Patent Publication Number: US-11378840-B2

Title: Image display

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/001,068 filed 19 Jan. 2016 which is a continuation of U.S. patent application Ser. No. 14/521,832 filed 23 Oct. 2014 now U.S. Pat. No. 9,270,956, which is a continuation of U.S. patent application Ser. No. 13/774,563 filed 22 Feb. 2013 now U.S. Pat. No. 8,890,799, which is a continuation of U.S. patent application Ser. No. 13/358,311 filed 25 Jan. 2012 now U.S. Pat. No. 8,446,351, which is a continuation of U.S. patent application Ser. No. 11/831,827 filed 31 Jul. 2007 now U.S. Pat. No. 8,125,425, which is a continuation of U.S. patent application Ser. No. 10/507,460 filed 10 Sep. 2004 now U.S. Pat. No. 7,403,332, which is the U.S. National Stage of International Application No. PCT/CA03/00350 filed 13 Mar. 2003, which claims the benefit of the filing date of U.S. provisional patent application No. 60/363,563 filed 13 Mar. 2002 and entitled HIGH DYNAMIC RANGE DISPLAY DEVICES. 
    
    
     The claimed invention was made as a result of activities undertaken within the scope of a joint research agreement as defined under 35 U.S.C. 103(c) between the National Sciences and Engineering Research Council of Canada, Lorne Whitehead of the University of British Columbia, Wolfgang Stuerzlinger and Hugh Wilson of York University, and Avi Chaudhuri of McGill University. 
     TECHNICAL FIELD 
     The invention relates to displays for displaying digital images. 
     BACKGROUND 
     Dynamic range is the ratio of intensity of the highest luminance parts of a scene and the lowest luminance parts of a scene. For example, the image projected by a video projection system may have a maximum dynamic range of 300:1. 
     The human visual system is capable of recognizing features in scenes which have very high dynamic ranges. For example, a person can look into the shadows of an unlit garage on a brightly sunlit day and see details of objects in the shadows even though the luminance in adjacent sunlit areas may be thousands of times greater than the luminance in the shadow parts of the scene. To create a realistic rendering of such a scene can require a display having a dynamic range in excess of 1000:1. The term “high dynamic range” means dynamic ranges of 800:1 or more. 
     Modern digital imaging systems are capable of capturing and recording digital representations of scenes in which the dynamic range of the scene is preserved. Computer imaging systems are capable of synthesizing images having high dynamic ranges. However, current display technology is not capable of rendering images in a manner which faithfully reproduces high dynamic ranges. 
     Blackham et al., U.S. Pat. No. 5,978,142 discloses a system for projecting an image onto a screen. The system has first and second light modulators which both modulate light from a light source. Each of the light modulators modulates light from the source at the pixel level. Light modulated by both of the light modulators is projected onto the screen. 
     Gibbon et al., PCT application No. PCT/US01/21367 discloses a projection system which includes a pre modulator. The pre modulator controls the amount of light incident on a deformable mirror display device. A separate pre-modulator may be used to darken a selected area (e.g. a quadrant). 
     There exists a need for cost effective displays capable of reproducing a wide range of light intensities in displayed images. 
     SUMMARY OF THE INVENTION 
     This invention provides displays for displaying images. One embodiment of the invention provides a display comprising: a light source comprising an array of light-emitting elements. Each of the elements has a controllable light output; and, a spatial light modulator comprising a plurality of controllable elements located to modulate light from the light source. A diffuser directs light from the light source which has been modulated by the spatial light modulator to a viewing area. 
     Another aspect of the invention provides a display comprising: a spatial light modulator comprising an array of controllable elements, each of the controllable elements providing a controllable light transmission; a light source comprising an array of solid state light-emitting elements each located to illuminate a plurality of corresponding controllable elements of the spatial light modulator and each having a controllable light output; and, a diffuser. Brightness of a point on the diffuser may be controlled by controlling the light output of one of the light-emitting elements corresponding to the point and controlling the light transmission of one of the controllable elements corresponding to the point. 
     A further aspect of the invention provides a display comprising: light provision means for providing light spatially modulated at a first spatial resolution; spatial modulation means for further spatially modulating the light at a second resolution different from the first resolution; and, means for controlling the first and second spatial modulation means to display an image defined by image data. 
     The invention also provides a method for displaying an image. The method comprises controlling an array of individually-controllable light-emitting elements to have brightnesses determined by a first set of image data; illuminating a face of a spatial light modulator with light from the array of light-emitting elements, the spatial light modulator comprising an array of elements, each of the elements having a controllable transmissivity; and, controlling the transmissivity of the elements of the spatial light modulator with a second set of image data. 
