Abstract:
A projection system includes an image-forming device and a polarizing beamsplitter. Illumination light illuminates the image-forming device via the first polarizing beamsplitter. A retardation element is disposed between the image-forming device and the polarizing beamsplitter. A bias controller applies a bias to pixels of the image-forming device in the dark state so as to substantially maximize contrast in image light reflected from image-forming device. In some arrangements, the volume between the polarizing beamsplitter and the image-forming device is sealed, with the retarding element being disposed within the sealed volume and being attached to either the polarizing beamsplitter or the image-forming device.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to image projection systems. More specifically, the invention relates to projection systems that use liquid crystal imaging panels for generating the image.  
       BACKGROUND  
       [0002]     Many image projection systems, such as might be used for projection televisions, are based on the use of liquid crystal display (LCD) imager panels. Some LCD panels operate in a reflective mode, in which incident illumination light is separated from reflected image light by using a polarizing beamsplitter in front of the reflective LCD panel. In such configurations, the illumination light is passed to the LCD panel via the polarization beamsplitter. The illumination light incident at the LCD panel is, therefore, polarized. The LCD panel operates by selectively adjusting the polarization modulation of the many pixels of the panel. Those pixels associated with dark areas of the image do not alter the polarization state of the light whereas those pixels associated with bright areas of the image do alter the polarization state of the light. When the illumination light is reflected to the LCD panel by the polarization beamsplitter as reflected light, that light which has polarization that has been changed to the polarization state that is orthogonal to the polarization state of the incident light is transmitted through the polarization beamsplitter. Ideally, only the light corresponding to pixels that actively modulate the incident light is transmitted through the polarization beamsplitter to the projector&#39;s lens system, while light reflected by pixels that correspond to dark areas of the image, i.e. pixels that are not actively modulating the light, is rejected by the polarization beamsplitter. Thus, the beamsplitter can be used to separate the polarization modulated image light from the unmodulated light, which results in an image beam that can be projected.  
         [0003]     An important characteristic of a display is the contrast ratio which, qualitatively, is a measure of how bright the bright pixels are compared to the dark pixels. The contrast ratio, R C , can be quantified as: 
 
 R   C   =I   b   /I   d , 
 
 where I d  and I b  correspond to the amount of reflected light in the dark and bright states respectively. Ideally, the value of I d  is as small as possible, so that the contrast ratio is large. 
 
         [0004]     However, in certain types of LCD image-forming device, such as twisted nematic (TN), super twisted nematic (STN) and vertically aligned nematic (VAN) panels, the liquid crystal manifests a residual birefringence even in the off (non-modulating) state. This residual birefringence increases the amount of light in the dark state, resulting in a reduction in the image contrast ratio.  
         [0005]     While a birefringent retardation plate with the same, but opposite retardation, may be used to compensate for this residual birefringence, a quarter-wave retardation plate, inserted between the polarizing beamsplitter and the LCD panel, can also be used to compensate for the residual birefringence. Furthermore, a quarter-wave retardation plate can also be used to compensate for birefringence in components of the polarizing beamsplitter. A compensating retardation plate is not suitable for compensating birefringence in components of the polarizing beamsplitter, and so the use of a quarter-wave retarding plate is more desirable.  
         [0006]     The quarter wave retardation plate is oriented so that the slow or fast axis is rotated away from being parallel to the polarization plane of the illumination light by a few degrees. One disadvantage of this compensation technique, however, is the very careful alignment required to achieve the optimum orientation of the quarter wave retarder. This alignment step increases the costs of producing a projection system.  
       SUMMARY OF THE INVENTION  
       [0007]     One embodiment of the invention is directed to a projection system that includes a first image-forming device and a first polarizing beamsplitter. Illumination light passes via the first polarizing beamsplifter to the first image-forming device. A first retardation element is disposed between the first image-forming device and the first polarizing beamsplitter. A bias controller is attached to the first image-forming device and applies a bias to pixels in the dark state so as to substantially maximize contrast in image light that has passed through the first polarizing beamsplitter from the first image-forming device.  
