Abstract:
A display system is disclosed that includes a light source, an integrator rod receiving a light beam from the light source and producing an integrated light beam, a beam splitting device splitting the integrated light beam into at least a first cone of light and a second cone of light, the cones of light having complementary intensity distributions, and at least a first spatial light modulator and a second spatial light modulator, wherein the first spatial light modulator is capable of selectively reflecting portions of the first cone of light in an ON direction and the second spatial light modulator is capable of selectively reflecting portions of the second cone of light in an ON direction. The display system further includes a combining device for combining the selectively reflected portions of the cones of light in the ON direction and projection optics for projecting the combined reflected cones of light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.K. provisional application Serial No. 0009590.1, filed Apr. 18, 2000, U.S. provisional application No. 60/198,614 filed on Apr. 20, 2000, and International Application No. PCT/US01/12613 filed on Apr. 18, 2001, the entire contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The field of the invention is image projection in general, and electronic image projection in particular. 
   BACKGROUND 
   U.S. Pat. No. 5,386,253 to Fielding, incorporated herein in its entirety by this reference, discusses exemplary projection systems utilizing one or more spatial light modulators (SLMs). As noted in the Fielding patent:
         Spatial light modulator devices include so-called “active matrix” devices, comprising an array of light modulating elements, or “light valves,” each of which is controllable by a control signal (usually an electrical signal) to controllably reflect or transmit light in accordance with the control signal. A liquid crystal array is one example of an active matrix device; another example is the deformable mirror device (DMD) developed by Texas Instruments . . . .
 
See Fielding, col. 1, 11. 13-21. Of course, yet other types of light “engines,” or sources, and projectors exist, and various of them may be used in connection with the inventions described herein.
       

   Regardless of the type of projector used, audiences frequently desire to see images high in detail and richness and low in objectionable artifacts. High resolution and image quality in particular facilitates suspension of disbelief of an audience as to the reality of the projected images. Such quality indeed often is an important factor in the overall success of the motion picture viewing experience among today&#39;s audiences. 
   Providing high resolution images to audiences can be prohibitively expensive in terms of producing the software, and in terms of the hardware necessary to show high resolution images. Imax Corporation, for example, the intended assignee of this application, utilizes not only specialized cameras and projectors, but also seventy millimeter, fifteen perforation film to increase the resolution and quality of projected images. 
   In some venues, it is desirable to be able to display high resolution moving picture images that are non-film based, such as computer generated graphics, or material captured with electronic cameras. It is particularly prohibitive to display these kinds of high resolution images using conventional electronic projectors (and especially those utilizing SLMs) because it is not technically or economically feasible to produce the necessary spatial light modulators (SLM) at sufficient resolution to match the high resolution of the source material. As well, such electronic projectors frequently fail to furnish the dynamic range and overall brightness of images provided by large-format films. 
   One approach for displaying high resolution images using electronic projectors is to use a number of smaller display devices arranged so that each individual device projects a portion of the total image. This is the principle behind so-called video walls, which feature a matrix of CRT displays, each one displaying only a subset of a high resolution image. This approach is commonly referred to as display tiling. Another approach is to combine or superimpose two or more sub-images by offsetting individual SLMs by 1/z of a pixel, so that the pixels of one spatial light modulator are positioned to lie between the spaces of the pixels of another SLM. This approach is discussed in U.S. Pat. No. 5,490,009. 
   While tiling can be accomplished using multiple projection systems, each having a light source, SLM and projection lens, in order to provide the most satisfactory tiling of images, and to accomplish superimposition where very high accuracy of image alignment is needed, it is desirable to combine the multiple SLMs into a single projection apparatus. However, it is also desirable for some components such as the light source and projection lens to be shared. Sharing the light source is desirable because a single light source can typically illuminate a larger solid angle than can be coupled into a single SLM. Sharing of projection lenses is further desirable in order to minimize variations in the image quality produced by multiple SLMs by reducing the number of optical components. In order to do this a means is required to efficiently sub-divide the light from the source and to recombine the output of the multiple SLMs prior to the projection lens. This invention provides systems and methods which enable high efficiency in distributing light from a single source to multiple SLMs and recombining the modulated light for projection onto a screen. 
