and light reflected from the lens back toward the polarizing beam splitter has a circular polarization of opposite handedness, resulting in light having an opposite linear polarization from the incident beam. The light then passes again through the quarter wave plate, resulting in light having a linear polarization with an axis opposite the original beam. This light is reflected away from the light valve by the polarizing beam splitter. 
     FIG. 1 illustrates a prior art liquid crystal light valve projection system, for example described in Ziedler, U.S. Pat. No. 5,268,775, expressly incorporated herein by reference, which is itself of the type generally shown in U.S. Pat. No. 4,343,535 to Bleha, Jr. and U.S. Pat. No. 4,650,286 to Koda, et al. This projection system embodies a high power light source, such as a high brightness arc lamp  10 , emitting unpolarized light that is transmitted through a collimating lens  12  which directs the light beam  14  to a polarizing beam splitter  18 , shown as an embedded version of a MacNeille prism. The polarizing beam splitter  18  includes an input window  28  through which it receives randomly polarized light from arc lamp source  10 . In general, the beam splitter transmits light of one polarization state, such as the “P” polarization state for example, and reflects light of another polarization state, such as the polarization state “S”, for example. Reflected light of S polarization state travels along a reflected beam  32  to a modulated liquid crystal light valve  34 . In one modulation state, the corresponding area of the light valve  34  remains in an off or dark state, and light is retroreflected from the dark part of the light valve  34  back to the polarizing beam splitter with its polarization state unchanged. Because the polarization of the light is unchanged from its original S state, light is again reflected from the beam splitter prism plate and returns to the light source  10 , and not to the projection lens  38 , so that the corresponding projected image regions remain dark. For those areas of the liquid crystal light valve that are modulated in the on state, some or all of the light reflected from such bright areas of the light valve  34  is rotated from S polarization slate to P polarization state. The retroreflected light of the polarization state P is transmitted from the liquid crystal light valve through the polarizing beam splitter  18 , passing through the beam splitter exit window  26  and projection lens  38  to form a bright image portion. 
     FIG. 2 is a simplified showing of the system of FIG. 1, schematically indicating transmission and reflection of light rays of the various polarized states and arranged to illustrate certain aspects of the manner in which a ghost image or decrease in contrast may occur in this reflective liquid crystal light valve projection system. In the simplified illustration of FIG. 2 no attempt is made to show angles of reflection of individual light rays, but arrows designated S or P with suitable subscripts are employed to illustrate the transmission and reflection of light components of polarization states S and P respectively. As schematically illustrated in FIG. 2, the high intensity arc lamp  10  transmits unpolarized light or light of random polarization indicated by the rays S 0  and P 0  to polarizing beam splitter  16 . The latter transmits light of polarization state P 0 , as indicated in the drawing by arrow P 0 , and reflects light of S state polarization, as indicated by the arrows S 0 . Where the S state light S 0  impinges upon a dark area (corresponding to a dark area of the cathode ray tube), such as area  42  of the liquid crystal light valve  34 , it is reflected without change of polarization, as indicated by the S 1  component, which is transmitted to the beam splitter from which it is reflected back toward the arc lamp. Where the light of polarization state S impinges upon a light area  44  of the liquid crystal light valve, such as indicated by S 0 , it is reflected with a polarization state P 1  to the beam splitter. The latter transmits the light component P 1  of polarization state P to lens  38 . This is the light that is intended to be projected by the lens system on to the screen. That is, all of the light reflected with polarization state P from the light areas of the liquid crystal light valve is desirably transmitted through the lens to the screen. However, as mentioned above, a small amount of light impinging upon the surfaces of the lens elements is reflected back toward the beam splitter, as indicated by component P 2 . This light of polarization state P is transmitted back through the beam splitter and may impinge upon various areas of the liquid crystal light valve, depending upon the curvature of the lens and the angle of the various light rays received by the lens. 
     Light of polarization state P reflected from the projection lens, as indicted by arrows P 2 , may impinge upon the dark areas, such as a dark area  46  of the liquid crystal light valve, from which it is retroreflected without change of polarization state as a light component indicated as P 3  in FIG.  2 . The components of retroreflected light P 3 , from the “dark” areas of the liquid crystal light valve, pass through the beam splitter back toward the lens system and are transmitted through the lens system (except for the very small percentage that once more is reflected from the lens element&#39;s surfaces) to the screen where they form a ghost image of the arc lamp and increase intensity of illumination of ideally darker areas on the projection screen, thereby diminishing contrast. 