         [0008]     Another embodiment of the invention is directed to a method of operating a projection system. The method includes illuminating an image-forming device with illumination light that has passed through a polarizing beamsplitter and through a retarding element disposed between the image-forming device and the polarizing beamsplitter. At least some of the illumination light is reflected as image light. The image light is substantially separated from non-image light using the polarizing beamsplitter. A compensating bias signal is applied to pixels of the image-forming device so as to substantially minimize dark state brightness of the image light.  
         [0009]     Another embodiment of the invention is directed to a projection device. The device includes a first sealed unit that has a first polarizing beamsplitter separated from a first image-forming device. A first seal connects between the polarizing beamsplitter and the image-forming device to enclose a sealed volume enclosed by the seal, the first polarizing beamsplitter and the first image-forming device. A retarding element is disposed within the sealed volume and is attached to one of the first polarizing beamsplitter and the first image-forming device.  
         [0010]     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The following figures and detailed description more particularly exemplify these embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0012]      FIG. 1  schematically illustrates an embodiment of a projection system that compensates for residual birefringence in the image-forming devices according to principles of the present invention;  
         [0013]      FIGS. 2A and 2B  schematically illustrate the effects of residual birefringence in an image-forming device;  
         [0014]      FIGS. 3A-3C  schematically illustrate the effects of residual birefringence with a compensating retarding element;  
         [0015]      FIGS. 4A and 4B  schematically illustrate the effects of residual birefringence with a compensating retarding element and a bias applied to the image-forming device, according to principles of the present invention;  
         [0016]      FIGS. 5A and 5B  schematically illustrate embodiments of sealed imaging units according to principles of the present invention; and  
         [0017]      FIG. 6  shows a graph of dark state brightness as a function of applied bias, for various orientation angles of the retarding element. 
     
    
       [0018]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0019]     The invention may be used in many different types of projection system. One exemplary embodiment of a multi-panel projection system  100  that may incorporate the invention described below is schematically illustrated in  FIG. 1 . The projection system  100  is a three-panel projection system, having a light source  102  that generates a light beam  104 , containing light in three different color bands. The light beam  104  is split by color splitting elements  106  for example, dichroic mirrors, into first, second and third beams  104   a ,  104   b  and  104   c  containing light of different colors. The beams  104   a ,  104   b  and  104   c  may be, for example, red, green and blue in color respectively. Beam steering elements  108 , for example mirrors or prisms, may be used to steer any of the beams  104 ,  104   a ,  104   b  and  104   c.    
         [0020]     The beams  104   a ,  104   b  and  104   c  are directed to respective image forming devices  110   a ,  110   b  and  110   c  which may be, for example, LCD-based reflective image-forming panels, such as liquid crystal on silicon (LCoS) panels. The light beams  104   a ,  104   b  and  104   c  are coupled to and from the respective image-forming devices  110   a ,  110   b  and  110   c  via respective polarizing beamsplitters (PBSs)  112   a ,  112   b  and  112   c . The image-forming devices  110   a ,  110   b  and  110   c  polarization modulate the incident light beams  104   a ,  104   b  and  104   c  so that the respective image beams  114   a ,  114   b  and  114   c  are separated by the PBSs  112   a ,  112   b  and  112   c  and pass to the color combiner unit  116 . In the illustrated exemplary embodiment, the illumination light beams  104   a ,  104   b  and  104   c  are reflected by the PBSs  112   a ,  112   b  and  112   c  to the image-forming devices  110   a ,  110   b  and  110   c  and the resulting image light beams  114   a ,  114   b  and  114   c  are transmitted through the PBSs  112   a ,  112   b  and  112   c . In another approach, not illustrated, the illumination light may be transmitted through the PBSs to the image-forming devices, while the image light is reflected by the PBSs.  
