Abstract:
An optical system for illumination spatial light modulators. The optical system includes a tall color splitting prism ( 16 ) with substantially symmetrical face bonding areas, thereby eliminating the need for external holding plates. The face bonding areas ( 62 ) are outside the optically active area. The tall prism ( 16 ) allows for better control of stray light, including a heat sink ( 41 ) for absorbing stray or OFF state light, preventing overheating of the optical assembly. The tall prism ( 16 ) also allows adjustments to be made to any other optical components such as projection lenses and TIR prisms ( 14 ).

Description:
This application claims priority under 35 USC §119(e)(1) of provisional application No. 60/085,357 filed May 13, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical systems, more particularly for optical systems for high resolution display systems. 
     2. Background of the Invention 
     Spatial light modulator-based display systems have become more prevalent in recent years. Spatial light modulators in these systems typically comprise an array of individually addressable elements laid out in an x-y grid. They elements form the image by turning ON and OFF selectively based upon control signals. The elements control the perceived intensity of the image pixels by controlling the amount of light that reaches the final display surface. 
     These systems generate color images in several different ways. Sequential color systems typically use one modulator and sequence the color of light that illuminates the modulator. A separate image for red, green and blue, for example, is generated and the integration of the human eye blends those colors and their respective intensities for each pixel into a final image. Another approach is to provide three differently colored sources of light or three different modulators or modulator panels, each with color filters that produce the colored images for each color. 
     The three modulator systems have several different implementations. In some systems, a prism is used that splits a white light source into red, green and blue. The light travels out of the prism to the respective modulators, where it is modulated to create the image and then transmitted back into the prism for recombination and projection to the final display surface. 
     Several problems exist with these prisms. Surface reflections, absorption and scattering characteristics affect image quality. The latter two reduce the efficiency and quality of light. Additionally, light absorbed at some optical surfaces can induce thermal gradients in the optical glass, distorting the assembly. This can result in misconvergence. Further, for telecentric systems, the prism size must increase with the increase of the resolution and size of the device. 
     Therefore, a solution is needed for multiple spatial light modulator systems that provides color splitting without increasing the size of the optics, as well as reducing the absorption and scattering characteristics. 
     SUMMARY OF THE INVENTION 
     An optical system for illumination of spatial light modulators is disclosed. The system includes a tall color splitting prism. The tall prism has symmetrical face bonding areas that provide enough stability to the prism components that external plates are not required. Additionally, a heat sink is used for OFF state or stray light for better thermal management of the optical system. The system provides enhanced control of all incoming and outgoing light in the system, increasing contrast ratio and improving image quality. 
     It is an advantage of the system in that it does not require external plates to hold prism components together for the color splitting prism. 
     It is an advantage of the system in that heat in the system is controlled, thereby allowing higher power light sources and preventing thermal expansion of optical components. 
     It is an advantage of the system in that it controls stray light in the system, thereby preventing stray light and scattering artifacts in the projection path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying Drawings in which: 
     FIGS. 1 a-c  show side views of a prior art prism optical system with a reflective spatial light modulator. 
     FIG. 2 shows a top view of one embodiment of a prism optical system in accordance with the invention. 
     FIGS. 3 a - 3   c  show side views of a prism optical system in accordance with the invention. 
     FIGS. 4 a - 4   c  show side views of prior art total-internal reflection (TIR) prism. 
     FIGS. 5 a - 5   c  show side views of a TIR prism in accordance with the invention. 
     FIG. 6 shows a prior art embodiment of face bonding areas. 
     FIG. 7 shows one embodiment of face bonding areas. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 a - 1   c  show a prior art prism optical system for a reflective spatial light modulator image system. The embodiment shown is typically used with three modulators, one each for red, green and blue. However, for discussion purposes, only one modulator  10  is shown. Light from the illumination path enters the first prism  14 , of a a total internal reflection (TIR) prism assembly  11 . 