     Further aspects of the invention and features of specific embodiments of the invention are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate non-limiting embodiments of the invention, 
         FIG. 1  is a schematic illustration of a display according to one embodiment of the invention; 
         FIG. 1A  is a schematic illustration of a specific implementation of the display of  FIG. 1 ; 
         FIG. 2  is a schematic illustration of a display according to an alternative embodiment of the invention comprising four spatial light modulators; 
         FIG. 3  is a schematic illustration of a rear-projection-type display according to a further embodiment of the invention; 
         FIG. 4  is a schematic illustration of a front-projection-type display according to a still further embodiment of the invention; 
         FIG. 5  is a drawing illustrating a possible relationship between pixels in a higher-resolution spatial light modulator and pixels in a lower-resolution spatial light modulator in a display according to the invention; 
         FIG. 5A  illustrates an effect of providing one light modulator which has lower resolution than another light modulator; 
         FIG. 6  is a schematic illustration of a front-projection-type color display having an alternative projector construction; 
         FIGS. 6A and 6B  are expanded cross-sectional views of portions of the front-projection screen of the color display of  FIG. 6 ; 
         FIG. 7  is a graph illustrating how light imaged onto a higher-resolution light modulator from pixels of a lower-resolution light modulator can overlap to yield a smooth variation in light intensity with position; 
         FIG. 7A  is a graph illustrating how the variation in light intensity with position for the image of a pixel of a light modulator can be represented as the convolution of a square profile and a spread function; 
         FIG. 8  is a schematic cross-section of a display according to an alternative embodiment of the invention and  FIG. 8A  is a schematic front view thereof; 
         FIG. 8B  is a schematic cross section of a display in which a spatial light modulator is spaced in front of an array of light sources; 
         FIG. 8C  is a schematic view of a display having a grid interposed between an array of light sources and a spatial light modulator; 
         FIG. 8D  is an isometric view of a hexagonal grid; 
         FIG. 8E  is a schematic representation of one channel through a grid illustrating reflected and non-reflected light components impinging on a spatial light modulator; 
         FIG. 8F  is a graph showing how reflected and non-reflected light components can sum to provide improved uniformity of illumination; 
         FIG. 8G  is a schematic representation of a display wherein internally reflecting members which form a grid are formed integrally with the material encapsulating LEDs; 
         FIGS. 9A and 9B  illustrate two possible configurations for an array of light emitting elements which could be used in the embodiment of  FIG. 8 ; 
         FIG. 9C  illustrates the use of light barriers to provide increased sharpness; 
         FIG. 10  is a schematic illustration of a projection-type display according to an alternative embodiment of the invention; 
         FIG. 11  is a block diagram of a calibration mechanism; 
         FIG. 11A  is a depiction of an LED illustrating paths by which stray light exits the LED; and, 
         FIGS. 11B, 11C, 11D and 11E  are schematic diagrams of alternative calibration mechanisms. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     This invention provides displays capable of rendering images with high dynamic ranges. Displays according to the invention comprise two light modulating stages. Light passes through the stages in series to provide an image which has an increased dynamic range. 
       FIG. 1  illustrates schematically a display  10  according to a simple embodiment of the invention. The sizes of elements and distances between them in  FIG. 1  are not to scale. Display  10  comprises a light source  12 . Light source  12  may, for example, comprise a projection lamp such as an incandescent lamp or an arc lamp, a laser, or another suitable source of light. Light source  12  may comprise an optical system comprising one or more mirrors, lenses or other optical elements which cooperate to deliver light to the rest of display  10 . 
     In the illustrated embodiment, light from light source  12  is directed toward a first light modulator  16 . Light source  12  preferably provides substantially uniform illumination of first light modulator  16 . Light modulator  16  comprises an array of individually addressable elements. Light modulator  16  may comprise, for example, a LCD (liquid crystal display), which is an example of a transmission-type light modulator or a DMD (deformable mirror device), which is an example of a reflection-type light modulator. Display driver circuitry (not shown in  FIG. 1 ) controls the elements of light modulator  16  according to data which defines an image being displayed. 
     Light which has been modulated by first light modulator  16  is imaged onto a rear-projection screen  23  by a suitable optical system  17 . Light from a small area of first light modulator  16  is directed by optical system  17  to a corresponding area on rear-projection screen  23 . In the illustrated embodiment, optical system  17  comprises a lens having a focal length f In general, the optical system  17  which images light modulated by first light modulator  16  onto rear-projection screen  23  may comprise one or more mirrors, lenses or other optical elements. Such an optical system has the function of imaging light modulated by the first light modulator onto a second light modulator. Optical system  17  may be termed an imaging means. 
     In the illustrated embodiment, rear-projection screen  23  comprises a second light modulator  20  and a collimator  18 . A main function of collimator  18  is to cause light which passes through rear-projection screen  23  to be directed preferentially toward a viewing area. Collimator  18  may comprise a Fresnel lens, a holographic lens, or, in the alternative, another arrangement of one or more lenses and/or other optical elements which operate to guide light in the direction of a viewing area. 
     In the illustrated embodiment, collimator  18  causes light to travel through the elements of second light modulator  20  in a direction which is generally normal to screen  23 . As light incident from collimator  18  travels through second light modulator  20  it is further modulated. The light then passes to a diffuser  22  which scatters the outgoing light through a range of directions so that a viewer located on an opposite side of diffuser  22  from first light modulator  16  can see light originating from the whole area of screen  23 . In general, diffuser  22  may scatter light to a different angular extent in the horizontal and vertical planes. Diffuser  22  should be selected so that light modulated by second light modulator  20  is scattered through a range of angles such that the maximum scatter angle is at least equal to the angle subtended by screen  23  when viewed from a desired viewing location. 
     Rear-projection screen  23  may differ in area from first light modulator  16 . For example, rear-projection screen  23  may be larger in area than first light modulator  16 . Where this is the case, optical system  17  expands the beam of light modulated by first light modulator  16  to illuminate a corresponding area on rear-projection screen  23  which is larger than first light modulator  16 . 