         [0021]     Retardation elements  111   a ,  111   b ,  111   c , for example quarter-wave retardation elements, are positioned between the image-forming devices  110   a ,  110   b ,  110   c , and their respective PBSs  112   a ,  112   b ,  112   c . The retardation elements  111   a ,  111   b ,  111   c  may be used for compensating for residual birefringence in the image forming devices  110   a ,  110   b ,  110   c  and, as is explained in greater detail below, for compensating birefringence in the PBSs  112   a ,  112   b ,  112   c . If the PBSs  112   a ,  112   b ,  112   c  are of the MacNeille type, then the retardation elements  111   a ,  111   b ,  111   c  may be used for compensating for skew ray effects as well as residual birefringence in the image forming devices.  
         [0022]     In the illustrated exemplary embodiment, the color combiner unit  116  combines image light beams  114   a ,  114   b  and  114   c  of different colors, for example using one or more dichroic elements. In particular, the illustrated exemplary embodiment shows an x-cube color combiner, but other types of combiner may be used. The three image beams  114   a ,  114   b  and  114   c  are combined in the color combiner unit  116  to produce a single, colored image beam  118  that may be directed by a projection lens system  120  to a screen (not shown).  
         [0023]     A controller  130  is connected to the three image forming devices  110   a ,  110   b ,  110   c . The controller  130  applies control signals to the image forming devices  110   a ,  110   b ,  110   c  that controls the image formed by each image forming device. Also, for reasons that are described further below, the controller  130  applies respective bias signals to each of the image forming devices  110   a ,  110   b ,  110   c  for maximizing the contrast in the respective image light beams  114   a ,  114   b ,  114   c . The controller may be coupled to a video source, such as a computer or television tuner, to receive a video signal. The video signal is processed to produce the control signals that are directed to each of the image forming devices  110   a ,  110   b ,  110   c.    
         [0024]     Other embodiments of projection systems may use one or more PBSs. For example, a projection system may use one or two image-forming devices, with respective PBSs, as is described in greater detail in U.S. patent application Ser. Nos. 10/439449 and 10/914,596, incorporated herein by reference. The maximum number of image-forming devices is not limited to three, and projection systems may use more or fewer than three image-forming devices. In addition, different types of light sources may be used, including white light sources, such as high-pressure mercury lamps, and colored light sources, such as light emitting diodes. The intention with the illustrated embodiment is not to limit how the illumination light reaching the image forming devices is generated, nor to limit how the light is processed before reaching the image forming devices.  
         [0025]     Some types of liquid crystal image-forming devices, such as the vertically aligned nematic (VAN) mode LCD devices, have of the order of 5 nm of residual retardation in the dark state. This can be compensated with a retarder that provides at most 5 nm retardation, oriented at 90° to the orientation of the residual retardation of the image-forming device. There are advantages, however, to using a quarter-wave retarder for compensating the residual birefringence. For example, a quarter-wave retarder is effective at compensating for stress birefringence in the glass prism of a polarizing beamsplitter, e.g. see U.S. patent application Ser. No. 11/088,153, incorporated by reference, and, in the case of the PBS being a MacNeille PBS, for compensating for skew ray birefringence.  
         [0026]     The use of an applied bias to control the residual birefringence of the image-forming device is now described with reference to  FIGS. 2-4 . An arbitrary co-ordinate system has been used in these figures to simplify the following explanation. The x-z plane lies parallel to the plane of the figure while the y-direction lies in a direction into the figure.  FIGS. 2A and 2B  illustrate a situation without a retardation element between the polarizing beamsplitter  202  and the image-forming device  204 . In  FIG. 2A , light  206  propagating in the z-direction from the polarizing beamsplitter  202  is assumed to linearly polarized, i.e. x-polarized, as illustrated by the double-headed arrow perpendicular to the propagation direction of the light  206 . In  FIG. 2B , the light has been reflected by the image-forming device  204  and is now propagating in the backwards direction, the negative z-direction. Since there is some residual birefringence in the image-forming device, the light  210  is no longer linearly polarized, but contains a combination of polarized components, i.e. the light contains a combination of x-polarized and y-polarized components. In the figure, the light is shown as being elliptically polarized, with the y-polarized component being detrimental to the contrast ratio of the projection system. It will be appreciated that in practical systems, it is difficult to obtain completely linearly polarized light propagating out of the polarizing beamsplitter. This description, however, is directed to that light that is polarized parallel to the x-direction.  