   SUMMARY OF THE INVENTION 
   The invention separates the light produced by a projection lamp into two discreet and essentially equal channels and then recombines them after modulation by SLMs. In the act of separating the light, a distinct intensity distribution is imposed on each channel and acts as a label for future recombination. The light in each channel carries on to uniformly illuminate each SLM surface. After modulation, means which rely on the labeling mentioned above recombine the two separate channels into one prior to passage through projection optics. 
   In one embodiment, light from a projection lamp is directed into a rectangular integrating bar and is then split into two complementary half cones of light intensity distribution by the use of a total internal reflection (TIR) prism after the integrating bar. Each half cone of light is then directed by the relay optics to a SLM where it is modulated in accordance with stored image information. The SLMs may be offset or tiled with respect to each other to increase the effective resolution of the projection apparatus. After modification by the SLMs the two beams are recombined using a second TIR prism. In another embodiment, separation is accomplished by a beam splitter positioned at the pupil of a telecentric relay. Similarly, the images of the two SLMs may be combined using a second telecentric relay (with the entrance pupil at infinity) with a second beam splitter positioned at a pupil or focal plane of the second relay. The illumination distribution at the pupils is directly related to the intensity distribution at the image and object planes. 
   The beam splitter combining element can have surface features with both reflecting mirrored portions and transparent portions, in approximately equal proportions. The mirrored and reflecting portions may be distributed in a number of possible patterns. An additional benefit of the invention is that for recombining the modulating light some patterns, for example a checkerboard pattern, allow the combining element to act as a spatial filter and remove to some degree high frequency spatial information, which contributes to image pixellation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustrating the operation of a spatial light modulator. 
       FIG. 2  is a schematic illustrating the method of separating the light by means of a knife edge mirror. 
       FIG. 3  is a schematic illustrating an exemplary distribution of a light beam and two cones of light according to the method of FIG.  2 . 
       FIG. 4  is a schematic illustrating the method of separating the light by means of a total internally reflecting (TIR) prism. 
       FIG. 5  is a schematic illustrating an exemplary distribution of a light beam and two cones of light according to the method of FIG.  4 . 
       FIG. 6  is a schematic illustrating focal plane recombination. 
       FIG. 7  is a schematic illustrating TIR recombination. 
       FIG. 8  is a schematic illustrating the incident angles of light in relation to a DMD. 
       FIG. 9  is a schematic illustrating the combination of two separate images. 
       FIG. 10  illustrates a checkerboard pattern of mirror distribution. 
       FIG. 11  illustrates an alternative embodiment in which the mirror pattern is a number of longitudinal strips. 
       FIG. 12  illustrates an alternative embodiment in which the mirror pattern is a series of concentric circles. 
   

   DETAILED DESCRIPTION 
   Referring first to  FIG. 1 , there is illustrated a spatial light modulator (SLM)  47  having an M×N array of pixels  42  arranged in rows and columns. A light beam  41  is directed at the SLM  47  and light is reflected off or transmitted by the pixels  42  in an ON direction or an OFF direction. SLM  47  could be a deformable mirror device (DMD), such as that sold by Texas Instruments, in which each of the pixels is actually a micro-steerable mirror that can be toggled between an off-state and an on-state in rapid succession, as is necessary to display an image on a projection screen. SLM  47  is connected to a driver circuit  44  that receives an electronic signal from a control circuit indicated generally as  46  and addresses each of the pixels. Alternatively, SLM  47  could be a liquid crystal array. 
   In  FIG. 2 , light produced by a projection lamp  1  is directed toward a rectangular integrating bar  2 , which serves the purpose of providing a beam of light with a rectangular profile matching that of the spatial light modulators (SLMs) of the projection device. Integrating bar  2  also serves to redistribute the light to produce a beam having a more uniform intensity distribution than that entering the integrating bar. The light is directed to an illumination relay system consisting of front end relay optics  3  and back end relay optics  6  and  8 . The illumination relay optics are designed so that the output will be telecentric (exit pupil at or near infinity) in order to provide the most uniform distribution of light to the SLMs. Maintaining consistent illumination angles over the field is particularly important for color systems that employ dichroic mirrors in combination with multiple SLMs to modulate the input light in color components (RGB). The relay is split into two parts to allow location of a knife edge mirror  4  at the pupil of the relay optics. The knife edge mirror  4  divides the illumination at the pupil into two symmetrical cones of light and reflects the first cone to first back end relay optics  6  and allows the second cone to be transmitted to second back-end relay optics  8 . The first back end relay optics  6  transmits the first cone of light  5  to a first SLM  31  and the second back end relay optics  8  transmits the second cone of light  7  to a second SLM  32 . Knife edge mirror  4 , may in fact be a beam splitter cube with one half of its internal surface mirrored. The two half cones will have complementary intensity distributions which can be exploited to achieve high efficiency recombination. 
