Abstract:
A reflection mode, ferroelectric liquid crystal spatial light modulating system, includes a light reflecting type spatial light modulator. The spatial light modulator has a light reflecting surface cooperating with a layer of ferroelectric liquid crystal light modulating medium switchable between first and second states so as to act on light in different first and second ways, respectively. A switching arrangement switches the liquid crystal light modulating medium between the first and second states and an illumination arrangement produces a source of light. An optics arrangement is optically coupled the spatial light modulator and the illumination arrangement such that light is directed from the source of light into the spatial light modulator for reflection back out of the modulator and such that reflected light is directed from the spatial light modulator into a predetermined viewing area. The optics arrangement includes a passive quarter wave plate positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. A compensator cell is also positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. The compensator cell has a layer of ferroelectric liquid crystal light modulating medium switchable between a primary and a secondary state so as to act on light in different primary and secondary ways, respectively.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to image generating systems including a reflective type, ferroelectric liquid crystal (FLC) spatial light modulator (SLM). More specifically, the invention relates to an optics arrangement including an FLC compensator cell for allowing the system to generate a substantially continuously viewable image while DC-balancing the FLC material of both the SLM and the compensator cell.  
           [0002]    FLC materials may be used to provide a low voltage, low power reflective spatial light modulator due to their switching stability and their high birefringence. However, a problem with FLC materials, and nematic liquid crystal materials, is that the liquid crystal material may degrade over time if the material is subjected to an unbalanced DC electric field for an extended period of time. In order to prevent this degradation, liquid crystal spatial light modulators (SLMs) must be DC field-balanced.  
           [0003]    Nematic liquid crystal materials respond to positive or negative voltages in a similar manner regardless of the sign of the voltage. Therefore, nematic liquid crystals are typically switched ON by applying either a positive or negative voltage through the liquid crystal material. Nematic liquid crystal materials are typically switched OFF by not applying any voltage through the material. Because nematic liquid crystal materials respond to voltages of either sign in a similar manner, DC balancing for nematic liquid crystal materials may be accomplished by simply applying an AC signal to create the voltage through the material. The use of an AC signal automatically DC balances the electric field created through the liquid crystal material by regularly reversing the direction of the electric field created through the liquid crystal material at the frequency of the AC signal.  
           [0004]    In the case of FLC materials, the materials are switched to one state (i.e. ON) by applying a particular voltage through the material (i.e. +5 VDC) and switched to the other state (i.e. OFF) by applying a different voltage through the material (i.e. −5 VDC). Because FLC materials respond differently to positive and negative voltages, they cannot be DC-balanced in situations where it is desired to vary the ratio of ON time to OFF time arbitrarily. Therefore, DC field-balancing for FLC SLMs is most often accomplished by displaying a frame of image data for a certain period of time, and then displaying a frame of the inverse image data for an equal period of time in order to obtain an average DC field of zero for each pixel making up the SLMs.  
           [0005]    In the case of an image generating system or display, the image produced by the SLM during the time in which the frame is inverted for purposes of DC field-balancing may not typically be viewed. If the system is viewed during the inverted time without correcting for the inversion of the image, the image would be distorted. In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of the pixels would appear to be half on. In the case in which the image is inverted at a frequency slower than the critical clicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image. Neither of these situations would provide a usable display.  
           [0006]    In one approach to solving this problem, the light source used to illuminated the SLM is switched off or directed away from the SLM during the time when the frame is inverted. This type of system is described in copending U.S. patent application Ser. No. 08-361,775, filed Dec. 22, 1994, entitled DC FIELD-BALANCING TECHNIQUE FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, which is incorporated herein by reference. However, this approach substantially limits the brightness and efficiency of the system. In the case where magnitude of the electric field during the DC field-balancing and the time when the frame is inverted is equal to the magnitude of the electric field and the time when the frame is viewed, only a maximum of 50% of the light from a given light source may be utilized. This is illustrated in FIG. 1 a  which is a timing diagram showing the relationship between the switching on and off of the light source and the switching of the SLM image data.  
           [0007]    As shown in FIG. 1 a , the light source is switched on for a period of time indicated by T 1 . During this time T 1 , the SLM is switched to form a desired image. In order to DC balance the SLM, the SLM is switched to form the inverse of the desired image during a time period T 2 . In order to prevent this inverse image from distorting the desired image, the light source is switched off during the time T 2  as shown in FIG. 1 a.    
           [0008]    In order to establish a convention to be used throughout this description, the operation of a given pixel  10  of a reflective type FLC SLM using the above mentioned approach of switching off the light source during the time the frame is inverted will be described with reference to FIGS. 1 b - d . FIG. 1 b  shows pixel  10  when it is in its bright state and FIG. 1 c  shows pixel  10  when it is in its dark state. As illustrated in both FIGS. 1 b  and  1   c , a light source  12  directs light, indicated by arrow  14 , into a polarizer  16 . Polarizer  16  is arranged to allow, for example, horizontally linearly polorized light, indicated by the reference letter H and by arrow  18 , to pass through polarizer  16 . However, polarizer  16  blocks any vertically linearly polarized component of the light and thereby directs only horizontally linearly polarized light into pixel  10 . This arrangement insures that only horizontally linearly polarized light is used to illuminate pixel  10 . For purposes of clarity throughout this description, the various configurations will be described using horizontally linearly polarized light as the initial input light for each of the various configurations.  