     An example path  12  shows the light entering the TIR prism  14 , passing to through the color prism assembly  13 . Since the other modulators for the other two colors are not shown, it appears that the light just passes directly through the color prism assembly  13 . However, the color prism assembly  13  actually splits the light into red, green and blue components. The modulator  10  shown in this embodiment is the green modulator, so the rays of light actually striking the modulator  10  are green. 
     If the elements of the modulator are ON, or deflected, the light will travel along path  18  back through the color prism assembly  13  to the TIR prism  14 . Since the path  18  is at a different angle, it strikes the TIR surface  20 , which causes the light to reflect towards the face of the prism in the projection path  22 . The light reflected along path  18  to face  22  will be projected onto the final imaging surface, such as a display screen. It is well known in the art that the illumination  12  and projection  22  paths may be reversed such that the illumination light is reflected by the TIR surface  20  while the projection light passes through the TIR surface  20 . 
     It must be noted that some reflective modulators use light from the OFF state to image the final picture for display, some use the ON state. While the example used here is to image with the ON state light, no limitations of the invention should be implied. 
     Additionally, some modulators have two states, ON and OFF, or ON and unaddressed. Others have three states, ON, OFF and unaddressed. To ensure a full discussion of all possibilities, the modulator in these examples are assumed to have all three states. Each state is shown separately, but in most systems, the modulator will have elements in each state at any given time. For the OFF and flat state drawings hereafter, the light will not be shown entering the TIR prism from the illumination system, but will be assumed. 
     FIG. 1 b  shows a similar arrangement as FIG. 1 a,  but shows the light for the unaddressed state, which for some modulators is the flat state. Light striking elements in this state on the modulator  10  will reflect the light in one of three directions in the prior art embodiment shown in FIG. 1 b.    
     A first example is shown by path  26 . Light is reflected back through the color prism and strikes the upper surface  15  of the TIR prism  14 . This light is reflected into the illumination path, which may cause scattering. A second example, shown by path  28 , is the desired case. Light is reflected out of the TIR prism away from both the illumination path and the projection path. 
     A third example is shown by path  24 . Light is reflected off the top surface  17  of the color prism assembly  13 , strikes the TIR surface  20  of the TIR prism  14  and is reflected, or transmitted, into the projection path. This is undesirable. In most systems, only the ON state light used for imaging is desired in the projection path. 
     The light from the flat state that enters the projection path causes contrast ratio degradation. As mentioned above, only ON state light is desired for imaging in this example. Having light enter the projection optics when not desired would occur when the image (or portion thereof) is supposed to be black. This makes the black levels lighter than desired. The contrast ratio is the ratio between the blacks and whites; a higher contrast ratio is desirable. By raising the black levels, the difference between the blacks and whites is lessened and raising the black levels lowers the contrast ratio. 
     The same problem occurs in the prior art embodiment when the modulator is in the OFF state. Light again travels along path  26  into the illumination path which is undesirable. Also, light travels along path  30 , reflecting off the top surface  17  of the color prism assembly  13  to the TIR surface  20  and into the projection path. This is very undesirable as discussed above. 
     The prior art embodiments shown in FIGS. 1 a-c  have significant other problems as well. Because of the size of the prisms, sufficient face area does not exist to bond the faces of the prisms directly together. Glass and/or metal plates hold the prisms together. These plates and adhesives have different thermal expansion rates and different dimensional tolerances that induce harmful stress and distortions. 
     Yet another problem with the prism shown in FIGS. 1 a - 1   c  results from its size. In telecentric optical systems, as the resolution increases the size of the modulator typically increases as well. In order to move from a lower resolution to a higher resolution, the prism size and volume increases. For example, a move from SVGA to SXGA could result in an increase of prism volume by as much as 4×. This also requires large, expensive elements in the projection path. 
     A system shown for three modulators in accordance with the invention that solves these problems is shown in FIG.  2 . The one modulator  10  discussed previously will now be referred to as  10   a.  Modulators  10   b  and  10   c  are the modulators for red and blue respectively. The color prism assembly  13  appears far different than in previous drawings because FIG. 2 shows a top view revealing prisms  16 ,  17 , and  19 , whereas all previous figures were of side views. 