     Second light modulator  20  may be of the same type as first light modulator  16  or a different type. Where first and second light modulators  16  and  20  are both of types that polarize light, second light modulator  20  should, as much as is practical, be oriented so that its plane of polarization matches that of the light incident on it from first light modulator  16 . 
     Display  10  may be a color display. This may be achieved in various ways including:
         making one of first light modulator  16  and second light modulator  20  a color light modulator;   providing a plurality of different first light modulators  16  operating in parallel on different colors; and,   providing a mechanism for rapidly introducing different color filters into the light path ahead of second light modulator  20 .
 
As an example of the first approach above, second light modulator  20  may comprise an LCD panel having a plurality of pixels each comprising a number of colored sub-pixels. For example, each pixel may comprise three sub-pixels, one associated with a red filter, one associated with a green filter and one associated with a blue filter. The filters may be integral with the LCD panel.
       

     As shown in  FIG. 1A , light source  12 , first light modulator  16  and optical system  17  may all be parts of a digital video projector  37  located to project an image defined by a signal  38 A from a controller  39  onto the back side of rear-projection screen  23 . The elements of second light modulator  20  are controlled by a signal  38 B from controller  39  to provide an image to a viewer which has a high dynamic range. 
     Controller  39  may comprise any suitable data processor. Controller  39  may comprise one or more microprocessors running suitable control software together with interfaces which permit controller  39  to control the operation of a display according to the invention. The general construction of such controllers and general techniques for programming such controllers to provide desired functions are well known to those skilled in the art and will not be described in detail herein. 
     As shown in  FIG. 2 , a display  10 A according to the invention may comprise one or more additional light modulation stages  24 . Each additional light modulation stage  24  comprises a collimator  25 , a light modulator  26  and an optical system  27  which focuses light from light modulator  26  onto either the next additional light modulation stage  24  or on collimator  18 . In device  10 A of  FIG. 2  there are two additional light modulation stages  24 . Devices according to this embodiment of the invention may have one or more additional light modulation stages  24 . 
     The luminance of any point on output diffuser  22  can be adjusted by controlling the amount of light passed on by corresponding elements of light modulators  16 ,  20  and  26 . This control may be provided by a suitable control system (not shown in  FIG. 2 ) connected to drive each of light modulators  16 ,  20  and  26 . 
     As noted above, light modulators  16 ,  20  and  26  may all be of the same type or may be of two or more different types.  FIG. 3  illustrates a display  10 B according to an alternative embodiment of the invention which includes a first light modulator  16 A which comprises a deformable mirror device. A deformable mirror device is a “binary” device in the sense that each pixel may be either “on” or “off”. Different apparent brightness levels may be produced by turning a pixel on and off rapidly. Such devices are described, for example, in U.S. Pat. Nos. 4,441,791 and 4,954,789 and are commonly used in digital video projectors. Light source  12  and first light modulator  16  (or  16 A) may be the light source and modulator from a commercial digital video projector, for example. 
       FIG. 4  illustrates a front-projection-type display  10 C according to the invention. Display  10 C comprises a screen  34 . A projector  37  projects an image  38  onto screen  34 . Projector  37  comprises a suitable light source  12 , a first light modulator  16  and an optical system  17  suitable for projecting an image defined by first light modulator  16  onto screen  34 . Projector  37  may comprise a commercially available display projector. Screen  34  incorporates a second light modulator  36 . Second light modulator  36  comprises a number of addressable elements which can be individually controlled to affect the luminance of a corresponding area of screen  34 . 
     Light modulator  36  may have any of various constructions. For example, light modulator  36  may comprise an array of LCD elements each having a controllable transmissivity located in front of a reflective backing. Light projected by projector  37  passes through each LCD element and is reflected back through the LCD element by the reflective backing. The luminance at any point on screen  34  is determined by the intensity of light received at that point by projector  37  and the degree to which light modulator  36  (e.g. the LCD element at that point) absorbs light being transmitted through it. 
     Light modulator  36  could also comprise an array of elements having variable retro-reflection properties. The elements may be prismatic. Such elements are described, for example, in Whitehead, U.S. Pat. No. 5,959,777 entitled Passive High Efficiency Variable Reflectivity Image Display Device and, Whitehead et al., U.S. Pat. No. 6,215,920 entitled Electrophoretic, High Index and Phase Transition Control of Total Internal Reflection in High Efficiency Variable Reflectivity Image Displays. 
     Light modulator  36  could also comprise an array of electrophoretic display elements as described, for example, in Albert et al., U.S. Pat. No. 6,172,798 entitled Shutter Mode Microencapsulated Electrophoretic Display; Comiskey et al., U.S. Pat. No. No. 6,120,839 entitled Electro-osmotic Displays and Materials for Making the Same; Jacobson, U.S. Pat. No. 6,120,588 entitled: Electronically Addressable Microencapsulated Ink and Display; Jacobson et al., U.S. Pat. No. 6,323,989 entitled Electrophoretic Displays Using Nanoparticles; Albert, U.S. Pat. No. 6,300,932 entitled Electrophoretic Displays with Luminescent Particles and Materials for Making the Same or, Comiskey et al., U.S. Pat. No. 6,327,072 entitled Microcell Electrophoretic Displays. 
     As shown in  FIGS. 6A and 6B , screen  34  preferably comprises a lens element  40  which functions to direct light preferentially toward the eyes of viewers. In the illustrated embodiment, lens element  40  comprises a Fresnel lens having a focal point substantially coincident with the apex of the cone of light originating from projector  37 . Lens element  40  could comprise another kind of lens such as a holographic lens. Lens element  40  incorporates scattering centers  45  which provide a desired degree of diffusion in the light reflected from screen  34 . In the illustrated embodiment, second light modulator  36  comprises a reflective LCD panel having a large number of pixels  42  backed by a reflective layer  43  and mounted on a backing  47 . 
     Where light modulator  36  comprises an array of elements having variable retro-reflection properties, the elements themselves could be designed to direct retro-reflected light preferentially in a direction of a viewing area in front of screen  34 . Reflective layer  43  may be patterned to scatter light to either augment the effect of scattering centers  45  or replace scattering centers  45 . 
     As shown in  FIG. 4 , a controller  39  provides data defining image  38  to each of first light modulator  16  and second light modulator  36 . Controller  39  could comprise, for example, a computer equipped with a suitable display adapter. The luminance of any point on screen  34  is determined by the combined effect of the pixels in first light modulator  16  and second light modulator  36  which correspond to that point. There is minimum luminance at points for which corresponding pixels of the first and second light modulators are set to their “darkest” states. There is maximum luminance at points for which corresponding pixels of the first and second light modulators are set to their “brightest” states. Other points have intermediate luminance values. The maximum luminance value might be, for example, on the order of 10 5  cd/m 2 . The minimum luminance value might be, for example on the order of 10 −2  cd/m 2 . 
     The cost of a light modulator and its associated control circuitry tends to increase with the number of addressable elements in the light modulator. In some embodiments of the invention one of the light modulators has a spatial resolution significantly higher than that of one or more other ones of the light modulators. When one or more of the light modulators are lower-resolution devices the cost of a display according to such embodiments of the invention may be reduced. In color displays comprising two or more light modulators, one of which is a color light modulator (a combination of a plurality of monochrome light modulators may constitute a color light modulator as shown, for example, in  FIG. 6 ) and one of which is a higher-resolution light modulator, the higher-resolution light modulator should also be the color light modulator. In some embodiments the higher- resolution light modulator is imaged onto the lower-resolution light modulator. In other embodiments the lower- resolution light modulator is imaged onto the higher-resolution light modulator. 
       FIG. 5  illustrates one possible configuration of pixels in a display  10  as shown in  FIG. 1 . Nine pixels  42  of a second light modulator  20  correspond to each pixel  44  of a first light modulator  16 . The number of pixels  42  of second light modulator  20  which correspond to each pixel  44  of first light modulator  16  may be varied as a matter of design choice. Pixels  44  of the higher-resolution one of first and second light modulators  16  and  20  (or  36 ) should be small enough to provide a desired overall resolution. In general there is a trade off between increasing resolution and increasing cost. In a typical display the higher-resolution light modulator will provide an array of pixels having at least a few hundred pixels in each direction and more typically over 1000 pixels in each direction. 
     The size of pixels  42  of the lower-resolution one of the first and second light modulators determines the scale over which one can reliably go from maximum intensity to minimum intensity. Consider, for example,  FIG. 5A  which depicts a situation where one wishes to display an image of a small maximum-luminance spot on a large minimum-luminance background. To obtain maximum luminance in a spot  47 , those pixels of each of the first and second light modulators which correspond to spot  47  should be set to their maximum-luminance values. Where the pixels of one light modulator are lower in resolution than pixels of the other light modulator then some pixels of the lower-resolution light modulator will straddle the boundary of spot  47 . This is the case, for example, in  FIG. 5A . 
     Outside of spot  47  there are two regions. In region  48  it is not possible to set the luminance to its minimum value because in that region the lower-resolution light modulator is set to its highest luminance value. In region  49  both of the light modulators can be set to their lowest-luminance values. If, for example, each of the first and second light modulators has a luminance range of 1 to 100 units, then region  47  might have a luminance of 100×100=10,000 units, region  48  would have a luminance of 100×1=100 units and region  49  would have a luminance of 1×1=1 units. 
     As a result of having one of the light modulators lower in resolution than the other, each pixel of the lower-resolution light modulator corresponds to more than one pixel in the higher-resolution light modulator. It is not possible for points corresponding to any one pixel of the lower-resolution light modulator and different pixels of the higher-resolution light modulator to have luminance values at extremes of the device&#39;s dynamic range. The maximum difference in luminance between such points is determined by the dynamic range provided by the higher-resolution light modulator. 
     It is generally not a problem that a display is not capable of causing closely-spaced points to differ in luminance from one another by the full dynamic range of the display. The human eye has enough intrinsic scatter that it is incapable of appreciating large changes in luminance which occur over very short distances in any event. 
     In a display according to the invention which includes both a lower-resolution spatial light modulator and a higher-resolution spatial light modulator, controller  39  may determine a value for each pixel of the lower-resolution spatial light modulator and adjust the signals which control the higher-resolution spatial light modulator to reduce artefacts which result from the fact that each pixel of the lower-resolution spatial light modulator is common to a plurality of pixels of the higher-resolution spatial light modulator. This may be done in any of a wide number of ways. 
     To take but one example, consider the case where each pixel of the lower-resolution spatial light modulator corresponds to a plurality of pixels of the higher-resolution spatial light modulator. Image data specifying a desired image is supplied to the controller. The image data indicates a desired luminance for an image area corresponding to each of the pixels of the higher-resolution spatial light modulator. The controller may set the pixels of the lower-resolution light modulator to provide an approximation of the desired image. This could be accomplished, for example, by determining an average or weighted average of the desired luminance values for the image areas corresponding to each pixel of the lower-resolution display. 
     The controller may then set the pixels of the higher-resolution display to cause the resulting image to approach the desired image. This could be done, for example, by dividing the desired luminance values by the intensity of light incident from the lower-resolution light modulator on the corresponding pixels of the higher-resolution light modulator. The intensity of light incident from the lower-resolution light modulator on a pixel of the higher-resolution light modulator can be computed from the known way that light from each pixel of the lower resolution spatial light modulator is distributed on the higher resolution spatial light modulator. The contributions from one or more of the pixels of the lower resolution spatial light modulator can be summed to determine the intensity with which any pixel of the higher resolution spatial light modulator will be illuminated for the way in which the pixels of the lower resolution spatial light modulator are set. 
     If the low-resolution pixels are too large then a viewer may be able to discern a halo around bright elements in an image. The low resolution pixels are preferably small enough that the appearance of bright patches on dark backgrounds or of dark spots on bright backgrounds is not unacceptably degraded. It is currently considered practical to provide in the range of about 8 to about 144, more preferably about 9 to 36, pixels on the higher-resolution light modulator for each pixel of the lower-resolution light modulator. 
     The sizes of steps in which each of pixels  42  and  44  can adjust the luminance of point(s) on the image are not necessarily equal. The pixels of the lower-resolution light modulator may adjust light intensity in coarser steps than the pixels of the higher-resolution light modulator. For example, the lower-resolution light modulator may permit adjustment of light intensity for each pixel over an intensity range of 1 to 512 units in 8 steps while the higher-resolution light modulator may permit adjustment of the light intensity for each pixel over a similar range in 512 steps. While pixels  42  and  44  are both illustrated as being square in  FIG. 5 , this is not necessary. Pixels  42  and/or  44  could be other shapes, such as rectangular, triangular, hexagonal, round, or oval. 
     The pixels of the lower-resolution light modulator preferably emit light which is somewhat diffuse so that the light intensity varies reasonably smoothly as one traverses pixels of the lower-resolution light modulator. This is the case where the light from each of the pixels of the lower-resolution light modulator spreads into adjacent pixels, as shown in  FIG. 7 . As shown in  FIG. 7A , the intensity profile of a pixel in the lower-resolution light modulator can often be approximated by gaussian spread function convolved with a rectangular profile having a width d 1  equal to the active width of the pixel. The spread function preferably has a full width at half maximum in the range of 0.3×d 2  to 3×d 2 , where d 2  is the center-to-center inter-pixel spacing, to yield the desired smoothly varying light intensity. Typically d 1  is nearly equal to d 2 . 
     In the embodiment of  FIG. 5 , each pixel  42  comprises three sub pixels  43 R,  43 G and  43 B (for clarity  FIG. 5  omits sub pixels for some pixels  42 ). Sub-pixels  43 R,  43 G and  43 B are independently addressable. They are respectively associated with red, green and blue color filters which are integrated into second light modulator  20 . Various constructions of LCD panels which include a number of colored sub-pixels and are suitable for use in this invention are known in the art. 
     For front projection-type displays (for example the display  10 C of  FIG. 4 ), it is typically most practical for first light modulator  16  to comprise a high-resolution light modulator which provides color information and for light modulator  36  to comprise a monochrome light modulator. Light modulator  36  preferably has reasonably small addressable elements so that the boundaries of its elements do not form a visually distracting pattern. For example, light modulator  36  may have the same number of addressable elements as projector  37  (although each such element will typically have significantly larger dimensions than the corresponding element in light modulator  16  of projector  37 ). 
     Projector  37  may have any suitable construction. All that is required is that projector  37  be able to project light which has been spatially modulated to provide an image onto screen  34 .  FIG. 6  illustrates a display system  10 D according to a further alternative embodiment of the invention. System  10 D comprises a screen  34  which has an integrated light modulator  36  as described above with reference to  FIG. 4 . System  10 D comprises a projector  37 A which has separate light modulators  16 R,  16 G and  16 R for each of three colors. Light modulated by each of light modulators  16 R,  16 G and  16 R is filtered by a corresponding one of three colored filters  47 R,  47 G and  47 B. The modulated light is projected onto screen  34  by optical systems  17 . A single light source  12  may supply light to all three light modulators  16 R,  16 G, and  16 B, or separate light sources (not shown) may be provided. 
     In the embodiments described above, light from a light source is spatially modulated by a first light modulator and then imaged onto a second light modulator. The inventors have realized that the functions of the light source and first light modulator can be combined by providing a light source comprising an array of light-emitting elements which each have a controllable brightness. The light-emitting elements may be solid state devices. For example, the light-emitting elements may comprise light-emitting diodes (LEDs). Each of the LEDs may be driven by a driver circuit which allows the current flowing through the LED, and consequently the brightness of the light emitted by the LED, to be controlled. The controller may also, or in the alternative, control a duty cycle of the corresponding LED. As discussed below, the driving circuit may monitor current being delivered to each LED or each group of LEDs and may generate an error signal if the magnitude of the current being delivered to each LED or each group of LEDs has an unexpected value. Such error signals may be used by a controller to compensate for failed LEDs. 
     In a preferred embodiment of the invention, the LEDs are of a type which emit white light. For example, the LEDs may comprise an array of tri-color LEDs. Tri-color LEDs which each include red, green and blue LEDs all encapsulated within a single housing are commercially available. One or more white LEDs may be used to illuminate each group of pixels of the second light modulator. 
       FIG. 8  shows a section through a display  60  according to an embodiment of the invention in which a rear-projection screen  53  comprising a diffusing layer  22  is illuminated by an array  50  of LEDs  52 . The brightness of each LED  52  is controlled by a controller  39 . Screen  53  includes a light modulator  20 . The rear face of light modulator  20  is illuminated by LED array  50 .  FIG. 8A  is a schematic front view of a portion of display  60  for a case where controllable elements (pixels)  42  of light modulator  20  correspond to each LED  52 . Each of the controllable elements  42  may comprise a plurality of colored sub-pixels. 
     LEDs  52  may be arranged in any suitable manner in array  50 . Two likely arrangements of LEDs  52  are shown in  FIGS. 9A and 9B .  FIG. 9A  illustrates a rectangular array  50 A of light sources  51 .  FIG. 9B  illustrates a hexagonal array  50 B of light sources  51 . Light sources  51  may comprise LEDs  52 . Where light sources  51  comprise discrete devices, a regular spacing between light sources  51  may be maintained by packing light sources  51  together as illustrated in  FIG. 9A or 9B , for example. 
     A diffuser  22 A in conjunction with the light-emitting characteristics of LEDs  52  causes the variation in intensity of light from LEDs  52  over the rear face of light modulator  20  to be smooth. 
     A similar effect can be obtained without a diffuser  22 A by spacing light modulator  20  away from LEDs  52 . Where light modulator  20  is spaced away from LEDs  52 , light from each LED  52  can contribute to illuminating edges of the areas of spatial light modulator  20  corresponding to neighboring LEDs  52 . 
     In cases where it is necessary that the display be viewable through a large range of angles, such spacing can cause a parallax problem. Where a viewer is not viewing a display head-on, as shown in  FIG. 8B , the viewer may see a pixel of spatial light modulator  20  illuminated by an LED  52  which does not correspond to the pixel. For example, in  FIG. 8B , area  21 A corresponds to LED  52 A and area  21 B corresponds to LED  52 B. However, due to parallax, the viewer sees pixels in area  21 A as being illuminated by LED  52 B. 
       FIG. 8C  shows an alternative construction which avoids the parallax problem illustrated by  FIG. 8B . In  FIG. 8C , a grid  122  of reflective-walled channels  123  is disposed between array  50  and spatial light modulator  20 . In a preferred embodiment, channels  123  are hexagonal in cross section and grid  122  comprises a honeycomb structure as shown in  FIG. 8D . Channels  123  could also have other cross sectional shapes such as square, triangular, rectangular or the like. The walls which define channels  123  are preferably thin. Grid  122  could comprise, for example, a section of aluminum honeycomb material. 
     Channels  123  may be, but are not necessarily hollow. Channels  123  may be provided by columns of light-transmitting material having walls at which light is internally reflected, preferably totally internally reflected. The columns may be separated by thin air gaps or clad in one or more materials which provide an interface at which light is internally reflected. The columns may be integral with the material in which LEDs  52  are encapsulated.  FIG. 8G  shows an embodiment of the invention in which columns  123 A having internally reflecting walls are integrally formed with LEDs  52 C. Columns  123 A may have various cross sectional shapes such as hexagonal, triangular, square or the like. 
     Light from each LED  52  passes through a channel  123 . As shown in  FIG. 8E , some light from an LED passes straight through channel  123  and some light is reflected from reflective walls  124  of channel  123 . The luminance at a point on spatial light modulator  20  is contributed to by both reflected and non-reflected light. The reflected component tends to be more intense around the edges of channel  123  while the non-reflected component tends to be more intense toward the center of channel  123 . The result is that the uniformity with which each LED  52  illuminates the corresponding portion of spatial light modulator  20  is improved by the presence of grid  122 . The increase in uniformity is illustrated in  FIG. 8F . 
     Grid  122  is spaced slightly away from spatial light modulator  20  by a gap  57  (see  FIGS. 8C and 8E ) to avoid perceptible shadows cast by the walls which separate adjacent channels  123  of grid  122 . 
     The geometry of channels  123  may be varied to achieve design goals. The width of each channel  123  largely determines the resolution with which the intensity of light falling on spatial light modulator  20  can be varied. For a given channel width and cross sectional shape, the uniformity of illumination provided by each channel  123  can be increased by making the channel  123  longer. This, however, reduces the efficiency with which light is passed to spatial light modulator  20 . 
     A reasonable trade off between efficiency and uniformity of illumination may be achieved by providing channels  123  which have lengths L such that near the channel edges non-reflected and once-reflected light components are each approximately half of the intensity of the non-reflected component on the axis of LED  52 . One way to approximately achieve this is to choose length L such that the angle ✓ between the axis of LED  52  and the edge of channel  123  is equal to the half angle ✓ 1/2  of the LED  52 . The half angle is the angle at which the illumination provided by LED  52  has an intensity equal to one half of the intensity of illumination in a forward direction on the axis of LED  52 . This condition is provided by making L satisfy the condition of equation (1), where R is the half-width of channel  123 . 
     
       
         
           
             
               
                 
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     It is generally desirable to provide one channel  123  for each LED or other light source. In some embodiments of the invention each channel  123  has a plurality of LEDs. In one embodiment of the invention each channel  123  has three LEDs of different colors, for example, red, green and blue. In such embodiments it is important that the channel  123  be long enough that light from each of the LEDs be uniformly distributed at spatial light modulator  20  as the human eye is sensitive to variations in color. 
     As described above, with reference to  FIGS. 7 and 7A , light modulator  20  is preferably illuminated in a manner such that the illumination of light modulator  20  by LED array  50  changes smoothly with position on light modulator  20 . This can be accomplished by providing LEDs  52  in LED array  50  which emit light in patterns which overlap somewhat on light modulator  20 . The light emitted by each LED  52  may be characterized by a spread function such that the variation of the intensity of light from an LED  52  incident on light modulator  20  is the convolution of a rectangular profile and the spread function. The spread function preferably has a full width at half maximum in the range of 0.3×d 2  to 3×d 2 , where d 2  is the center-to-center spacing on light modulator  20  between the illumination patterns of adjacent LEDs  52  on light modulator  20 . A diffuser  22 A (shown in dashed lines  FIG. 8 ) may be interposed between array  50  and light modulator  20  to broaden the illumination patterns of LEDs  52  on light modulator  20 . 
     For some applications it may be desirable to provide a display on which the level of illumination of closely spaced pixels may be greatly different. This may be achieved, at the cost of some smoothness, by confining light originating from each of the light sources of array  50  so that the illumination patterns of adjacent light sources on light modulator  20  do not overlap significantly. This may be achieved, for example, by providing light barriers  56  which limit the spread of light from each of the light sources of array  50  as shown in  FIG. 9C . With light barriers  56 , each light source of array  50  illuminates only corresponding pixels of light modulator  20 . This may also be achieved by providing light sources  52  which project substantially non-overlapping illumination patterns onto light modulator  20 . In either case, the resulting image displayed to a viewer may appear somewhat sharper than in embodiments wherein light from each light source  52  is permitted to spread sufficiently that it provides significant illumination to some pixels corresponding to adjacent light sources. In many cases, limitations of the human eye will make this increased level of sharpness unnoticeable. 
     Light modulator  20  may be a monochrome light modulator. In the alternative, light modulator  20  may be a high resolution color light modulator. Light modulator  20  may comprise, for example, a LCD array. Display  60  can be quite thin. For example, display  60  may be 10 centimeters or less in thickness. 
       FIG. 10  shows a projection-type display  70  which is similar to display  60  of  FIG. 8  except that an array  50  of light sources  52  is imaged onto a light modulator  20  by a suitable optical system  17 . 
     A controller  39  may control the elements of array  50  to provide a low-resolution version of an image to be displayed on spatial light modulator  20 . Controller  39  may control the elements of spatial light modulator  20  to supply features having a high spatial resolution and to otherwise correct the image provided by array  50  as described above. 
     One problem with using LEDs  52  as light sources in a high resolution high quality display is that the brightness of light emitted at a specific current level can vary significantly between individual LEDs. This variation is due to manufacturing process variations. Further, the brightness of light that a LED  52  will produce tends to slowly decrease in an unpredictable manner as the LED ages. It is therefore desirable to provide a mechanism for calibrating an LED array  50  to compensate for differences in brightness between different LEDs  52  in array  50 . 
     One calibration mechanism  78  which is illustrated schematically in  FIG. 11  provides a light detector  80  which detects light emitted by each of LEDs  52 . Light detector  80  may be moved into different positions for capturing light from different LEDs  52 . In the alternative, a suitable optical system may be provided to direct light from LEDs  52  to light detector  80 . Controller  39  receives a signal  81  from light detector  80 . Signal  81  indicates the brightness of light emitted by each LED  52  in array  50  for a given current. If the brightness of light emitted by an LED  52  differs from a desired value then controller  39  determines a correction to be applied to the current applied to each LED  52 . Controller  39  subsequently applies the correction. Calibration mechanism  78  may be used for initial calibration of a display. Calibration mechanism  78  may optionally include a calibration controller  39 A which performs some calibration tasks, such as determining a correction to be applied to the current applied to each LED  52 , and making the resulting calibration information available to controller  39 . 
     It is desirable to provide a calibration mechanism that does not interfere with the normal operation of a display. One way to achieve this is to detect light which is emitted by an LED in a direction other than the forward direction.  FIG. 11A  shows a typical LED  52 . Most light emitted by LED  52  is directed in a forward direction as shown by arrow  55 A. A very small fraction of the light emitted by each LED  52  is emitted sideways as indicated by arrows  55 B or rearwardly as indicated by arrow  55 C. Light emitted in a direction other than the forward direction may be termed “stray light”. One or more light detectors  80 A may be located to detect stray light from each LED  52 . 
     A calibration mechanism  90  according to one embodiment of the invention is shown in  FIG. 11B . In calibration mechanism  90 , small optical waveguides  82  carry stray light from LEDs  52  to a light detector  80 . Only a small fraction of the light emitted by each LED  52  is captured by waveguides  82 . As long as the coupling between a waveguide  82  and the corresponding LED  52  does not change, the proportion of the light emitted by an LED  52  which is captured by waveguide  82  remains constant. One light detector  80 A or a few light detectors  80 A may be located at convenient locations such as at edges of array  50 . 
       FIG. 11C  shows a calibration mechanism  90 A according to another embodiment of the invention. In mechanism  90 A, individual optical waveguides  82  are replaced by a planar optical waveguide  82 A. Power leads for LEDs  52  pass through holes  83  in waveguide  82 A. One or more light detectors  80 A are located at edges of optical waveguide  82 A. Light emitted in the rearward direction by any of LEDs  52  is trapped within optical waveguide  82 A and detected by light detector(s)  80 A. 
       FIG. 11D  shows another optical calibration mechanism  90 B wherein a planar optical waveguide  82 B collects light emitted by LEDs  52  in sideways directions and carries that light to one or more light detectors  80 A. 
       FIG. 11E  shows another optical calibration mechanism  90 C wherein a planar optical waveguide  82 C collects a small fraction of the light emitted by LEDs  52  in the forward direction and carries that light to one or more light detectors  80 A. Waveguide  82 C is constructed so that some light passing through it in the forward direction is trapped in waveguide  82 C and carried to light detector(s)  80 A. To achieve this, one surface of waveguide  82 C, typically the surface facing LEDs  52  may be roughened slightly to scatter some light generally into the plane of waveguide  82 C or some scattering centers may be provided in the material of waveguide  82 C. In the illustrated embodiment, waveguide  82 C acts as a spacer which maintains a gap  57  between a grid  122  and spatial light modulator  20 . Calibration mechanism  80 C has the advantage that optical waveguide  82 C does not need to be penetrated by holes  83  which can interfere with the propagation of light to light detector(s)  80 A. 
     In operation, an array  50  is first factory calibrated, for example, with a calibration mechanism  78  ( FIG. 11 ). After, or during, factory calibration LEDs  52  are turned on one at a time with current at a calibration level. Light detector(s)  80 A are used to measure stray light for each LED  52 . Information about the amount of stray light detected for each LED  52  may be stored as a reference value. Over the life of LED array  50 , mechanism  90  can be used to monitor the brightness of each LED  52 . Depending upon the application, such brightness measurements may be made at times when the display is initialized or periodically while the display is in use. Brightness measurements of one or more LEDs  52  may be made in intervals between the display of successive image frames. 
     If mechanism  90  detects that the brightness of an LED  52  has changed over time (typically as indicated by a decrease in the amount of stray light detected by light detector(s)  80 A in comparison to the stored reference value) then controller  39  can automatically adjust the current provided to that LED  52  to compensate for its change in brightness. 
     A calibration mechanism  90  can also be used to detect failures of LEDs  52 . Although LEDs  52  tend to be highly reliable they can fail. Calibration mechanism  90  can detect failure of an LED  52  by detecting no light from LED  52  when controller  39  is controlling LED  52  to be “ON”. Certain failure modes of an LED  52  or a row of LEDs  52  may also be detected by LED driving electronics associated with controller  39 . If the driving electronics detect that no current, or a current having an unexpected value, is being delivered at a time when current should be passing through one or more LEDs  50  then the driving electronics may generate an error signal detectable by controller  39 . 
     Where controller  39  detects a failure of one or more LEDs  52 , controller  39  may compensate for the failure(s) by increasing brightness of one or more neighboring LEDs  52 , adjusting the elements of spatial light modulator  20  which correspond to the failed LED  52  to provide greater light transmission, or both. In fault tolerant displays according to this embodiment of the invention, after failure of an LED  52 , spill over light from adjacent LEDs  52  illuminates the area corresponding to the failed LED  52  sufficiently to make the image visible in the area. 
     Where controller  39  is configured to increase the brightness of neighboring LEDs  52 , controller  39  may determine the amount of increase based in part upon the image content of the area of spatial light modulator  20  corresponding to the failed LED. If the image content calls for the area to be bright then the brightness of neighboring LEDs may be increased more than if the image content calls for the area to be dark. The resulting image quality will be degraded but catastrophic failure will be avoided. 
     In some embodiments of the invention each LED  52  is dimmed or turned off during those times when the corresponding elements of spatial light modulator are being refreshed. Some spatial light modulators refresh slowly enough that the refresh can be perceived by a viewer. This causes an undesirable effect called “motion blur”. 
     With proper timing, at those times when each row of spatial light modulator  20  is being refreshed, corresponding LEDs  52  can be off or dimmed At other times the corresponding LEDs  52  can be overdriven sufficiently that a viewer perceives a desired brightness. The viewer&#39;s eye cannot perceive rapid flickering of LEDs  52 . Instead, the viewer perceives an average brightness. It is typically desirable to multiplex the operation of LEDs  52 . Where LEDs are operated in a multiplexed manner, correcting for motion blur can be performed by synchronizing the multiplexing of LEDs  52  with the refreshing of spatial light modulator  52 . 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
         diffuser  22  and collimator  18  could be combined with one another;   diffuser  22  and collimator  18  could be reversed in order;   multiple cooperating elements could be provided to perform light diffusion and/or collimation;   the function of diffuser  22  could be provided by another element which both diffuses light and performs some other function. In such cases, the other element may be said to comprise a diffuser and an apparatus comprising such an element comprises a diffuser;   the order in screen  23  of second light modulator  20  collimator  18  and diffuser  22  could be varied;   the signal  38 A driving first light modulator  16  may comprise the same data driving second light modulator  20  or may comprise different data.   Instead of or in addition to providing measuring light output for fixed calibration currents, calibration mechanisms  78  and/or  90  could adjust current to a LED  52  until the LED  52  provides a desired brightness.
 
Accordingly, the scope of the invention includes, but is not limited to, the substance defined by the following claims.