     The following discussion may aid in understanding the problem presented by lens element reflection. The normal screen image is composed of light that emanates from the arc lamp, illuminates the light valve surface and is reflected selectively through the projection lens and on to the screen. The objectionable ghosts, with which the present invention is concerned, are unwanted reflections of the modulated image at the projection screen, and are most noticeable in black areas of the screen when only a portion of the screen is white. 
     Unlike the classical ghost image, which is the unwanted image of a star like point object, this ghost image is that of an extended source at the screen. The unwanted ghost is that portion of light re-reflecting from the air-glass interface of a lens element&#39;s surface and which then forms a focused image of the back-reflected modulated image on the light valve surface. This image is then reflected by the valve through the lens and continues on to the screen. Even though broad band anti-reflection coatings help in controlling ghost images, the brightness of the arc lamp is so great that even a fraction of a percent of reflection is visibly noticeable at the screen. 
     According Ziedler, U.S. Pat. No. 5,268,775, expressly incorporated herein by reference (hereinafter referred to as “Ziedler”), the problem described above is resolved, as illustrated in FIG. 3, by interposing a quarter wave plate  50  between the beam splitter and the lens  38 . The plate  50  is a broadband one-quarter wave plate with its axis oriented at 45° to the linear polarization axis, to provide a circular polarization of the light upon transmission. It is active upon a broadband of visible frequencies. Light of P state polarization, indicated by components P 1  are retroreflections, with changed polarization state, of the S polarization state light S 0  impinging upon light areas  44  of the liquid crystal light valve  34 . These components P 1 , as in the arrangement of FIG. 2, pass through the beam splitter and thence through the quarter wave plate  50  (FIG.  3 ). The quarter wave plate converts the P polarization state light to a circular polarization state, to provide light components designated in the drawing of FIG. 3 as C 1 . A small portion of light of the polarization state C 1  then is reflected from the rear surfaces of elements of lens  38 , in a manner similar to that previously described, an opposite circular polarization. This is indicated in the drawing of FIG. 3 as component −C 1 . The component −C 1  reflected from the lens passes through the quarter wave plate  50  a second time, and converted to linear polarized light having a 90° polarization shift, which effectively changes the polarization state to S, as indicated in the drawing by the component S 2 . Components of polarization state S 2  are not transmitted through the beam splitter but, to the contrary, are reflected away from the liquid crystal valve and out of the system. Thus by changing the polarization state of the light from the liquid crystal light valve after the light passes through the beam splitter, even these faint reflections from the surfaces of the lens elements are prevented from reaching the liquid crystal light valve. The polarization state of light reflected from the lens is changed so that it cannot be transmitted through the beam splitter. Accordingly, this source of ghost image and contrast diminution is eliminated. 
     As stated above, the contrast of the known optical system is improved by quarter wave plate incorporating a thin birefringent layer or layers placed between the projection lens and the polarizing beam splitter 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 system toward the lens and then reflected back through the film a second time will have its polarization direction effectively rotated by 90° and thus will be removed from the optical system by the polarizing cube. 
     FIG. 4 shows an alternate prior art system including a polarizing beamsplitter (PBS) prism. In this system, discussed further below, the projection lens optics generate reflections which are imaged on the surface of the image panel, and thus are further projected in like manner to the intended image information. These reflected images are seen as ghost images on the projection screen. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a quarter wave foil is inserted into the projection path before the projection lens. The axis of the foil is at 45 degrees to the, projected polarization axis. The quarter wave foil along with the PBS and optional post polarizer, serve to prevent reflections from the projection lens from reaching the RLCD panel. This eliminates a source of ghost images and contrast limiting back reflections. 
     Ziedler differs from the present invention in that, while Ziedler seeks to block reflections from the projection lens, it does not abate reflections from the quarter wave plate itself, and these would be produced, to some extent, despite addition of an antireflective coating. Since the quarter wave plate is not in the focal plane, these reflections are seen as a loss of contrast and increase in black level. 
     The data provided in Ziedler clearly infer that, indeed, significant reflections remain. For example, Ziedler cites an increase in contrast from 36:1 to 105:1, 43:1 to 110:1, and 38:1 to 54:1. In fact, the data demonstrates between a 1.5:1 and 3:1 improvement in contrast, and therefore leaving approximately between about 33%-66% of stray light. This contrast data, however, does not address ghost images directly, since the stray light is not uniformly present in the projected image, and which ghosts were found to be objectionable in a prototype system lacking the improvements according to the present invention. 
     The present invention therefore proposes that the quarter wave foil be laminated to the face of the prism, or to a linear polarization filter in the optical path. This lamination eliminates a separate air-interface surface, and therefore effectively eliminates reflections that would be generated at such an interface. Therefore, the present invention suppresses the rays reflected from the projection lens system without substantially creating reflections of its own. Thus, the ghosts that were present in the prototype system were essentially eliminated. This goal was achieved without an antireflective coating on the quarter wave foil, and without introducing other apparent artifacts or limitations in the system. 
     It is therefore an object according to the present invention to provide an image projection system having a polarized beamsplitting prism, a reflective imaging device, and a projection device, characterized in that a quarter wave plate is provided between the polarized beamsplitting prism and projection lens in such manner as to suppress reflections from the projection lens from reaching the reflective imaging device while minimizing reflections from its own surfaces reaching the reflective imaging device. 
     Preferably, the quarter wave plate is laminated to the polarizing beamsplitter exit face or to an optional linear polarizer sheet, to avoid an air interface. Preferably, all refractive air interfaces in the system transmitting light are antireflection coated. 
     It is a further object of the invention to provide a method of reducing internal reflections within a PBS image projection system, comprising the steps of laminating a polarizing filter to a face of the PBS with an optical quality adhesive to thereby eliminate a pair of refractive air interfaces. 
    
    
     These and other objects will be apparent from a review of the drawings and detailed description of the preferred embodiments. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art MacNeille prism liquid crystal light valve projection system; 
     FIG. 2 shows a diagram of the system depicted in FIG. 1, showing various polarization states of the light reflected among the system elements; 
     FIG. 3 shows a prior art liquid crystal projection system with a quarter wave polarization plate interposed between the polarizing beam splitter and the projection lens; 
     FIG. 4 shows a prior art polarizing beam splitter prism liquid crystal image projection system; 
     FIG. 5 shows an improved polarizing beam splitter prism liquid crystal image projection system according to the present invention having a quarter wave foil and optional linear polarizer sheet located between the polarizing beam splitter prism and the projection lens; and 
     FIG. 6 shows a detailed cross sectional view of a polarizing beamsplitter and polarizing filters laminated together to eliminate excess refractive air interfaces. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A common arrangement for a reflective LCD projection system is shown in FIG.  4 . This system, and other implementation details, are described in more detail in U.S. Pat. No. 5,532,763, Janssen, et al. (Jul. 2, 1996), expressly incorporated herein by reference. Light of one polarization (as shown “p” polarized) transmits through the PBS  105  and illuminates the RL,CD panel  102 . In this case the panel  102  is bright in one area  103  and black  104  in the other. In the bright area  103 , the polarization of the incident light is changed to “s” and is reflected from the PBS  105  to the projection lens  107  and onto the viewing screen  108 . In the dark region  104  of tie picture, the polarization state is unchanged. The reflected p-polarized light passes through the PBS  105  back towards the illumination system  101 . 
     The projection lens  107  typically consists of many elements. Each element reflects some light backwards. Anti-reflection coatings are used to reduce the amount of reflection but there is always a finite reflection. Reflection  125  from a lens surface is shown in FIG.  4 . The reflection  125  maintains the s-polarization. Reflected light from the lens is passed through the PBS  105  back onto the panel  102 . When light strikes a dark region  104 , die panel  102  will reflect the light with no change in polarization state. This light is still s-polarized and will be passed by the PBS  105  back through the projection lens  107  and onto the viewing screen  108 . If the lens  107  surface is nearly confocal to the panel  102 , a reasonably coherent ghost image of the bright area is seen on the opposite side of the image in the black area. For surfaces that are not confocal, background light is generated over the image, raising the black level and degrading intra-scene contrast. 
     The present invention addresses these reflections  125  by adding a quarter-wave foil  110  into the projection path after the PBS  105  and before the projection lens  107 , as shown in FIG.  5 . The post-polarizer  106  passes s-polarized light. The quarter wave foil  110  is inserted after the post-polarizer  106 , and before the projection lens  107 . The axis of the quarter wave foil  110  is inclined 45° relative to the post-polarizer  106  or s-axis. The combination of a polarizer  106  and quarter wave foil  110  serves as an optical isolator, which eliminates reflection  125 . The projected s-polarized light becomes, for example, left hand circular polarized. Light reflected from a projection lens  107  surface becomes right hand circular polarized. The right hand circular polarized light becomes P-polarized (90° shifted) alter passing back through the quarter wave foil  110 . The p-polarized light is then absorbed at the post-polarizer  106 , or in its absence, is transmitted away from the RLCD panel  102  by the PBS  105 . 
     There is effectively no penalty for implementation of this solution. The transmission efficiency of a quarter wave foil  110  is very high, so there is virtually no insertion loss. When laminated to the PBS  105  surface or to a post-polarizer  106 , there are no additional air interface surfaces. The only effect is that the projected light is now circularly polarized rather than linearly polarized. 
     The arrangement according to the present invention has been tested. The prototype system, absent the quarter wave foil, exhibited a strong ghost image mirrored about the optical axis. This ghost was quite noticeable in some scenes and degraded picture quality. By inserting the quarter wave as described herein, this ghost image disappeared. This disappearance of the ghost by insertion of the quarter wave foil from a dark test pattern implies a diminution in reflection intensity of at least one order of magnitude, significantly higher than the overall contrast improvement seen by Ziedler. 
     Alternatives to the arrangement described above are possible. The system may not have a post polarizer  106 . In this case the PBS  105  itself in combination with the quarter wave foil  110  produces the desired effect. The quarter wave foil  110  could be laminated to the exit surface of the PBS  105 . In this case the reflection from the PBS  105  exit surface is also eliminated. The arrangement works equally well if the illumination path is upon reflection and the projected light is transmitted through the PBS  105 . 
     Therefore, it is apparent that the present invention provides an optical isolator to the exit of the polarizing beamsplitter prism, to prevent once reflected light from the projection lens from reaching the imaging device. Multiply reflected light will be greatly attenuated, and is thus of lesser importance. 
     As shown in FIG. 6, the quarter wave foil  110  may be laminated to the polarizing beamsplitter (PBS)  105 . This embodiment eliminates refractive air interfaces, and thus reduces problematic reflections. According to the embodiment of FIG. 6, a linear polarizer sheet  106  is laminated to the PBS  105  using an optical adhesive or pressure-sensitive adhesive (PSA)  118 . This face of the PBS  105  is preferably uncoated, while the remaining external faces are preferably antireflection coated  122 ,  115 ,  113 ,  124 . Further, the quarter wave foil  110  is laminated to the linear polarizer sheet  106  using an optical adhesive or PSA  119 . The quarter wave foil  110  is further laminated to an antireflective-coated cover sheet  121 , using an optical adhesive or PSA  120 . Thus, since the PBS  105  exit face is preferably antireflective coated in any case, no additional coated surfaces are required, and no additional refractive air interfaces are present. 
     Normally, a MacNeille PBS  105  has limited efficiency, and thus a small portion of light having an undesired polarization axis exits. In addition, the PBS  105  has angular sensitivity. By providing a linear polarizer  106  within the light exit path, a net improvement in contrast is seen, even without employing a quarter wave foil  110 . By combining this linear polarizer  106  with the quarter wave foil  110 , the reflection  125  is extinguished with high efficiency. 
     The method for laminating polarizer to the PBS comprises the steps of providing a PBS  105  or PBS-half prism  117 , with an uncoated face, providing an optical adhesive or optical-grade pressure sensitive adhesive  118  either on the polarizer  106  or on the PBS  105  surface, and adhering the polarizer sheet  106  to the PBS  105 . The PBS is constructed in known manner from two prism halves,  117 ,  126  having an intervening coating  114  on one of the halves  117 ,  126 . These steps may be performed multiple limes to create a stack of optical elements  105 ,  106 ,  110 ,  121 . In order to provide an antireflective optical coating  122  on the ultimate refractive air interface, the final sheet may be antireflective coated, or a cover sheet  121  provided as one of the layers in the laminate, having an optical antireflective coating  122 . 
     Thus, illumination  101  from the projection lamp having p-polarization  116  passes through the PBS  105 . On the other hand, s-polarized light is reflected by the coating  114  toward the RLCD panel  102 , which forms a reflected modulation pattern  112 . Since this light  127  has a changed polarization state, it passes directly through the PBS  105  without reflection, and through the layers  106 ,  110 ,  121  on the exit face of the PBS  105 , toward the projection lens  107 . A portion  123  of this light is reflected from surfaces of the projection lens  107 , while the remainder  108  is directed toward the projection screen. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.