     The modulators  10   a-c  have the elements set out in the ON position. From this perspective one can see the functioning of the overall system. Light enters TIR prism assembly  11 , which again looks different from the top view than the side, and is split by the color prism assembly  13  into the appropriate color components for each modulator  10   a-c.  The paths shown are only for the incoming light to the modulators. 
     An example of one such path is shown by incoming path  32 . The light enters the TIR prism assembly  11  and strikes the red/blue face  38  of the color prism at point  34 . The red light is reflected along path  36  to the red modulator  10   b.  The blue/green light passes to the blue/green face  42  and strikes at point  40 . The blue light is reflected along path  44  to eventually reach blue modulator  10   c.  The remaining green light eventually passes to the green modulator  10   a.  It should be noted at this point that the use of three modulators is not a restriction on this system. 
     In a two-modulator system, such as that shown in U.S. Pat. No. 5,612,753, a color wheel is used to sequence the light such that one modulator receives only one color of light while the other modulator receives two colors in sequence. In such a case, the modulator receiving two colors would only be able to process each color for half of the time that the other modulator processes its one color. However, this can be used to balance the spectrum of the incoming white light. 
     In this embodiment of the invention, the color prism is considerably taller than in systems previous to this invention and some supplemental techniques can be combined with the taller prism for optimal system design. These combinations have several key effects on the overall system design. All light rays are able to pass through a minimum number of prism faces without the possibility of re-entering the illumination or projection elements. The taller prisms have more face area that allow for sufficient bonding without the use of external plates and adhesives. The marginal rays at the corners of the modulator are controlled such that a much smaller prism can be used; and an increase in resolution will not necessarily result in an increase in prism size, although it can. 
     For a more detailed discussion, the system of FIG. 2 will be addressed in terms of one modulator out of the three shown. As noted above, it is possible to have only two modulators. FIGS. 3 a-c  show side views of the modulator system shown in top view of FIG.  2 . For consistency, the green modulator remains in the drawing. 
     FIG. 3 a  shows the taller color splitting prism assembly  21  with the green modulator  10  in the ON state. Comparing FIG. 3 a  with FIG. 1 a,  it can be seen that the color prism assembly  21  is considerably taller than that of FIG. 1 a.  A further beneficial effect of this change in dimension is the positioning of the modulator  10  with regard to the prism assembly  21 . In FIG. 1 a  the modulator has to be positioned off center of the prism. However, because of more optimal angles available in the taller prism, the modulator can be positioned more centrally to the taller prism. 
     FIG. 3 b  shows the taller prism in the flat or unaddressed state light. Again, comparing FIG. 3 b  to FIG. 1 b,  the dramatic difference can be seen. Ray  24  in FIG. 1 b  entered the projection optics, an undesirable effect. Ray  24  in FIG. 3 b  strikes the top surface of the TIR prism and is reflected away if a coated surface is used, or absorbed, depending upon the coating. Ray  26  in FIG. 1 b  entered the illumination optics, but now exits the TIR prism at an angle such that such effect is avoided, similar to ray  28  in both diagrams. A further technique used with regard to the TIR prism is discussed with reference to FIGS. 5 a - 5   c.    
     A heat sink could be used at the top of the diagram, shown by  41  in FIG. 3 b.  This heat sink would receive the rays that are passing through the TIR prism, such as rays  26  and  28 . FIG. 3 c  shows the modulator in its OFF state. 
     FIG. 3 c  shows that the rays  30  and  26  no longer travel into the projection path or the illumination path as shown in FIG. 1 c.  All of the light impinging upon the OFF elements of modulator  10  pass through the color splitting prism assembly  21  or the color splitting prism assembly  21  and the TIR prism assembly  23  away from the illumination and projection optics. Again, a heat sink  41  could be used here to absorb the heat from the OFF state or flat state light. 