           [0009]    As also illustrated in FIGS. 1 b  and  1   c , pixel  10  includes a reflective backplane  22  and a layer of FLC material  24  which is supported in front of reflective backplane  22  and which acts as the light modulating medium. The various components would typically be positioned adjacent one another, however, for illustrative purposes, the spacing between the various components is provided. In this example, the FLC material has a thickness and a birefringence which cause the material to act as a quarter wave plate for a given wavelength. In this example, the FLC material is typical of those readily available and has a birefringence of 0.142. Therefore a thickness of 900 nm causes the SLM to act as a quarter wave plate for a wavelength of approximately 510 nm.  
           [0010]    FLC material  22  has accompanying alignment layers (not shown) at the surfaces which have a buff axis or alignment axis that controls the alignment of the molecules of the FLC material. For this example of a reflective mode SLM, the SLM is oriented such that the alignment axis is rotated 22.5 degrees relative to the polarization of the horizontally linearly polarized light being directed into the SLM. The FLC also has a tilt angle of 22.5 degrees associated with the average optic axis of the molecules making up the FLC material. Therefore, when FLC material  24  of the pixel is switched to its first state, in this case by applying a +5 VDC electric field across the pixel, the optic axis is rotated to a 45 degree angle relative to the horizontally linearly polarized light. This causes the pixel to act as a quarter wave plate for horizontally linearly polarized light at 510 nm. Alternatively, when the pixel is switched to its second state, in this case by applying a −5 VDC electric field across the pixel, the optic axis is rotated to a zero degree angle relative to the horizontally linearly polarized light. This causes the pixel to have no effect on the horizontally linearly polarized light directed into the pixel. In other words, the tilt angle is the angle that the FLC optic axis is rotated one side or the other of the buff axis when the FLC material is switched to its first and second states.  
           [0011]    Now that the configuration of the pixel for this example has been described, its effect on the light as it passes through the various elements will be described. Initially, it will be assumed the light is monochrome at the wavelength at which the SLM acts as a quarter wave plate, in this case 510 nm. As illustrated in FIG. 1 b , when the FLC material is switched to its first state, which will be referred to hereinafter as its A state, FLC material  24  converts the 510 nm wavelength horizontally linearly polarized light directed into the pixel and indicated by arrow  18  into circularly polarized light indicated by the reference letters C and arrow  26 . Reflective backplane  22  reflects this circularly polarized light as indicated by arrow  28  and directing it back into FLC material  24 . FLC material  24  again acts on the light converting it from circularly polarized light to vertically linearly polarized light as indicated by reference letter V and arrow  30 . The vertically linearly polarized light  30  is directed into an analyzer  32  which is configured to pass vertically linearly polarized light and block horizontally polarized light. Since analyzer  32  is arranged to pass vertically linearly polarized light, this vertically linearly polarized light indicated by arrow  30  passes through analyzer  32  to a viewing area indicated by viewer  34  causing the pixel to appear bright to the viewer.  
           [0012]    Alternatively, as illustrated in FIG. 1 c,  FLC material  24  has no effect on the horizontally linearly polarized light directed into the pixel when the pixel is in its second state, which will be referred to hereinafter as its B state. This is the case regardless of the wavelength of the light. Therefore, the horizontally linearly polarized light passes through FLC material  24  and is reflected by reflective backplane  22  back into FLC material  24 . Again, FLC material  24  has no effect on the horizontally linearly polarized light. And finally, since analyzer  32  is arranged to block horizontally linearly polarized light, the horizontally linearly polarized light is prevented from passing through to viewing area  34  causing the pixel to appear dark.  
           [0013]    Although the polarization state of the light is relatively straight forward when the light is assumed to be at a wavelength at which the SLM acts as a quarter wave plate, it becomes more complicated when polychromatic light is used. This is because even if the birefringence An of the FLC were constant, the retardance of the SLM in waves would vary with wavelength; furthermore, the birefringence of the FLC material also varies as the wavelength of the light varies. In display applications, this becomes very important due to the desire to provide color displays. FIG. 1 d  illustrates the effects the SLM has on visible light ranging in wavelength from 400 nm to 700 nm as a function of the wavelength of the light assuming typical FLC birefringence dispersions. Solid line  36  corresponds to the first case when the pixel is in its A state as illustrated in FIG. 1 b  and the dashed line  38  corresponds to the second case when the pixel is in its B state as illustrated in FIG. 1 c.  As is illustrated in FIG. 1 d , the resulting output of this configuration varies substantially depending on the wavelength of the light as indicated by line  36 . In fact, only a little more than 50% of the horizontally linearly polarized light at 400 nm that is directed into the SLM is converted to vertically linearly polarized light using this configuration.  
           [0014]    The above described configuration makes use of crossed polarizers. That is, polarizer  16  blocks vertically linearly polarized light and analyzer  32  blocks horizontally linearly polarized light. This means that polarizer  16  and analyzer  32  must be different elements. If both polarizer  16  and analyzer  32  were configured to pass the same polarization of light, they would be referred to as parallel polarizers and could be provided by the same element.  
           [0015]    In an alternative system configuration, a polarizing beam splitter may be used to replace both the polarizer and the analyzer. FIGS. 1 e  and  1   f  illustrate such a system when pixel  10  is in its A and B states respectively. In this alternative system, light from light source  12  is directed into a polarizing beam splitter (PBS)  40  as indicated by arrow  42 . PBS  40  is configured to reflect horizontally linearly polarized light as indicated by arrow  44  and pass vertically linearly polarized light as indicated by arrow  46 . The horizontally linearly polarized light indicated by arrow  44  is directed into SLM  24 .  
           [0016]    When pixel  10  is in its A state as illustrated in FIG. 1 e , SLM  24  acts as a quarter wave plate as described above converting the horizontally linearly polarized light to circularly polarized light and reflective backplane  22  reflects this light back into SLM  24 . Again, SLM  24  converts this circularly polarized light into vertically linearly polarized light as described above for FIG. 1 b  and as indicated by arrow  48 . Since PBS  40  is configured to pass vertically linearly polarized light, this light passes through PBS  40  into viewing area  34  causing pixel  10  to appear bright.  
           [0017]    When pixel  10  is in its B state as illustrated in FIG. 1 f , SLM  24  has no effect on the horizontally linearly polorized light. Therefore, the horizontally linearly polarized light that is directed into SLM  24  as indicated by arrow  44  remains horizontally linearly polarized light as it passes through SLM  24 , is reflected by backplane  22 , and again passes through SLM  24 . However, since PBS  40  is configured to reflect horizontally linearly polarized light, this light is reflected back toward light source  12  as indicated by arrow  50  causing pixel  10  to appear dark.  
           [0018]    As mentioned above, in the configuration currently being described, the light source is turned off during the time in which the image is inverted for purposes of DC field-balancing the FLC material as illustrated in FIG. 1 a . This substantially reduces the brightness or efficiency of the display. In order to overcome this problem of not being able to view the system during the DC field-balancing frame inversion time, compensator cells have been proposed for transmissive SLMs such as those described in U.S. Pat. No. 5,126,864. These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted state. FIG. 2 a  illustrates a transmissive mode system  200  which includes an SLM  202 , a compensator cell  204 , a polarizer  206 , and an analyzer  208 .  
           [0019]    As described above for the FLC material of the SLM of the previous configuration, SLM  202  and compensator cell  204  each include an FLC layer which is switchable between an A and a B state. This results in four possible combinations of states for the SLM and compensator cell. For purposes of consistency in comparing various configurations described herein, these four cases will be defined as follows:  
           [0020]    Case  1 —compensator cell in B state, SLM pixel in A state  
           [0021]    Case  2 —compensator cell in B state, SLM pixel in B state  
           [0022]    Case  3 —compensator cell in A state, SLM pixel in B state  
           [0023]    Case  4 —compensator cell in A state, SLM pixel in A state  
           [0024]    For this configuration, Cases  1  and  2  correspond to the normal operation of the system during which the compensator cell is in its B state and the SLM pixels are switched between their A and B states to respectively produce a bright or dark pixel. This is illustrated in the first half of FIG. 2 b  which is a timing diagram showing the states of the light source, the SLM, and the compensator cell. As shown in FIG. 2 b , the light source remains ON throughout the operation of the system. During the first half of the time illustrated in FIG. 2 b , the pixels of the SLM are switched between their A and B states to produce a desired image. Cases  3  and  4  correspond to the time during which the frame is inverted for purposes of DC field balancing (i.e. the SLM pixel states must be reversed) and the compensator cell is switched to its A state to compensate for the inversion. This is illustrated by the second half of the diagram of FIG. 2 b . To properly DC field-balance the display as well as allow the display to be viewed continuously, Case  1  and Case  3  must give the same results and Case  2  and Case  4  must give the same results. That is, for this configuration, Cases  1  and  3  must both produce a bright pixel and Cases  2  and  4  must both produce a dark pixel.  
           [0025]    In this example of a transmissive mode system, both the FLC layer of the SLM pixel and the compensator cell are 1800 nm thick which causes them to act as a half wave plate for a wavelength of 510 nm when in the ON state. In this configuration, the polarizer and analyzer perform the functions performed by polarizer  16  and analyzer  32 , or alternatively PBS  40 , of the reflective mode systems described above. Polarizer  206  is positioned optically in front of compensator cell  204  and the SLM pixel  202  such that it allows only horizontally linearly polarized light to pass through it into compensator cell  204 . Also, analyzer  208  which only allows vertically linearly polarized light to pass through is positioned optically behind SLM  202 .  
           [0026]    [0026]FIGS. 2 c  and  2   d  illustrate the net result the above described transmissive system configuration has on light directed in to the system. FIG. 2 c  shows the results for Case  1  and  2  during which the compensator cell is in its B state and the SLM is switched between its A state for Case  1  and its B state for Case  2 . Case  1  is indicated by solid line  210  and Case  2  is indicated by dashed line  212 . FIG. 2 d  shows the results for Case  3  and  4  during which the compensator cell is in its A state and the SLM is switched between its B state for Case  3  and its A state for Case  4 . Case  3  is represented by solid line  214  and Case  4  is represented by dashed line  216 .  
           [0027]    As clearly shown by FIGS. 2 c  and  2   d , this transmissive configuration produces identical results, that is a bright pixel, for Case  1  and  3  as indicated by lines  210  and  214 , respectively. It also produces identical results for Cases  2  and  4  as indicated by lines  212  and  216 , respectively. It should also be noted that this configuration produces relatively good results over the entire wavelength range from 400 nm to 700 nm. The worst results are at 400 nm where approximately 80% of the horizontally linearly polarized light is converted to vertically polarized light.  
           [0028]    Although the compensator cell approach works well for a transmissive SLM as described above, applicant has found that this same general approach does not work as well for a reflective type SLM. To illustrate this difference, and referring to FIG. 3 a , a reflective type display system  300  including a reflective type SLM  302  having a reflective backplane  303 , a compensator cell  304 , a polarizer  306 , and an analyzer  308  will be described. Compensator cell  304  is positioned adjacent to SLM  302 . As described above for FIGS  1   b  and  1   c , polarizer  306  is positioned to direct only horizontally linearly polarized light into compensator cell  304 . Because the light passes through the SLM and the compensator cell twice in a reflective mode system, the FLC material of SLM  302  and compensator cell  304  are configured to act as quarter wave plates for a wavelength of 510 nm rather than half wave plates as described above for the transmissive system of FIG. 2 a.    
           [0029]    In this example, the FLC materials of both SLM  302  and compensator cell  304  are 900 nm thick and both have a tilt angle of 22.5 degrees. The buff axis of the SLM is aligned with the horizontally linearly polarized light directed into the system by polarizer  306 . Also, the buff axis of compensator cell  304  is positioned perpendicular to the buff axis of SLM  302 . FIGS. 3 b  and  3   c  illustrate the net result that system  300  has on light directed in to the system. FIG. 3 b  shows the results for Case  1  and  2  during which the compensator cell is in its B state and the SLM is switched between its A state for Case  1  and its B state for Case  2 . Case  1  is indicated by solid line  310  and Case  2  is indicated by dashed line  312 . FIG. 3 c  shows the results for Case  3  and  4  during which the compensator cell is in its A state and the SLM is switched between its B state for Case  3  and its A state for Case  4 . Case  3  is represented by solid line  314  and Case  4  is represented by dashed line  316 .  
           [0030]    As clearly shown by FIGS. 3 b  and  3   c , system  300  produces identical results, that is, a bright pixel for Case  1  and  3  as indicated by lines  310  and  314 , respectively. It also produces identical results for Cases  2  and  4  as indicated by lines  312  and  316 , respectively. However, this configuration does not produces very good results over the entire wavelength range from 400 nm to 700 nm. The worst results are at 400 nm where only approximately 5% of the horizontally linearly polarized light is converted to vertically polarized light. At a wavelength of about 500 nm about 50% of the horizontally linearly polarized light is converted to vertically linearly polarized light. The best results are at 700 nm where about 80% of the horizontally linearly polarized light is converted to vertically linearly polarized light. Since the point to adding the compensator cell is to increase the efficiency or brightness of the system, this arrangement does not improve the efficiency or brightness for the lower wavelength range when compared to the system of FIG. 1 b  and  1   c  which simply turns OFF the light source during the DC field-balancing time.  
           [0031]    As can be clearly seen when comparing FIGS. 3 b - c  to FIGS. 2 c - d , the effects on the light caused by the various components of the reflective configuration of FIG. 3 a  are very much different from the effects on the light caused by the transmissive configuration of FIG. 2 a . That is, the reflective configuration of FIG. 3 a  is not optically equivalent to the transmissive configuration of FIG. 2 a  even though it may initially seem as though they should be optically equivalent. These two configurations are optically different from one another because the light must pass through the SLM and compensator cell twice in the reflective configuration with the first pass through the compensator being before the two passes through the SLM and the second pass through the compensator cell being after the two passes through the SLM.  
           [0032]    Due to this difference in the transmissive and reflective configurations, it has proved difficult to provide a reflective type system which is DC field-balanced and is substantially continuously viewable while providing improved efficiency or brightness compared to a system which simply turns off the light source during the DC field-balancing portion of the frame. The present invention provides arrangements and methods for overcome this problem.  
         SUMMARY OF THE INVENTION  
         [0033]    As will be described in more detail hereinafter, a reflection mode, spatial light modulating system and methods of operating the system are herein disclosed. The reflection mode, ferroelectric liquid crystal spatial light modulating system, includes a light reflecting type spatial light modulator. The spatial light modulator has a light reflecting surface cooperating with a layer of ferroelectric liquid crystal light modulating medium switchable between first and second states so as to act on light in different first and second ways, respectively. A switching arrangement switches the liquid crystal light modulating medium between the first and second states and an illumination arrangement produces a source of light. An optics arrangement is optically coupled the spatial light modulator and the illumination arrangement such that light is directed from the source of light into the spatial light modulator for reflection back out of the modulator and such that reflected light is directed from the spatial light modulator into a predetermined viewing area. A compensator cell is also positioned in the optical path between the light source and the viewing area. The compensator cell has a layer of ferroelectric liquid crystal light modulating medium switchable between a primary and a secondary state so as to act on light in different primary and secondary ways, respectively.  
           [0034]    In one embodiment, the optics arrangement includes a passive quarter wave plate positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. In this embodiment, the compensator cell is positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The features of the present invention may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings.  
         [0036]    [0036]FIG. 1 a  is a timing diagram illustrating the timing at which a light source for a prior art DC-balanced display system is switched ON and OFF.  
         [0037]    FIGS  1   b  and  1   c  are diagrammatic cross sectional views of a pixel of a prior art reflective type SLM display system illustrating how the pixel acts on light when the pixel is in the ON and OFF states.  
         [0038]    [0038]FIG. 1 d  is a graph illustrating the effects the system of FIG. 1 b  and  1   c  has on light after it passes through the system.  
         [0039]    FIGS  1   e  and  1   f  are diagrammatic cross sectional views of a pixel of a prior art reflective type SLM display system including a polarizing beam splitter.  
         [0040]    [0040]FIG. 2 a  is a diagrammatic cross sectional view of a prior art transmissive SLM display system.  
         [0041]    [0041]FIG. 2 b  is a timing diagram illustrating the timing at which a light source for a prior art DC-balanced display system is switched ON and OFF.  
         [0042]    [0042]FIGS. 2 c  and  2   d  are graphs illustrating the effects the system of FIG. 2a has on light after it passes through the system.  
         [0043]    [0043]FIG. 3 a  is a diagrammatic cross sectional view of a prior art reflective SLM display system.  
         [0044]    [0044]FIGS. 3 b  and  3   c  are graphs illustrating the effects the system of FIG. 3 a  has on light after it passes through the system.  
         [0045]    [0045]FIG. 4 a  is a diagrammatic cross sectional view of a first embodiment of a reflective SLM display system designed in accordance with the present invention.  
         [0046]    [0046]FIGS. 4 b - c  are graphs illustrating the effects the system of FIG. 4 a  has on light after it passes through the system.  
         [0047]    [0047]FIG. 5 a  is a diagrammatic cross sectional view of a second embodiment of a reflective SLM display system designed in accordance with the present invention.  
         [0048]    FIGS  5   b - c  are graphs illustrating the effects the system of FIG. 5 a  has on light after it passes through the system.  
         [0049]    [0049]FIG. 5 is a diagrammatic cross sectional view of a third embodiment of a reflective SLM display system designed in accordance with the present invention.  
         [0050]    [0050]FIGS. 7 a - b  are diagrammatic cross sectional views of a fourth embodiment of a reflective SLM display system designed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]    An invention is described for providing methods and apparatus for producing a substantially continuously viewable reflective type SLM display system which is DC field-balanced and which is more efficient or brighter than would be possible using a reflective type SLM display system which simply turns off the light source during the DC field balancing portion of each image frame. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, based on the following description, it will be obvious to one skilled in the art that the present invention may be embodied in a wide variety of specific configurations. Also, well known processes for producing various components and certain well known optical effects of various optical components will not be described in detail in order not to unnecessarily obscure the present invention.  
         [0052]    Referring initially to FIG. 4 a , the present invention will be described with reference to a first embodiment of the invention which takes the form of a reflective type SLM display system generally designated by reference numeral  400 . As illustrated in FIG. 4 a , system  400  includes an SLM  402  having a reflective backplane  403 , a compensator cell  404 , a polarizer  405 , and an analyzer  406 . Alternatively, in the same manner as described above, crossed polarizer  405  and analyzer  406  may be replaced with a polarizing beam splitter.  
         [0053]    System  400  is configured in a manner similar to that described above for system  300  of FIG. 3 a . That is, compensator cell  404  is positioned adjacent SLM  402 . Also, polarizer  405  is positioned to direct only horizontally linearly polarized light into compensator cell  404 . Similarly, analyzer  406  allows only vertically linearly polarized light to pass through it and into the viewing area after the light directed in to the system has passed through compensator cell  404  and SLM  402  and been reflected back through SLM  402  and compensator cell  404 . However, in accordance with the invention, system  400  also includes a static quarter wave plate  408  positioned optically between compensator cell  404  and polarizer  405  and analyzer  406 .  
         [0054]    As would be understood by those skilled in the art, SLM  402  may be made up of an array of any number of individually controllable pixels which are individually switchable between two states. For purposes of consistency, it will be assumed that each pixel is switched to its A state by applying a +5 VDC electric field through the pixel and each pixel is switched to its B state by applying a −5 VDC electric field through the pixel. It should be understood that the present invention is not limited to these specific voltages and would equally apply regardless of the voltages used to switch the pixels.  
         [0055]    System  400  further includes a light source  410  for directing light into the system in a manner similar to that described above for FIGS  1   b  and  1   c . With this configuration, light source  410  directs light into polarizer  405  as indicated by arrow  412 . Polarizer  405  blocks any vertically linearly polarized portions of the light from passing through polarizer  405  an allows only horizontally linearly polarized portions of the light to pass through polarizer  405  into static quarter wave plate  408 . This light passes through static quarter wave plate  408 , compensator cell  404 , and SLM  402  and is then reflected by reflective backplane  403  back through SLM  402 , compensator cell  404 , and static wave plate  408  to analyzer  406  as illustrated in FIG. 4 a . Analyzer  406  then blocks any horizontally linearly polarized portions of the light and allows only vertically linearly polarized portions of the light to pass through it to a viewing area indicated by viewer  416 . Since polarizer  405  blocks vertically linearly polarized light and analyzer  406  blocks horizontally linearly polarized light, this type of system is referred to as using crossed polarizers.  
         [0056]    For this embodiment and as described above for system  300 , because the light passes through the SLM and the compensator cell twice in a reflective mode system, the FLC material of SLM  402  and compensator cell  404  are configured to act as quarter wave plates for a wavelength of 510 nm. In this configuration, the FLC materials of both SLM  402  and compensator cell  404  are 900 nm thick and both have a tilt angle of 22.5 degrees. In this specific embodiment, the buff axis of the SLM is positioned at a 22.5 degree angle relative to the horizontally linearly polarized light directed into the system. Also, for this embodiment, the buff axis of compensator cell  404  is positioned perpendicular to the buff axis of SLM  402 .  
         [0057]    Although the buff axis of the SLM is described as being positioned at 22.5 degrees relative to the horizontally linearly polarized light directed into the system, this is not a requirement. In fact, this configuration works equally as well regardless of the orientation of the SLM buff axis relative to the horizontally linearly polarized light directed into the system so long as the buff axis of the compensator cell is oriented perpendicular to the buff axis of the SLM. This freedom in orienting the buff axis of the SLM relative to the horizontally linearly polarized light directed into the system makes this overall system easier to produce than other conventional systems because only the orientation of the SLM relative to the compensator cell must be precisely controlled.  
         [0058]    The orientation of the static quarter wave plate relative to the horizontally linearly polarized light directed into the system is also important. Generally, static quarter wave plate  408  has a primary axis which is oriented at a 45 degree angle to the horizontally linearly polarized light directed into the quarter wave plate.  
         [0059]    Although the tilt angles of SLM  402  and compensator cell  404  are described as being 22.5 degrees, this is not a requirement. The configuration described above for this embodiment works regardless of the tilt angle of the FLC material of the SLM and the compensator cell, but works best when the tilt angles of the two components are the same. Therefore, it should be understood that the present invention would equally apply to systems using SLMs and compensator cells having tilt angles other than 22.5 degrees. With this configuration, the bright states obtained by the system remain bright regardless of the tilt angle used provided the tilt angles match. However, the use of tilt angles in the range of 22.5 to 25.5 degrees provides optimum dark state extinction, with the choice of tilt angle at the low end of the range providing best extinction over a narrow range of wavelengths centered on the wavelength for which the SLM and compensator have quarter-wave retardance and with the choice of tilt angle towards the upper end of the range providing good extinction over a more extended range of wavelength. Increasing the tilt angle past 25.5 degrees eventually reduces dark state extinction.  
         [0060]    Now that the physical configuration of system  400  has been described, its effect on light directed into system  400  will be described. FIGS. 4 b  and  4   c  illustrate the net result that system  400  has on light directed in to the system. FIG. 4 b  shows the results for Case  1  and  2  during which the compensator cell is in its B state and the SLM is switched between the A state for Case  1  and the B state for Case  2 . Case  1  is indicated by solid line  420  and Case  2  is indicated by dashed line  422 . FIG. 4 c  shows the results for Case  3  and  4  during which the compensator cell is in its A state and the SLM is switched between the B state for Case  3  and the A state for Case  4 . Case  3  is represented by solid line  424  and Case  4  is represented by dashed line  426 . Cases  1 - 4  correspond to Cases  1 - 4  for the systems described above in the background.  
         [0061]    As illustrated in FIGS. 4 b  and  4   c , because of quarter wave plate  408  is included in the configuration of system  400 , Cases  1  and  3  result in a dark pixel rather than a bright pixel and Cases  2  and  4  result in a bright pixel rather than a dark pixel. This is the opposite of the results described in the background. However, this inversion of the bright and the dark states may be compensated for in a variety of ways such as reversing the A and the B states for the SLM (i.e. using a −5 VDC to switch the pixel to the A state and using a 5 VDC to switch the pixel to the B state). The important thing is that the results of Cases  1  and  3  are identical and the results of Cases  2  and  4  are identical.  
         [0062]    For system  400 , static quarter wave plate  408  is preferably a readily providable achromatic quarter wave plate. The use of an achromatic static quarter wave plate provides the best results over a broad color spectrum because it flattens out the curves  422  of FIG. 4 b  and  426  of FIG. 4 c  representing the bright states obtained by Case  1  and Case  2 . This flattening out of the curve improves the optical throughput of system  400  by increasing the amount of light which passes through the system for a given pixel when the combination of that pixel and the other elements are switched to produce a bright state.  
         [0063]    In one embodiment of the invention which reverses the bright and dark states described above for FIGS. 4 a - c , parallel polarizers are used instead of crossed polarizers. FIG. 5 a - c  illustrate a system  500  which utilizes parallel polarizers. As described above for system  400 , system  500  includes a SLM  502 , a reflective backplane  503 , a compensator cell  504 , a polarizer  505 , a static quarter wave plate  508 , and a light source  510 . Light source  510  directs light into polarizer  505  which blocks any vertically linearly polarized light and allows only horizontally linearly polarized light to pass through. This horizontally linearly polarized light then passes through and is acted upon by static quarter wave plate  508 , compensator cell  504 , SLM  502 , and reflective backplane  503  in the same way as described above for FIG. 4 a . However, in this embodiment, polarizer  505  also acts as the analyzer for the system. This use of polarizer  505  for both the polarizer and the analyzer is what makes this system a parallel polarizer system.  
         [0064]    In the configuration of FIG. 5 a , polarizer  505  acts as the analyzer by blocking any vertically linearly polarized light and allowing any horizontally linearly polarized light to pass into the viewing area. This is the opposite of the polarizations of light blocked and passed by analyzer  406  in system  400 . This has the effect of reversing the bright and dark states of the system and results in the net effects illustrated in FIGS. 5 b  and  5   c . FIG. 5 b  shows the results for Case  1  and  2  during which the compensator cell is in its B state and the SLM is switched between the A state for Case  1  and the B state for Case  2 . Case  1  is indicated by solid line  520  and Case  2  is indicated by dashed line  522 . FIG. 5 c  shows the results for Case  3  and  4  during which the compensator cell is in its A state and the SLM is switched between the B state for Case  3  and the A state for Case  4 . Case  3  is represented by solid line  524  and Case  4  is represented by dashed line  526 . Cases  1 - 4  correspond to Cases  1 - 4  for the systems described above in the background and Cases  1 - 4  described above for FIG. 4.  
         [0065]    As clearly shown by FIGS. 5 b  and  5   c , system  500  produces identical results, that is, a bright pixel for Case  1  and  3  as indicated by lines  520  and  524 , respectively. It also produces identical results for Cases  2  and  4  as indicated by lines  522  and  526 , respectively. This configuration also produces very good results over the entire wavelength range from 400 nm to 700 nm. In fact, as illustrated by lines  522  and  526 , this configuration provides substantially uniform blockage of the entire range of wavelengths of the light that is directed into the spatial light modulator. Also, in both Cases  1  and  3 , a large portion of the horizontally linearly polarized light passes through the system for the entire range of 400 nm to 700 nm. Since the point to adding the compensator cell is to increase the efficiency or brightness of the system, this arrangement dramatically improves the efficiency or brightness of system  500  over the complete wavelength range when compared to the system of FIG. 1 b  and  1   c  which simply turns OFF the light source during the DC field-balancing time. This also substantially improves the efficiency of the system compared to system  300  of FIG. 3 described above which does not include the static quarter wave plate. Furthermore, since essentially no light from the light source passes through the system to the viewing area when the elements are switched to produce a dark state as indicated by lines  522  and  526 , this configuration also provides an excellent contrast ratio.  
         [0066]    In another embodiment similar to system  400  of FIG. 4 a , a birefringent element may be added to system  400  in order to provide results very similar to the results obtained by system  500  of FIG. 5 a . Using like reference numerals to represent like components, FIG. 6 illustrates a system  600  including SLM  402 , reflective backplane  403 , compensator cell  404 , polarizer  405 , analyzer  406 , static quarter wave plate  408 , and light source  410 . As described above for FIG. 4, polarizer  405  and analyzer  406  are crossed polarizers. However, in accordance with this embodiment of the invention, system  600  further includes an additional birefringent element  612  which can be positioned between SLM  402  and compensator cell  404 , as shown here, or alternately, can be positioned between compensator cell  404  and static quarter wave plate  408 .  
         [0067]    In this embodiment, birefringent element  612  is a commercially available polycarbonate film having a retardance of approximately one half of the wavelength of the light for which the system is optimized, for example a wavelength of 510 nm. Alternatively, birefringent element  612  may be any birefringent material capable of providing the desired retardance such as poly vinyl alcohol or any other optically clear birefringent material.  
         [0068]    In this embodiment, the buff axes of SLM  402  and compensator cell  404  are parallel to one another and birefringent element  612  has a primary axis which is oriented perpendicular to the buff axis of both SLM  402  and compensator cell  404 . As describe above for system  400 , polarizer  405  directs horizontally linearly polarized light into quarter wave plate  408  and quarter wave plate  408  is oriented at a 45 degree angle to the horizontally linearly polarized light. SLM  402 , compensator cell  404 , and birefringent element  612  may be oriented in any way relative to quarter wave plate  408  so long as the buff axes of SLM  402  and Compensator cell  404  are parallel to one another and the primary axis of birefringent element  612  is perpendicular to the buff axes of SLM  402  and compensator cell  404 .  
         [0069]    The addition of the birefringent element causes Case  1  and Case  3  for this embodiment to result in a bright state in which the throughput varies only slightly over the range of the wavelengths similar to curves  520  and  524  of FIGS. 5 b  and  5   c . Also, the addition of the birefringent element causes Case  2  and Case  4  for this embodiment to result in a substantially more uniform dark state similar to lines  522  and  526  of FIGS. 5 b  and  5   c . This results in a system that is able to provide a high contrast ratio while maintaining a relatively high throughput for the entire wavelength range even though crossed polarizers are utilized.  
         [0070]    Although the above described embodiments have been described as having the static quarter wave plate positioned between the polarizer and the compensator cell, this is not a requirement. Instead, the static quarter wave plate may be located between the compensator cell and SLM and still remain within the scope of the invention.  
         [0071]    In another embodiment, an off axis system may be utilized in order to provide a continuously viewable DC field-balanced reflective display system. FIGS. 7 a  and  7   b  illustrate one embodiment of an off axis display system  700 . As illustrated in FIGS. 7 a  and  7   b , system  700  includes a SLM  702 , a reflective backplane  703 , a compensator cell  704 , a polarizer  705 , an analyzer  706 , and a light source  710 . In this embodiment, the light is directed into the SLM at an angle and reflected back into a viewing area indicated by viewer  720  such that the light directed into the system only passes through the compensator cell once rather than passing through the compensator cell twice as described above for the previously described embodiments.  
         [0072]    Since the light only passes through compensator cell  704  once, the thickness of compensator cell  704  is configured to be twice the thickness of the SLM. Generally, SLM  702  has a thickness which causes SLM  702  to act as a quarter wave plate when switched to its A state and compensator cell  704  has a thickness which causes it to act as a half wave plate when it is switched to its A state. Therefore, in the case in which an FLC material is used for both the SLM and compensator cell that has a birefringence of 0.142, the thickness FLC material for the SLM would be approximately 900 nm and the thickness of the FLC material for the compensator cell would be approximately 1800 nm. Both SLM  702  and compensator cell are configured to have substantially no effect on the polarization of the light passing through them when they are switched to their B states.  
         [0073]    For the configuration being described, polarizer  705  is configured to allow only horizontally linearly polarized light to be directed into the system. Analyzer  706  is configured to allow only vertically linearly polarized light to pass into the viewing area. Also, for this embodiment, the buff axis of compensator cell  704  is oriented perpendicular to the buff axis of SLM  702  and the buff axis of SLM  702  is advantageously oriented parallel to horizontally linearly polarized light directed into the system. Other orientations of the buff axes are also effective provided that the SLM and compensator cell buff axes remain perpendicular to one another.  
         [0074]    As described above for the previous embodiments, the off axis configuration shown in FIGS. 7 a  and  7   b  provide identical results for Cases  1  and  3  and Cases  2  and  4 . This configuration also provides good results over a broad spectrum similar to the results illustrated in FIGS. 5 b  and  5   c . Therefore, system  700  is also able to provide a continuously viewable system which more effectively utilizes light from the light source when compared to the conventional reflective systems illustrated in FIGS. 1 b - c  and FIG. 3 a.    
         [0075]    Although only certain specific embodiments of the present invention have been described in detail, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. For example, although the systems have been described above as using horizontally linearly polarized light as the initial input light polarization, this is not a requirement. Instead, it should be understood that the initial input light polarization may alternatively be vertically linearly polarized light. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.