     FIG. 3  illustrates an exemplary light distribution for a beam of light and two half cones of light according to the system of FIG.  2 . Here the distribution of light at the pupil might be as shown at  20 . The hole in the center  23  is due, for example, to the characteristics of the light source (a collection mirror with a hole at the center to clear a lamp electrode). The knife edge  4  acts to split the distribution of the light at the pupil into two symmetrical distributions shown at  21  and  22 . The outputs of the back end relay optics  6  and  8  are ½ cones of light  5  and  7  respectively which telecentrically illuminate two SLM devices. 
   While the preferred embodiment discloses splitting the light beam into two cones of light, it will be appreciated by those of skill in the art that the light beam could be split into multiple cones of light for multiple SLMs. 
   In  FIG. 4 , the output of a projection lamp  1  is directed toward a rectangular integrating bar  2  which serves the same purpose described above. The light emerging from  2  is then directed toward a total internal reflection (TIR) prism  10 . TIR  10  is designed to transmit light rays at angles less than 45 degrees to its boundary surface  11  and to reflect light rays at angles greater than 45 degrees. By suitably arranging the angles of incidence in each light path, the TIR prism divides the larger light cone of light from the integrating bar into to ½ cones. This effect is similar to that obtained by the method of using a knife-edge mirror at the pupil as described above. However, in the case of the knife edge mirror located at a pupil the resulting illumination distribution in each ½ cone is the same as the original overall input illumination distribution. In the case of the TIR prism  10  the illumination distribution is actually a symmetrical ½ of the distribution of the total. Referring now to  FIG. 5 , the use of TIR allows the output of the integrating bar  23  to be divided into two complimentary and symmetrical distributions  24  and  25  without requiring the splitting zone to be at a focal plane, eliminating the need for the front end relay optics. Referring again to  FIG. 4 , the two half cones of light from the TIR prism are converted to telecentric illumination for the SLMs  31  and  32  by relay optics  6  and  8 . 
     FIG. 6  illustrates how the light from two SLM devices illuminated by the light splitting system of  FIG. 2  is re-combined in a relay system that is essentially the inverse of that used to split the light. That is, it is a telecentric relay with the entrance pupil located at infinity. Again, this relay is split into two parts, front end parts  35  and  36 , and back end part  38  so that a second knife edge or partial beam splitter can be located at a focal plane in the relay. In  FIG. 6 , a first half-cone of light  33  is reflected from the surface of a surface light modulator (SLM)  31  and is directed through first front-end relay optics  35 . A second half cone of light  34  is reflected from the surface of a second SLM  32 , and is directed toward second front-end relay optics  36 . The half cone intensity distribution at the SLM manifests itself as an illumination distribution at the pupil which is located at the beam splitter  37 . The beam splitter  37  can be a beam splitter cube with one half of its internal surface mirrored. The beam splitter  37  allows low loss combination by reflecting all the light, in the ON direction, from SLM  32  and transmitting all of the light, in the ON direction, from SLM  31 . The combined beam passes through the back end of the relay optics  38  prior to passage through projection optics  39 . The combined beam is projected by the projection optics  39  onto a screen (not shown). 
     FIG. 7  similarly illustrates how the light split by the TIR prism in FIG.  4  and directed to two SLMs is re-combined. In  FIG. 7 , a TIR prism  40  combines the two half cones of light  33  and  34  reflected, in the ON direction, from the surfaces of SLMs  31  and  32  respectively. The combined light is directed towards projection optics  39 . This arrangement has the advantage of not requiring optics between the SLMs and the combining element since the TIR method does not require a focal plane for performing the combination and thus does not suffer from concerns of having matching optical elements, as is the case in the embodiment of FIG.  6 . 
   A secondary benefit of these systems and methods of light separation and combination is an improvement in system contrast due to the smaller pupil each SLM sees. The smaller pupil reduces the amount of light scattering within the projecter and leads to an improvement in contrast. In addition, the distribution of scattered light at the pupil is generally not uniform, and by suitable positioning of the recombining relay optics with respect to the knife edge or beam splitter located at the pupil one can reduce the amount of scattered light that propagates through the rest of the system. This can result in a contrast improvement. For DMDs, in particular, improvements may also be realized by orienting the illuminating half cones with respect to the surface of the DMDs so that the average incident angle is maximized. Referring now to  FIG. 8 , principle ray  26  of the illuminating light strikes DMD mirrors  28 ,  29  and  30  at an angle of 20 degrees with respect to a normal  27  to the mirror plane. As shown in  FIG. 8  the illuminating cone around principle ray  26  consists of rays +/−10 degrees either side of this principle ray. Specular reflection of the desired imaging light will occupy a range of angles about the principle reflected ray  72 . Only the desired image rays should enter the aperture  71  of the output relay or projection lens. If the angular relationship of this aperture  71  is correct with respect to principle ray  26  and the accompanying cone of light, then scattered rays such as those reflected from the substrate below the mirrors like  73  will not enter the aperture  71  of the output relay or projection lens. This helps to ensure that any unwanted reflections from the DMD, which may contribute to reducing contrast on the screen, will be directed away from the projection axis of the system. 
   The beam splitter  37  used with the embodiments illustrated and described with respect to  FIG. 6  for the recombination of the two half cones may be formed in a number of possible patterns of reflective and transparent portions. For example,  FIG. 9  illustrates schematically the combination of two separate images  50  and  52  using the combining element  37 , which has both mirrored portions and transparent portions in substantially equal portions. Image light  50 , from a first SLM, passes through the transparent portions of combining element  37 , whereas image light  52 , from a second SLM is reflected off of the mirrored portions of the combining element. The two SLMs may be offset from each other by ½ pixel, horizontally, vertically or both to increase the apparent resolution of the images on the screen. Combining element  37  is located at an intermediate plane within the relay optics of the projector, and is preferably located at the pupil of said relay optics. Similarly, a beam splitter element  4  with mirrored portions and transparent portions in equal proportions could be used. 
     FIGS. 10-12  illustrate different possible patterns of mirror portions  56 ,  60 ,  66 , and transparent portions  58 ,  62 ,  64 . The combining element or splitter element can be manufactured as a standard beam splitting cube, with the mirror pattern(s) deposited on the diagonal plane of the cube.  FIGS. 10 ,  11 , and  12  illustrate a checkerboard pattern, a longitudinal strip pattern, and a concentric circular pattern respectively and are examples of some possible mirror patterns. Other patterns exist, which are also effective. 
   An additional benefit of using the inventive method of beam combination is that some mirror-portion patterns may act as a spatial filter to remove unwanted higher spatial frequencies associated with the edges of pixels. The checkerboard pattern illustrated in  FIG. 10  is one such pattern. It breaks the cross section of a light beam down into a set of small apertures, each of which passes a narrow bandwidth of spatial frequencies and subsequently removes high spatial frequency information. In the case of incoherent illumination, the bandwidth of the entire system is equal to that of each individual smaller aperture. Experiments have shown that changing the size of the squares in a checkerboard pattern changes the filtering characteristics, allowing one to select an optimal mirror pitch for each particular system. 
   This system may also be implemented for a three color projection system where typically three SLMs are used per channel, one for modulation each of red, green and blue light. Here it is common to use a color splitting and combining prism employing TIR and dichroic mirrors. In the foregoing descriptions, each of the single SLMs could be replaced by such a prism, each having three SLMs for modulation of red, green and blue light. The corresponding light source of course requires the appropriate spectral distribution, and the associated relay optics or TIR beam splitters must be designed to minimize chromatic aberrations. A three color projection system is generally described in U.S. Pat. No. 5,865,520, which is incorporated herein in its entirety by this reference. 
   The foregoing is provided for purposes of explanation and disclosure of preferred embodiments of the present invention. For instance, a preferred embodiment of this invention involves using two deformable mirror devices as the spatial light modulators. It is expected that such capabilities or their equivalent will be provided in other standard types of spatial light modulators and that more than two spatial light modulators could be used, in which case the preferred embodiment of this invention may be easily adapted for use in such systems. Further modifications and adaptations to the described embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of the invention and the following claims.