     The overall effect of the taller prism results in better control of the light. This prevents light from the OFF and unadressed states from interfering with the image quality and contrast ratio of the overall system. The taller prism can be combined with other techniques that result in even more improvement. The angles of the color splitting dichroics can be better optimized. This results in a minimized back working distance, making design of the projection lens easier. Other angles could be adjusted as well. For example, the angles of the TIR prism could be changed. Additionally, the size of the prism necessary for a predetermined modulator size can be reduced. 
     FIGS. 4 a - 4   c  show a prior art embodiment of the TIR prism for each of the three states of the modulator. The modulator here is a single modulator, not one of a multiple modulator system as discussed previously. Therefore, there is no need to show the color prism. In some systems using only one modulator, a color wheel is used to sequence the light for the single modulator. The TIR prism controls the light impinging upon and being imaged by the modulator. 
     As can be seen in FIGS. 4 a - 4   c  the top surface  15  of the TIR prism is relatively flat. Ray  42  strikes this flat top surface  15  in the unaddressed state of FIG. 4 b.  This causes an undesirable reflection into the illumination optics. This is shown for the OFF state by ray  44  (shown in FIG. 4 c ). 
     FIGS. 5 a - 5   c  show a TIR prism assembly used in combination with the present invention. The top angle  25  of the prism has been changed from relatively flat to an angle of 20-30 degrees. Experiments have shown that a top angle of approximately 27.5 degrees provides optimal performance. As can be seen from all the states of the modulator elements shown in FIGS. 5 a - 5   c,  this eliminates reflection from rays that otherwise would have reflected off the top surface  15  of the TIR prism assembly if it had been flat. 
     A further technique that provides even better system performance is the use of vignetting in the illumination path. Apertures are used in the illumination paths that eliminate any marginal rays that would strike the corners of the modulator. Such apertures are set forth in U.S. patent application Ser. No. 60/071,243, commonly owned by the assignee and incorporated by reference herein. 
     One concern in increasing the height of the prism is a corresponding increase in the volume. However, a combination of angles exists that minimize the volume of the prism based upon the modulator panel size. As will be seen with regard to FIGS. 6 and 7, any increase in cost will more than likely be offset by the savings made by eliminating the plates. 
     FIG. 6 shows the area of the red/blue face of a typical color prism  16  used prior to this invention. The region  60  is the area of the face through which light will pass during system operation. The two areas  62  are the available spaces for bonding adhesive. As can be seen by this, very little space is available, which gives rise to the need for the external holding plates. 
     FIG. 7 demonstrates how the taller color prism  16  eliminates the need for the plates. The area through which light will pass is shown by area  60 , the optically active area. As mentioned above, the modulator can be centered on the prism with the relaxation of the tolerances caused by the taller prism. Additionally, more area is available for the bonding areas  62 . In this case, the plates would not be necessary, reducing the cost and complexity of the system, while avoiding the thermal problems discussed above. The substantially symmetrical bonding areas  62  provide the necessary area for bonding. 
     In summary, the use of a taller color splitting prism solves several problems including those related to light scattering, contrast ratio and heat absorption. The taller prism can be used in combination with other techniques such as angling the top of a TIR prism and vignetting in the illumination path. However, the taller prism can also be used without those techniques and will still improve system performance. 
     The discussion to this point has been focused on reflective light modulators. However, the taller prism with the symmetrical bonding areas, as well as the heat sink, could be used in transmissive modulators as well. Both reflective and transmissive modulators transmit light to an imaging surface, either by direct transmission or by reflection. The taller prism and the heat sink would be useful in either case. The only aspect that is different is that the heat sink in a reflective modulator system absorbs OFF state light, but in a transmissive modulator system it would absorb stray light. None of the discussion above is intended to limit the application of these concepts to reflective modulators. 
     Thus, although there has been described to this point a particular embodiment for a color splitting prism in an imaging system, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims.