Liquid crystal (LC) technology has been applied in projection displays for use in projection televisions, computer monitors, point of sale displays, and electronic cinema to mention a few applications. A more recent application of LC devices is the reflective LC display on a silicon substrate, forming a liquid crystal on silicon (LCOS) structure. Silicon-based reflective LC displays often include an active matrix array of complementary metal-oxide-semiconductor (CMOS) transistors/switches that are used to selectively rotate the axes of the liquid crystal molecules. As is well known, by application of a voltage across the LC cell, the direction of the LC molecules can be controlled and the state of polarization of the reflected light is selectively changed. As such, by selective switching of the transistors in the array, the LC medium can be used to modulate the light with image information. This modulated light can then be imaged on a screen by polarization and projection optics thereby forming the image or 'picture.' In many LCD systems, the light from a source is selectively polarized in a particular orientation prior to being incident on the LC layer. The LC layer may have a voltage selectively applied to orient the molecules of the material in a certain manner. The polarization of the light that is incident on the LC layer is then selectively altered upon its traversing the LC layer. The LCD system often includes a polarization beam splitter (PBS). The light that is reflected by the LCD and that has a certain polarization state will be reflected from or transmitted through the PBS, depending on the structure of the system, and is then incident on the projection optics. This light forms the bright-state pixels of the image. Contrastingly, light of a polarization that is orthogonal to the polarization state of the bright state pixels is prevented from reaching the projection optics, and becomes the dark state light. In this manner, an image may be formed from a plurality of bright and dark state pixels, where a scrolling image may be formed by scrolling the input signals to the matrix of transistors in the LCD. Beneficially, the LC device (the 'device') behaves like a mirror. That is, in the "dark" state the polarization of the reflected light is unchanged and will hence be redirected towards the light source. In practice, however the LC device has a residual retardance in both magnitude and orientation. The residual retardance of the device changes the polarization state and hence a fraction of the light is directed towards the projection optics deleteriously impacting the contrast. Accordingly, the undesired residual retardance should be cancelled. Thus, the contrast is of the projected image may be compromised by the residual retardance of the device. Ultimately, the dark state pixels on the screen may not be sufficiently absent of light to provide a high contrast/high quality image. However, these compensators, which are to mitigate the effects of the residual retardance, may be non-uniform in retardance across their useful area and thus the phase compensation is incomplete. This results in regions of 'lighter' pixels across an imaging surface. Of course, this is deleterious to the image contrast and quality. Furthermore, compensators are often comprised of a layer of optically birefringent material, such as a stretched polymer material, sandwiched between two layers of glass. In the fabrication process, contaminants such as particulates may be trapped between the retarder material and the glass. Being in close proximity to the device, these contaminants interfere with the projection of light onto pixels of the device, and are thus image at a corresponding location of the imaging screen. Accordingly, known compensation techniques to address residual retardance in LCD projection devices have certain drawbacks and shortcomings that make them less than desirable. In accordance with an example embodiment, a light projection system includes an LC device, a polarization beamsplitter and a projection optic. The projection system also includes a compensator located adjacent to the projection optic, and the compensator has a position-dependent retardance across an area of the compensator that transforms a polarization state of light at each location of the area. In accordance with another example embodiment, a method of compensating residual retardance in a light projection system to provide dark state light includes providing a compensator adjacent to an illumination optic. The compensator has a retardance and orientation that is dependent on a position over an area of the compensator. The method also includes altering a polarization state of light incident on the compensator so as to prevent the dark state light from reaching the projection optic. The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig. 1 is a schematic view of an LCD projection display device in accordance with an example embodiment. Fig. 2a is a pupil compensator, which provides varying degrees of optical retardance and different slow/fast axis orientation used to provide dark state light to the projection optics of an LCD projection system in accordance with an example embodiment. Figs. 2b and 2c show different grating structures that may be used in a pupil compensator in accordance with an example embodiment.

In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the example embodiments. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention. Finally, wherever practical, like reference numerals refer to like features. Fig. 1 shows an LCD projection display device (LCD display) 100 in accordance with an example embodiment. It is noted that the LCD projection display device 100 may be incorporated into a variety of types of projection display systems, illustratively a front projection display device. The LCD display 100 includes an illumination system (or illumination unit) 101 , which include a gas discharge light source (e.g., a high pressure mercury lamp, a noble gas arc lamp or metal halide lamp), an LED array or other known light source for emitting light comprising red, green and blue (R, G, B) light that is used in display devices. Moreover, the illumination system 101 may include a parabolic mirror, an illumination optic or similar device. Light from the illumination system is incident on a PBS 102, which reflects light of a particular polarization state (p-state), and transmits light of another polarization state. This selective transmission and reflection selectively provides the suitable dark state and bright state light to form the needed contrast in an image; and selectively provides the colors required to form an image. As the principles and function of the PBS 102 are well- known in the image projection art, further details are not described so as to not obscure the description of the example embodiments. As described previously, this polarization- dependent selection by the PBS 102 is less than perfect, and some residual geometric depolarization from a glass MacNeille PBS, a common PBS, can deleteriously impact the contrast and quality of the image at the screen. While a skew angle compensator may be used to address the residual retardance geometric depolarization of the PBS, in an example embodiment (e.g., Fig.1) a wire-grid PBS is may be used, and does not require skew angle compensation. As will become clearer as the present description continues, example embodiments reduce the residual retardance and thus improve the image quality and contrast across the screen, In the example embodiment shown in Fig. 1, three point sources at the illumination unit 101 are depicted as light cones 1 13, 114,1 15. The light cones 1 13-1 15 traverse the PBS 102. The solid lines of cone 1 14 represent telecentric collimated light at the device 104; whereas the dotted (1 13) and dashed (1 15) lines represent light that is incident on the device 104 at angles other than parallel to the normal to the device 104. As such, the solid lines of cone 114 are reflected and focus at a center focal 1 10 point. It is noted that the device 104 may be one of a variety liquid crystal devices, such as an LCoS device. It is noted, however, that other types of reflective retarders, commonly used in light valve applications may be used as the light valve device 104. The illumination optics (not shown) forms a non-telecentric pupil image 101. This light traverses the PBS and is converted into telecentric illumination by field lens 103. Hence each point on the display is illuminated by a cone of light whose central (principal) ray is parallel to the display 104 normal. Upon reflection from the device 104 the field lens 103 forms an image at a pupil 105. Light is incident on the device 104 and some of the light is reflected toward the projection optic as shown. The light undergoes a polarization transformation upon reflection from the device, this light and is then partially reflected by the PBS 102 towards to the pupil 105, where a compensator 106 in accordance with an example embodiment is shown. As mentioned previously, the light valve, like the PBS, may have a residual retardance that must be compensated to improve the contrast and image quality in general. As will become clearer as the present description continues, the compensator 106 significantly compensates for or substantially eliminates the affects of the residual retardance from the device 104 and the PBS 102. In the bright state, the device 104 transforms the polarization such that upon reflection from the device; the PBS 102 directs most of the light towards the projection optics 108. Light that is incident on the compensator 106 is selectively compensated by the compensator 106 so that its polarization state is either substantially orthogonal or substantially parallel to the polarizer 107 transmission axis. As can be readily appreciated, the former results in dark state light and the latter results in bright state light at the image screen. Accordingly, the bright state light (not shown) traverses the polarizer 107 and is projected onto the image screen by the projection optic 108. Conversely, the dark state light is absorbed by the polarizer 107 back into the system 100. As referenced previously, the device 104 often has a residual retardance, which impacts the uniformity brightness level of the darkness across the image screen. Locating the compensator adjacent to the device 104 results in the imaging local area non-uniformity by the device onto the image screen. To this end, in known structures, because the device 104 forms pixels at the imaging screen from the light incident on the device 104 from the compensator adjacent thereto, any non-uniformities in the light from the compensator are mapped to a corresponding location on the imaging screen; and these non-uniformities would be imaged and thus pronounced on the image screen. Similarly, contaminants in the compensator, if located adjacent to the device 104, are mapped from a location on the device to a particular location on the image screen, forming bright images on the image screen. Contrastingly, a point source or focal point on the pupil 105 of the projection optics 108 maps to all locations on the imaging screen. Therefore, by locating the compensator 106 at the pupil 105, each point on the pupil (e.g., focal points 1 10-1 12) maps to all points on the image screen. Thus, any uncompensated residual retardance, which is manifest as non-uniformity in the contrast (or dark state), can be 'spread out' over the image screen. Likewise, in accordance with example embodiments, any images of contaminants are spread over the screen. Thus, instead of being projected bright spots as in the known art, the image of the contaminants is diffused over the image screen. Accordingly, it is exceedingly beneficial to provide the compensator 106 at the pupil 105 as shown in Fig. 1. It is noted that pursuant to the example embodiments, the reflection of dark state light may be used to form a particular color. To wit, if a color at a pixel(s) at the image screen requires no red light, the device 104 would alter the polarization state of this device so it is absorbed extinguished by the combined action of PBS rejection 102 and polarizer 107 absorption. Of course, in most scanning projection device, this is not needed. In addition to the placement of the compensator 106 at the location of the pupil 105 of the system 100, according to an example embodiment the compensator 106 provides selective retardance that is spatially dependent across the pupil. To this end, and as will become clearer as the present description continues, the light from cone 1 14 that is focused at the center of the pupil 105 requires little, if any compensation for residual retardance from the device 104. (It is noted that the zero compensation at the pupil center is for the LC mode 90TN0 of one example embodiment of Fig. 1. In general, other LC modes will require compensation even at the pupil center location). This is directly related to the zero angle of incidence at the device 104. However, the light from cones 1 13 and 1 15 have a finite angle of incidence at the device 104 are not telecentric, and as is well known, there is a dependence of the phase change (retardance) value imparted by the LC and as a function of the angle of incidence. To wit, because the light from cones 1 13 and 1 15 are incident and not telecentric it undergoes a phase change due to the device that differs from the phase change imparted to the required to provide dark state light of cone 1 14 (in the present embodiment) or bright state light. This light focuses at the pupil 105 at off-center foci of 1 1 1 and 1 12 in the present example; and with respective polarization states that will not be fully absorbed or extinguished by the polarizer 107. Accordingly, without the position dependent polarization transformation provided by the compensator 106, the light from this residual retardance will traverse the polarizer 107 and adversely impact the contrast uniformity, if not other aspects of the image quality. Finally, it is noted that in other types of some non-LC-based light valves (other than LC-based devices) the phase change (retardance) value imparted by the light valve device also depends on the angle of incidence. Hence, such devices may benefit from the methods and apparati of the example embodiments. In accordance with an example embodiment, the compensator 106 has a retardance that varies in magnitude and orientation across the area of the compensator 106. To this end, because of the angular dependence of the device residual retardance and the angle of incidence of light, and the non-uniform retardance across the device 104 as well as other factors, the light incident at non-telecentric finite angles of incidence light will focus at the pupil 105 with a less than desired polarization state. Thus, at each location in the plane of the pupil, there is a unique polarization state associated with the corresponding ray angle in the illumination cone at the panel. In accordance with example embodiments, the compensator 106 has a spatially varying fast axis orientation and a spatially varying retardance across its area in the plane of the pupil. This spatial variation in retardance and orientation provides a retarder that has a magnitude and orientation at each point of the compensator that will transform the "dark- state" polarization state at that point to a state that is substantially absorbed by the polarizer 107. For example, suppose the polarization state of the light at point 1 12 is a vector sum of two linear polarization states and the polarizer blocks only one of these states. In this case, the linear state that will be absorbed by the analyzer 107 is transformed by the compensator to the other linear state that is so absorbed. In furtherance to this example, if the light at a particular point on the pupil is a vector sum of S-polarized light and P-polarized light, and the polarizer 107 transmits S-polarized light, the orientation of the fast axis and retardance at point 1 12 will transform the S/P polarized light at point 1 12 to P-polarized light. Likewise, because the light incident at the point 1 10 is collimated at the device, the compensator 106 will have no effect on the light at this point. Again, it is noted that the present illustrative example pertains to the 90TN0 LC mode, which does not required compensation for "collimated light" at the device. Rather this LC mode only requires compensation for off-axis light. However, it is noted that there are many other LC modes that require compensation even for "collimated" light. In an example embodiment, the spatial variation of the compensator 106 for a particular system 100 may be determined by measuring the spatial variation of the polarization of the light across the pupil 105. Next, a retarder having a spatial variation of retardance and fast axis orientation that at every point on the pupil negates the polarization component that will not be absorbed by the polarizer 107. It is noted that in accordance with another example embodiment, a compensator 1 16 may be disposed adjacent to the device 104. If this compensator fully compensates the device, the compensator 106 may be used to compensate for any geometrical phase defects due to the PBS 102, which have not been compensated. To this end, the device may be partially compensated for by the compensator 116, and full compensation is achieved by the combination of compensators 1 16 and 106. For example, the 45TN0 LC mode requires compensation for both "collimated" and "off-axis" light; and compensator 116 could perform the "collimated" compensation and compensator 106 the "off-axis" compensation. Alternatively, the roles of the compensators could be switched. It is noted that the example embodiment that includes the 90TN0 LC mode only requires an "off-axis" compensator. In the example embodiment described in connection with Fig. 1, the polarization state at each point in the pupil 105 is measured and the spatial variation of the retardance and fast axis required for the retarder is determined. To wit, as described previously the spatial variation of the retardance and fast axes over the compensator are chosen to transform the light incident at each point on the compensator 106 into the polarization state that is absorbed by of the polarizer 107. Fig. 2a is a top view of a compensator 200 in accordance with an example embodiment. As will be appreciated, this compensator 200 may be used as the compensator 106 discussed above. The compensator 200 is useful in a system that incorporates a 90TN0 LCoS mode panel as the light valve (e.g., device 104). It is noted that the present device does not compensate for the non-ideal polarization properties of the PBS of the system. This should be compensated in such a way that its reflection and transmission are substantially uniform across the illumination cone angle. It is further noted that a wire-grid PBS inherently does not produce strong phase shifts as a function of cone angle. Furthermore, a glass-based MacNeille PBS, which may be used in example embodiments, requires a "skew angle" compensator to achieve uniform reflection/transmission characteristics. Normally, this compensation is effected using quarter wave retarder having a fast axis aligned with the p-polarization axis of the PBS. I llustratively, the compensator 200 may be a C-plate, which is well known in the physical optics arts. A known c-plate consists of a retarder having a fast (or slow) axis that points in a direction orthogonal to the plane of the substrate. The c-plate only induces a phase change for light rays that are "off-axis." The vectors 201 in the Fig. 2a indicate the orientation of the slow axis and magnitude of the retardance of the device across the plane of the compensator 200. As can be appreciated, about the outer regions of the compensator, the magnitude of the retardance is rather large. For example, at the extreme of the pupil (e.g., F/2.4) the retardance required to provide adequate compensation is on the order of lOOnm. It is noted that the compensator 200 may impact both dark state and bright state light. Moreover, the bright state as shown in the diagrams produces mostly s-polarized light after reflection from the device and the PBS. Without the compensator 106 of the example embodiments this light would be wholly transmitted by the analyzer 107. However, the compensator 106 will also transform substantially all of this s-state light into p-state light, which in turn will be absorbed by the analyzer 107 reducing the brightness. In an example embodiment, the compensator 200 is comprised of gratings. For example, these gratings may be sub-wavelength phase grating structures that form birefringent retarders. The retardance values over the plane of the compensator would be adjusted by selectively altering the duty cycle, height or refractive index of the grating. The orientation of the retarder fast axis as a function of position would be defined by the orientation of the grating as a function of position. As such a plurality of gratings may be used to realize a compensator 200 having a spatially dependent fast axis and a spatially dependent retardance. The gratings 202 and 203 shown in Figs. 2b and 2c, respectively, may be subwavelength phase gratings such as described in Formation of Pancharatnum-Berry Phase Optical Elements with Space-Variant Subwavelength Gratings to E. Hasman, et al. (Optics and Photonics News December 2002) p. 45. This article is specifically incorporated in its entirety herein by reference. It is noted that the grating 202 has a 50% duty-cycle and the grating 203 has a 70% duty-cycle. The maximum birefringence is obtained for a duty cycle of approximately 50%, and by increasing or decreasing the duty cycle, the birefringence may be reduced. In an example embodiment sub-wavelength gratings are fabricated by direct e-beam patterning of a photoresist on a substrate of a suitable material. For example, photoresist may be disposed over a glass substrate. A plurality of gratings may be formed over the surface to form a compensator such as compensator 200, which can be used as compensator 106. Each spatial location on the compensator has a retardance magnitude and fast axis orientation dictated by the requirements of the light valve, or PBS, or both. For example, the orientation of the fast axes may be set by the orientation of the grating on the substrate. The magnitude of the retardance is set by fixing one or more of the height of the grating, the duty cycle and the material used. After the grating parameters are determined, a direct e-beam patterning may be effected across the area of the substrate. This may be automated by programming the orientations of the gratings, as well as the duty cycle and the height in an e-beam writer. All of the gratings may be fabricated by stepping across the photoresist. The photoresist in then washed. It is noted that this grating structure may be used as a form for others. For example, nickel shims may be made to write replicate other gratings by known techniques. Finally, it is noted that the use of grating-based compensators is merely illustrative, and other grating technologies may be used to effect the compensator 200. For example, LC retarders could be used to fabricate a spatially dependent compensator. The retarder orientation may be defined by UV phototechniques that define the alignment of the LC material. Thus, the alignment conditions may be patterned across the retarder surface. The example embodiments having been described in detail, it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure. Such modifications and variations are included in the scope of the appended claims.