         [0027]     In  FIG. 3A , the light  306  propagates from the polarizing beamsplitter  302 , through the retarding element  308 , to the image-forming device  304 . At position A the light  306  is linearly polarized and then passes through the retarder element  308  to become elliptically polarized, at position B. The light  306  is shown having clockwise elliptical polarization. After experiencing the residual birefringence of the image-forming device  304 , the reflected light  310 , shown in  FIG. 3B , is also elliptically polarized, but is now elliptically polarized in the counter-clockwise direction. If the retarding element  308  provides exactly enough retardation to compensate for the residual retardation in the image-forming device  304 , then the light  312  that has passed through the retarding element  308  is linearly polarized before reaching the polarizing beamsplitter  302 . The orientation condition for exact compensation of the residual birefringence is that the retarding element is oriented to an angle, θ, that is equal to θ C , the angle that provides exact compensation. In this situation, the value of θ is critical to achieving high contrast: orienting the retarding element to an angle other than θ C  can result in a significant drop in contrast. For example, in some LCD projection systems, the contrast can drop by 30% or more when the value of θ is about ±0.5° from the value of θ C . For example, tests for this effect in a commercially available JVC G1000 projector, supplied by JVC Corp., Yokohama, Japan, have shown that contrast falls by 30% when the compensator is rotated by 0.5° from its preset optimum rotation angle. In a projector system that uses a Brillian type 720p VAN mode image-forming device, supplied by Brillian Corp., Tempe Ariz., the contrast falls by 40% when the compensator is rotated from the optimum setting by 0.7°.  
         [0028]     According to an embodiment of the present invention, another approach to compensating for the residual birefringence of the image-forming device is to bias the birefringence of the image-forming device, while also using a retarding element. This alters the overall birefringence of the image-forming device and the bias can easily be adjusted to a value that maximizes the contrast: electrically adjusting the bias is significantly less time consuming than manually adjusting the retarding element, and is easier to maintain at a constant value over time. Thus, the processes of assembling the projection device and maintaining high performance are simplified and the device become more manufacturable.  
         [0029]     An exemplary arrangement for carrying out the invention is schematically illustrated in  FIGS. 4A and 4B . In  FIG. 4A , light  406  propagates towards the image-forming device  404  from the polarizing beamsplitter  402 . The light  406  is assumed to be substantially linearly polarized on exiting from the polarizing beamsplitter  402 . The light  406  passes through the retarding element  408  and becomes elliptically polarized light  410 . The retarding element  408  is oriented to an angle θ that is not equal to θ C . A bias controller  412  applies a bias to the image-forming device  404 . The light  414  reflected from the image-forming device  404  has a polarization state that can be adjusted by adjusting the bias to the image-forming device. The polarization state is adjusted so that the reflected light  416  passing out of the retarding element  408  to the polarizing beamsplitter  402  has a substantially maximized contrast ratio, which corresponds to the light  416  having a polarization state that is substantially linear. This condition occurs when the bias voltage, V b , applied by the controller  412  is substantially equal to V c , the value required for maximum contrast.  
         [0030]     Another way of understanding this compensation technique is to consider that the light experiences three independent sources of birefringence during the round trip between the polarizing beamsplitter and the image-forming device, namely the birefringence from the retarding element, b e , the residual birefringence, b r  and the bias birefringence, b b . The residual birefringence, b r  and the bias birefringence, b b , both increase the retardance seen by the incident polarized light ray. Maximum contrast occurs when the orientation of b e  is such that overall retardation is minimized.  
         [0031]     Since there is always some stray birefringence in the image-forming device,  404 , another way of understanding this compensation technique is to consider that a compensator  408  may be oriented with its optic axis at a non-zero angle, θ C , to the input polarization such that the amount of retardation the polarized light ray will experience in its round trip through the compensator  408  and image forming device  404  is minimized. When the compensator  408  is rotated to an angle θ&gt;θ C , the light ray  416  becomes overcompensated and the light ray  416  traveling back to the PBS  402  contains a larger y-polarization component than is desired. However, the bias applied by the image-forming device  404 , through application of the bias voltage V c , can largely compensate for this added retardation. In this way the projection system may be adjusted to best contrast through an electronic adjustment rather than a mechanical one.  
         [0032]     Using the present compensating technique, the core of a projection system can be aligned using the following steps: 
        i) The quarter-wave retarder is attached to either the image-forming device or the polarizing beamsplitter. It may be desired to attach the quarter-wave retarder using a substantially index matching optical adhesive. This reduces the number of reflecting surfaces, which results in lower losses and reduced image ghosting.     ii) The image-forming devices and polarizing beamsplitters are arranged around the color combiner.     iii) The six axes of each image-forming device (three positions, x, y, z, three angles, pitch, roll and yaw) are adjusted so that each is in focus, oriented properly in the horizontal and vertical directions and centered in the optical system and with the other image-forming devices     iv) The dark state bias voltage, V b , of each image-forming device is adjusted for the lowest light output from the color channel controlled by that image-forming device.        
 
         [0037]     In addition to reducing the complexity of the manufacturing process and reducing the number of reflecting surfaces, this technique also permits a configuration that reduces the amount of dust or other unwanted particles settling on optical surfaces, particularly the image-forming device. Dust on the image-forming device can strongly affect both the dark and bright states of the image-forming device. In the bright state, the dust shows up as a more-or-less focused spot of the complementary color to the color channel of the image-forming device. For example, for an image-forming device in the green channel of a three-channel projection system, the dust particle produces a magenta spot. In the dark state, the dust appears as a spot of the same color as the color channel. For example, for an image-forming device in the green channel of a three-channel projection system, the dust particle produces a green spot. It has been found to be complicated and often expensive to develop attachment methods for the image-forming devices and compensators that produce a dust-tight seal enclosing the faces of the polarizing beamsplitter, the image-forming device and both sides of a free-standing compensating retarder. By attaching the compensating retarder to either the image-forming device to the polarizing beamsplitter, the problem is significantly simplified: there is now only one gap to seal, rather than two, and the reduced number of attachment points facilitates simple sealing structures. These two possibilities are schematically illustrated in  FIGS. 5A and 5B . In each case, a seal  508  is formed between the polarizing beamsplitter  502  and the image-forming device  504  for excluding dust from the optical surfaces. In  FIG. 5A  the retarding element  506  is attached to the polarizing beamsplitter  502  and in  FIG. 5B  the retarding element  506  is attached to the image-forming device  504 . The compensating technique described above in which a bias is applied to the image-forming device  504  permits the attachment of the retarding element  506  to either the beamsplitter  502  or the image-forming device  504  to be made simply, without high precision orientation of the retarding element  506  being needed to achieve an orientation angle of exactly θ c .  
         [0038]     It will, of course, be appreciated, that the retarding element  506  need not be attached to either the polarizing beamsplitter  502  or the image-forming device  504 , but may be positioned between the polarizing beamsplitter  502  or the image-forming device  504  and supported using some other support structure.  
       EXAMPLE  
       [0039]     The technique was tested on a type 720p VAN mode liquid crystal on silicon (LCoS) image-forming device, supplied by Brillian Corp, Tempe, Ariz. A quarter-wave retarder on a precision rotation stage was positioned between a polarizing beamsplitter and the image-forming device. The system was illuminated with green light from an arc lamp at f/2.3. The illumination light was reflected by the polarizing beamsplitter to the image-forming device. A projection lens was mounted to project the light reflected by the image-forming device that was transmitted by the polarizing beamsplitter. A light meter was placed 10 cm in front of the projection lens to measure the projected light flux. The angle of rotation of the quarter-wave retarder required to compensate this imager optimally is around 0.25°, corresponding to a residual birefringence of approximately 0.75 nm.  
         [0040]     The retarder was aligned so that an optimally dim dark state was achieved with the image-forming device set to a greyscale of zero (GS=0), at which point the light level was measured as 7.5 lux. The angle for this condition is designated as θ=0°. When the retarder was rotated through an angle of 0.125°, the dark state brightness increased to 8 lux, corresponding to a contrast degradation of around 6.5%.  
         [0041]     Next, the voltage level applied to the image-forming device was adjusted to obtain the lowest possible brightness when the retarder was oriented at θ=0.125°. The bias voltage is measured in terms of the greyscale level for a Gamma of 2.0, for the image-forming device with the quarter-wave retarder as originally oriented at θ=0°. This set of grey scales numbered  256 , corresponding to 8-bit color. This was done for actual convenience. In a practical system, the bias voltage may be introduced as equal to the voltage level corresponding to the grey scales referenced. For this rotation of the retarder, at 0.125°, the dimmest dark state was measured at the 20 th  grey scale, at a level of 7.7 lux. This is not quite as dark as the dark value achieved at θ=0°, but is 60% better than without the bias voltage, and results in a reduction in the contrast from the optimal value by only 2.5%.  
         [0042]     Practical considerations of angular tolerance determine the size of the offset. For example, if the polarizing beamsplitter orientation is controlled to within ±0.1° and the axis of the retarder is controlled to within ±0.05°, then it becomes desirable to place the retarder in the system at a minimum offset angle of 0.3°.  
         [0043]     In order to test other angular placements, the effects of offsetting the retarder by 0.375° were explored. In this case, the GS=0 dark state (zero bias) condition resulted in a measured light output of 11.5 lux, reducing the contrast by 50% from that obtained with θ=0°. Adjusting the bias resulted in a minimum dark state light intensity of 8.3 lux, obtained at a grey scale level of about 47. Thus, 80% of the reduction in contrast was recovered, and the achieved contrast was only about 10% lower than with θ=0°. Since the overall contrast level of such image-forming devices has been measured to exceed 3000:1, a 10% reduction still results in a contrast ratio of 2700:1, which is still acceptable for many commercial projection systems.  
         [0044]     A graph shown the brightness of the dark state is shown in  FIG. 6  for various orientations of the retarder, as a function of the bias voltage applied to the image-forming device. The bias voltage is presented in terms of the greyscale. It can be seen that rotation of the retarder in the negative direction results in a brighter GS=0 dark state, and that in this case the application of the bias voltage only increases the brightness.  
         [0045]     All angles described above are relative to the best angle for orientation of the retarder, i.e. that orientation that produces the least bright dark state when the image-forming device is unbiased. For the particular image-forming device used here, this orientation corresponds to the fast or slow axis of the retarder being set at an angle to 0.247°±0.017° (one standard deviation) relative to the polarization axis of the incident light. Thus, the retarder may be oriented so that the fast or slow axis of the retarder is set at about 0.25°+0.375°=0.625°, although other angles may be selected, depending on the manufacturing tolerances of the components used.  
         [0046]     For other VAN mode image-forming devices, for example having an unbiased residual birefringence of around 5 nm, the fast or slow axis of the retarder is set to about 1.75° for relative to the polarization plane for θ=0°, i.e. θ c =1.75°. Thus, the slow or fast axis of a retarder used with these image-forming devices may be set at an angle of about 1.75°+0.375°=2.125°, although other angles may be selected.  
         [0047]     In different embodiments, the fast or slow axis of the retarder may be set at an angle whose value differs from the value of θ c  by less than 1°, less than 0.5° or less than 0.25°.  
         [0048]     Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.