Abstract:
A system and method for addressing and synchronizing a spatial light modulator (SLM) device and a scrolling color recovery (SCR) illumination system. This method applies all the colors to a single SLM simultaneously and recaptures light rejected by the color filters. The recaptured light is reapplied to the color filters and, if passed by the color filter, directed to the SLM The SCR concept requires multiple colors to be imaged on to an SLM array simultaneously. As the color bands scroll across the SLM, the data applied to elements of the SLM changes to remain appropriate for the color being received by that element. The data a lied to the SLM elements ma be loaded into the SLM by reset group with each reset group load delayed by a skew time relative to the previous group.

Description:
This application is a Divisional of application Ser. No. 10/335,313, filed Dec. 31, 2002, which claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/345,702, filed Dec. 31, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates to spatial light modulator (SLM) projection systems and more specifically to the electronic timing method for systems with scrolling color optics. 
   BACKGROUND OF THE INVENTION 
   In conventional SLM projection systems, the entire device (modulator) is sequentially exposed to uniform colors for relatively long periods of time, with brief, spatially distributed transition periods between colors, called spokes (in reference to the physical spokes between filters on a color filter wheel), while the pulse width modulation (PWM) process is carried out for each respective color frame.  FIG. 1  is an example illustrating this process, where the white illumination is passed through a color filter wheel  100 , having red  102 , green  104 , and blue  106  primary color filter segments, producing sequential red  108 , green  110 , blue  112 , red  114 , etc. beams of light that expose the SLM. Other examples can include secondary color filters and/or include a white (clear) filter segment. These relatively long color periods with globally defined temporal boundaries allows the PWM bits to be turned ON and OFF either globally or phased by reset groups over the entire device load time. Since the PWM bits can be thought of as beginning and ending more or less simultaneously over the entire array, the PWM design process could be performed while treating the SLM as a whole, as long as certain design rules were followed. 
   The traditional PWM design rules developed over the years allowed a designer to size and arrange bit times to enhance performance and to adhere to predetermined bit weights. This meant that the designer could effectively place the resets that turn bits ON and OFF into the video frame timeline with confidence that, if the rules were followed, the loading of device data could be subsequently inserted into the same timeline without conflict. Furthermore, the designer would create one timeline for the entire array, knowing that the design rules would allow time phasing by reset group. However, although this illumination method is quite effective, two-thirds of the illumination is filtered out and lost at the color wheel, limiting the overall brightness of the projector. 
   With the introduction of the scrolling color recovery (SCR) optics method of illuminating a SLM, illustrated in  FIG. 2 , the concept of global bits is no longer applicable. This concept produces red  218 , green  220 , blue  222 , and optional white color bands, which scroll across the SLM  216  so that multiple colors are applied to the SLM array simultaneously. This optical system is comprised of a light source  200  that supplies white light  202  to an integrator rod  204 , with light from the integrator rod passing through a color filter wheel  206 , through a condenser lens  214 , on to the SLM  216 . The filters are spiral shaped so as to produce three simultaneous red  208 , green  210 , and blue  212  bands of light. This concept also recovers a portion of the light that is not passed by a color filter segment by reflecting the secondary CYM light  224  back into the integrator rod  204  where it is recovered and sent back as RGB  226  light to the appropriate primary color filters. 
   Since the nature of the SCR method requires multiple colors to be applied to the SLM array simultaneously, what is needed is an electronic addressing method to accommodate the use of this illumination scheme in SLM devices. The present invention meets this need by dividing and controlling the SLM device at independent reset group levels, which are synchronized with the scrolling color bands. This method displays the bits in such a way that exactly match the rolling bands of color across the device and assure that the bits all add up in a way to provide PWM linearity. As a result, this approach provides a significant increase in the brightness in single-SLM projection systems. 
   SUMMARY OF THE INVENTION 
   The present invention discloses an electronic method for addressing and synchronizing a single spatial light modulator (SLM) device when used with color scrolling recovery (SCR) illumination. This method applies all the colors to a single SLM simultaneously and recaptures secondary light, redirecting it along the primary color paths to significantly improve the brightness in single-chip display applications. The SCR concept requires multiple colors to be imaged on to an SLM array simultaneously. This requires that the SLM be divided into reset groups so that separate groups of pixels can project bits from the different colors at the same time. As these color bands scroll across the array, the various reset groups track the color bands and change content to match. This requires that the reset boundaries and the color region boundaries be close to parallel, so that a reset group is contained within a color band and the displayed bit represents that color. 
   The method requires that the device be divided and controlled at an independent reset group level, consisting of color cycles. In operation, bits are constantly loaded in identical sequence in a group-to-group manner, with each group being delayed by a group — skew time from the previous group. This group — skew is computed from the number of groups, the group load time, and the device load time. The load interval for a device is chosen such that a color cycle has an integral multiple of load intervals, where the integral multiple is the number of reset groups plus one. In addition, it requires that there be an integral number of color cycles in a PWM frame. This method assures that all colors are exactly on the SLM simultaneously, assuring that the color mix is the same at all times. 
   This approach also deals with the transition time between colors bands where the device is exposed to mixed colors. In this case, when multiple colors of light are present on a group, it is said to be a virtual spoke time. However, spoke time light does not have to be discarded, but is added together as white light that can further increase the brightness of the system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram for a conventional color filter wheel, which sequentially illuminates an entire spatial light modulator with colors. 
       FIG. 2  is a block diagram of an optical system used in the present invention to provide scrolling color bands on a spatial light modulator. This optics recombines a portion of the filtered secondary light and redirects it back into the appropriate primary color band. 
       FIG. 3  is an exploded view illustrating how the scrolling color bands used in the present invention are all present on the spatial light modulator simultaneously. 
       FIG. 4   a  is a drawing illustrating the need to have all light of each color on the spatial light modulator surface at all times to prevent intensity changes in the various colors. 
       FIGS. 4   b  and  4   c  are drawings illustrating how the need to have all light of each color exactly on the spatial light modulator surface at all times is met. 
       FIG. 5  is a diagram that illustrates the traverse time for a scrolling color virtual spoke width component. 
       FIG. 6  is a timing diagram illustrating how the spatial light modulator is divided into multiple reset groups. 
       FIG. 7  is timing diagram illustrating how bit-data is loading into a spatial light modulator for a scrolling color system. 
       FIG. 8  is a timing diagram for a loads-first color scrolling approach illustrating equidistant device load times, which is equal to the group — skew time. 
       FIG. 9  is a timing diagram showing each reset group in  FIG. 8  being delayed the group — skew time, making all the device loads line up and thereby avoiding memory conflicts. 
       FIG. 10  is a color cycle template for the SCR method of the present invention, showing the color and virtual spoke bands, and the load times for a spatial light modulator in a scrolling color system application. 
       FIG. 11  is a timing diagram defining no-reset-zones for loading a spatial light modulator in a scrolling color system application. 
       FIG. 12  is a block diagram of a single spatial light modulator projection system, which uses the scrolling color band optics and the electronic addressing and synchronizing scheme of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention discloses an electronic method for addressing and synchronizing a spatial light modulator (SLM) device when used with color scrolling recovery (SCR) illumination. This method applies all the colors to a single SLM simultaneously and recaptures secondary light, redirecting it along the primary color paths to significantly improve the brightness in single-chip display applications. This requires that the SLM be divided into reset groups so that separate groups of pixels can project bits from the different colors at the same time. As these color bands, and boundary regions between bands scroll across the array, the various reset groups track the bands and change content to match. 
     FIG. 3  is a view illustrating how the scrolling color bands used in the present invention are all present on the spatial light modulator simultaneously. The rotating color wheel  300  is made up of color filter segments (red  302 , green  304 , blue  306 , optional white, red  308 , green  310 , blue  312 , and optional white) arranged in the form of an Archimedes spiral, which sweep across an area  314  representing an SLM device. This illustrates how the total of all the colors are present on the device at all times. Since the illumination bands have a slight curvature and the device is rectangular, the colors will be split at the top comers and bottom center of the device. Here a red band  302  is just moving on to the top of the device  314  while a previous green band  310  is moving off the bottom of the device. In this capture view, the blue color  318  is fully contained in blue band  306  in the center of the device, the green color  316  consists of color from the green band  304  at the top of the device and the green band  310  just moving off the center bottom  326  of the device, and the red color  320  consists of colors from the red band  308  at the bottom of the device and the red band  302  at the upper comers  322 ,  324  of the device. 
   The SCR method of illuminating an SLM requires that the multiple colors be imaged on the device array simultaneously. This means that at any point in time there is an equal amount of red, green, blue, and optional white light on the surface of the SLM. As a result, this requires that the SLM be divided into reset groups of separately addressed pixels, which can show bits from the different colors at the same time. As the color segments and boundary regions scroll across the array, the various reset groups track these regions and change content to match. It is necessary that the curvature of the color filter segments (bands) and reset group boundaries be made as close as possible to parallel. Otherwise, it would be difficult for a reset group to be completely contained within a color region and thus could never show a bit for that color. 
   Another aspect of the SCR method is that the recycled light spectrum is a product of the mix of color filter components in the integrator rod&#39;s reflection window. If that filter mix is allowed to vary, then the intensity and/or color of the various color regions will also vary, which is unacceptable. An example of this is shown in  FIG. 4   a . Here, the color filter is producing red, green, blue, and white color bands. First, the SLM device frame  401  is exactly aligned with the blue  400 , green  402 , and red  404  color bands. As the red band  404  moves off the bottom of the device in frame  403 , the white band  406  moves on to the top of the device, so that the red-green-blue color sequence is interrupted by a white band  406  between the red  404  and blue  400  bands. In frame  405  the red band  404  has moved completely off and the white band  406  has move completely on to the device. In frame  407  the green band  402  is moving off the bottom of the device and a new red band  408  is moving on to the top of the device. In frame  409 , the device is illuminated with the blue band  400 , the white band  406  and the new red band  408 . In frame  411  the blue band  400  is moving off the bottom of the device and a new green band  410  is moving on to the top of the device. Continuing, in frame  413  the device is illuminated with the white band  406 , red band  408 , and the new green band  410 . In frame  415  the white band  406  is moving off the bottom of the device and a new blue band  412  is moving on to the top of the device. Finally, the cycle repeats with new red  408 , new green  410 , and new blue  412  bands exactly illuminating the device. However, as the white segment scrolls on to, across (down), and off of the device, the intensities of the red, green, and blue colors go up and down while the color of the white segment takes on various shades of color. All the changes happen gradually as the color bands move down the device, thereby preventing any reset group from obtaining uniform light. This can cause observable artifacts in the image. For example, in frame  403  where there is less red light, the blue  400  and green  402  intensities go down and the white band  406  takes on a redish shade. In frame  405  where the red band has moved completely off the device, the blue  400  and green  402  intensities are further down and the white band has more red color content. Similarly, the intensities vary up and down and the white segment takes on greenish and blueish shades in frames  409 , 411  and  413 , 415 , respectively. As a result, the SCR method requires that the color filter mix be constant; i.e., whatever color is scrolling off one edge of the array must be simultaneously scrolling on to the opposite edge of the array, as is illustrated in  FIGS. 4   b  and  4   c.    
     FIGS. 4   b  and  4   c  are drawings illustrating cases where the need to have all light of each color on the spatial light modulator surface at all times is met. In  FIG. 4b , frame  460  consists of red  462 , green  464 , and blue  466  color bands, exactly aligned with the device. As the bands moves down the device in frame  470 , the blue band  466  moves off the bottom of the device, a new blue band  468  enters at the top of the device, so that there is always exactly the same mix of red, green, and blue light on the device. In the case where a white band is added, the frame  480  consists of white  482 , red  484 , green  486 , and blue  488  color bands, exactly aligned with the device. Again, as the bands moves down the device in frame  490 , as the blue band  488  moves off the bottom of the device, a new blue band  489  enters at the top of the device, so that there is always exactly the same mix of red, green, blue, and white light on the device. 
   As mentioned earlier, the SCR method of the present invention requires that the SLM be divided into reset groups with boundaries more or less parallel to the color band boundaries that move down the device. Anytime the projection of a color band is certain to encompass an entire reset group that reset group can display data for the encompassing color bits. However, anytime a reset group has mixed colors, or might have mixed colors due to some system uncertainty, then that group cannot display data from a single color and is said to be in a virtual spoke. There are no physical spokes as in the case of a conventional rotating color filter wheel, thus the name virtual spoke. The light incident on a reset group during a spoke period is variable both spatially, due to the color difference on the opposite sides of the boundary, and temporally, since the boundary is moving. 
   Fortunately, light during the virtual spoke time does not have to be discarded. Using spoke light recapture techniques developed for traditional SLM systems, these spoke times can be combined in a way such that the total integrated light on the reset group is uniform and treated as white light. This is true even thought the contribution of individual spokes is generally non-uniform on different pixels within a reset group. So the pulse width modulation software must display white data bits during spokes times and show individual color data only while a rest group&#39;s incident light is uniform. Since all spoke time must be treated as white, the duration of the spokes directly affects how much light can be used for saturated color intensity, but has no effect on total white intensity. 
   The length of time devoted to spokes has several components as shown in  FIG. 5 . This shows the case for top-to-bottom scrolling with the components being grouped into architectural and optical classes. The architectural component is simply the height of the reset group  500 . The optical components consist of the curvature and alignment width  502  of the color boundary as imaged on the SLM, the focus blur  504 , and the position error  506  in the color wheel control loop around the nominal color boundary  508 . All the optical components put together comprise a band  510 , having an upper boundary  512  and a lower boundary  514  extending across the width  516  of the SLM, in which the color can be spatially non-uniform. The total optical width  518  is the distance in the center of the band, taking the curvature into account, from the lowest point of the lower boundary  514  to the highest point of the upper boundary  512 . The total spoke time  520  is then the time it takes for this optical band to move all the way across the reset group  500 . In other words, the spoke time begins when the leading edge  514  of the optical band  510  reaches the top edge  522  of the reset group  500  and ends when the trailing edge  512  reaches the bottom edge  524  of the reset group  500 . 
   As discussed earlier, since the mix on the device must be held constant, the color sequence on any pixel must be a repeating string, such as RGBRGBRGB or RGBWRGBWRGBW, where one repetition of this string represents a color cycle. For pulse width modulation purposes, a color cycle includes both the color bands and the pokes between colors. In the SCR method, each reset group is exposed to the same color cycle except that the cycle is offset in time from every other reset group. This offset in time between color cycles in one reset group and the color cycles of an adjacent reset group is called the group — skew. The concept of color cycle and group — skew are illustrated in the timing diagram of  FIG. 6 . This shows approximately one-third of a progressive TV frame  600  (approximately 1/180 sec) with 16 reset group (numbered 0 to 15), the red  602 , green  604 , and blue  606  color bands (shaded), and the group — skew time  608 . Each reset group receives the same color cycle but delayed in time by the group — skew time. For example, the red color band  610  in reset group  1  is delayed by the group — skew time  612  from the red color band  602  in the previous reset group  0 , etc. 
   The actual time occupied by one color cycle is inversely proportional to the rotational speed of the color filter wheel. Assuming equivalent control loop performance for the color wheel at different speeds, the percentages of the color cycle taken up by the individual colors, virtual spokes, and group — skew do not change with varying color cycle length. The cycle length can therefore be optimized without affecting overall color content. 
   An additional restriction on the color cycle length is that the cycles must fit exactly into PWM frames. One PWM frame is one cycle through the PWM program, where all the bits are modulated and integrated together to make up a perceptible picture. In order to make the PWM display consistent, each frame must contain an integral number of color cycles. The PWM designer must therefore select a color cycle period equal to an integral fraction of the PWM frame time. 
   The length of the group — skew time is a critical factor in the visibility of a type of PWM artifact. When the time difference between the same significant bit being displayed on adjacent reset groups becomes large enough, a viewer can see a temporal anomaly while moving the viewing focus across the group boundary. This phenomenon is caused by a mis-integration of light from two different PWM patterns during the brief period of eye motion. Based on measurements and the state of the technology, it is prudent to limit these reset group time differences to approximately 100 μsec with current technology. This restriction has two implications:
         1) The maximum group — skew time  608 , shown in  FIG. 6 , must be held to approximately 100 μsec, and   2) The PWM bit sequence displayed during a frame must be the same for every reset group.
 
The latter ensures that the difference in time for any bits of the same significance in any two adjacent reset groups is the same and that this difference in time is equal to the group — skew time. The PWM program must therefore execute resets in each reset group, offset in time by the group — skew time from the adjacent reset groups. The offset applies not only to the resets that begin and end each color or spoke, but also to any intermediate resets that divide colors into bit times.
       

   With traditional PWM and phased reset timing, adjacent reset groups are typically offset from each other by the time required to load data into one group. This time is usually less than 30 μsec. However, in the example of  FIG. 6  a significant bit cannot be activated consecutively over all reset groups. For example, while the initial red bit  602  is being turned on as the red band scrolls down the array, other reset groups are activating green, blue, spoke, and even other significant red bits. Since all the data for these other bits must also be loaded, the first red bit data load must be separated in time to avoid conflicts. This separation extends the red bit spacing and thus the group — skew time to many times what is required for one group load. The relationship of the group — skew (S) to the color cycle is:
 
 S=C/G , and  (1)
         C=S*G, where C is the length of the color cycle and G is the number of reset groups. For example, in order to meet the group — skew requirement of 100 μsecs, the color cycle length should be no longer than 1600 μsec for a SLM with 16 reset groups.       

   With conventional PWM, the designer distributes resets into the frame timeline to produce the desired bit times and order. By adhering to certain rules, the associated data loads can be inserted later without conflict. In addition, the group loads for each PWM bit are guaranteed to be contiguous and in reset group order, thereby simplifying memory access hardware design. However, as discussed earlier, SCR PWM bits must be displayed and thus loaded in some interleaved manner to enable simultaneous display of multiple bits on different reset groups. This new scenario must be accommodated in the SCR sequence design process. 
   Even though SCR PWM could load groups randomly, to be effective for use with existing SLM devices, reset group-order data loading is used. The solution to meeting the group-order requirement and the bit interleave requirement is to mandate sequential group loading cycles with different PWM bits.  FIG. 7  is a timing diagram  700  illustrating how these requirements for bit-data loading in a spatial light modulator using SCR is satisfied. By delaying the loading of the same PWM bit (example bit d)  702  on one reset group from the previous reset group by the group — skew time  704 , the loading of that data bit  702  occupies the same position in the timeline in every reset group; i.e., between bits c and e. In order to keep the group — skew relationship and also load the reset groups sequentially, the time between full load starts (full load interval)  706  must be slightly shorter than the group — skew time  704  and must be regularly spaced in time. Conversely, the time delay from group load  708  to group load (group load interval) must also be fixed. However, this strictly limits the latitude of placing data loads in the timeline. 
   An important relationship derived from  FIG. 7  is
         (2) S=L F +L G , where L F  is the full load interval  706  and L G  is the group load interval  708 .       

   Solving the equation for L F  and substituting from equation (1)
 
 L   F =( C/G )− L   G , and  (3)
 
 C=S*G=G ( L   F   +L   G ).
 
The fact that data loading with SCR has to be carried out on a fixed and regular time schedule is a significant change from the traditional PWM approach. Currently, loads happen when necessary and there can be long periods of idle time for long bits. But with the SCR method of this invention, the sequence program must at least account for the time where a group load would not ordinarily be needed. For example, in  FIG. 7  if the load bit e  710  is not required because the bit pattern called for a long bit time, the program must still do something to advance the SLM row address between group loads bit f  712  and bit d  702 . In this case, the program could simply reload the previous bit for that group (in this example bit d  702 ) or a “dummy” loading mode could be used where the row address advances but no data is loaded.
 
   Another critical point relative to the SCR method is that data load can no longer be inserted into the timeline after all the bit lengths and order have been determined. Instead, the bit times must now accommodate the data loading. This changes the PWM design process so that designers might insert resets where they can, combine bit times into significant bit weights, and then feed that (non-binary) weighting scheme back into data processing and degamma designs. 
   The method for generating a sequence in the SCR method of the present invention is more or less backwards from that of conventional PWM methods. In order to assure a linear, continuous light transfer function, the bits are designed into the sequence according to how they fit, so as to avoid conflicts, then bit weights are assigned, and the degamma maps are used to generate the resulting codes. One way of accomplishing this is to prioritize the short bits first, fitting them into the sequence, and then adjacent bit segments are set to what is left over. The method for generating the sequence is as follows:
         (1) Determine the color scroll group — skew,   (2) Design a sequence as if there were no color scroll involved, as shown in  FIG. 8 , but rather as if there were a color wheel whose global effect is like that of the color scroll effect on the first block. Thus the color sequence timing of the first color group, called the prototype group, is the same the timing for every block in the entire device. This shows five reset groups  800 – 808  and two color bands  810 ,  814  with a spoke  812  between them. For example, each bit consists of a load  800 , reset  822 , and a data display time  816  until the next bit is reset  824 . (This shows reset  832  being further delayed to accommodate a short bit  826 ),   (3) Apply the constraint that each device load must be equidistant by the time of the color scroll group skew from the previous load. The key to this approach is that the bit lengths are determined by resets, with the constraint that no extra time may be distributed as is the case with conventional PWM techniques, and then the bit-weights  816 ,  818 ,  820 , extending between resets  822 ,  824 ,  826 , respectively, are derived within these constraints rather than them being user defined,   (4) Next, each reset group is skewed by exactly one color scroll group — skew time  910  as shown in  FIG. 9 . This makes all the loads  900 – 908  line up, as shown, retaining the colors bands and avoiding memory conflicts,   (5) Then, reset conflicts are dealt with by adjusting bit lengths where necessary until all conflicts are resolved, and   (6) Finally, the bit lengths are used with gamma table codes to derive the various linear light output levels.       

     FIG. 10  is a color cycle template  1000  for the SCR method of the present invention, showing the color and virtual spoke bands, and the load times for a spatial light modulator in a scrolling color system application. In this diagram the outer circle  1002  shows the color  1004 ,  1006 ,  1008  and spoke  1010 ,  1012 ,  1014  boundaries for the prototype group and thus the placement of the required reset times  1016 ,  1018 ,  1020  and  1022 ,  1024 ,  1026 , respectively, with no reset conflicts. The inner circle  1028  shows the 17 (for 16 reset groups) group loads for each color cycle. These include color loads  1030  and spoke time loads  1032 . 
   Within each reset group, there is a period of time associated with each group load when no resets are allowed. These are called no-reset zones (NRZ) and are defined as shown in  FIG. 11 . This NRZ is a fixed time before, during, and after each group load. The total NRZ  1100  is made up of the reset pulse time  1102 , the data hold time  1104 , the group load time  1106 , and the data setup time  1108 . These times are only dependent on the system and the four SLM parameters  1102 – 1108  and not on the color cycle period or frame rate. Also, since the group NRZ applies only to resets in the same group, a load does not generate any imputed NRZ&#39;s from other reset groups. In other words, each load results in only one NRZ and since all loads occupy the same relative time slot in each reset group timeline, the group NRZ&#39;s can be considered in the prototype group, thereby allowing a single prototype group timeline to be designed and applied to all reset groups. For meeting the requirements for SCR operation, the inner  1028  and outer  1002  circles of  FIG. 10  are rotated with respect to each other until a solution is found where none of the required resets line up inside a no reset zone. 
     FIG. 12  is a block diagram of a projection display with scrolling color optics, which incorporates the PWM sequence generation method of the present invention. In this system  1200 , a white light from a light source  1202  is coupled through condensing optics  1204 , through an integrator rod  1206 , and focused on to the surface of a rotating color wheel  1208 , which has a set of color filters arranged in the form of an Archimedes spiral  1210 . Red  1216 , green  1218 , blue  1220  (and optional white) bands of light coming through the color wheel are then coupled through relay optics  12   12  on to the surface of an SLM  1214 . The system&#39;s electronic controller  1222  controls and synchronizes pulse width modulation timing of the SLM and the filter wheel rotation so that there is exactly all the colors present on the surface of the device at all times. Modulated light from the ON pixels of the device is then reflected through a projection lens  1224  on to a display screen  1226 . This means that at any point in time there is an equal amount of red, green, blue, and optional white light on the surface of the SLM. In operation, the SLM is divided into reset groups of separately addressed pixels, which can simultaneously show bits from the different colors. As the color segments and boundary regions scroll across the array, the various reset groups track these regions and change content to match. The reset group boundaries and the color band boundaries are made to closely match so that the various image bits can be displayed for each color in such a way as to assure PWM linearity. 
   This method applies all the colors to a single SLM simultaneously and recaptures a portion of the secondary light, redirecting it along the primary color paths to significantly improve the brightness in single-chip display applications. 
   While this invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.