Abstract:
1266-03028 A color display system includes a color light separator that separates incident white illumination light into red, green and blue wavelength bands to be directed to distinct color component sub-pixels (sometimes called dots) that are arranged in a dot-matrix, color triad arrangement (e.g., stripe or delta) to form individual picture elements (pixels) on a pixelated electronic image device (e.g., LCD of DMD). The entire picture is optically shifted from one set of color component sub-pixels to another in a 3-field sequence. As a result, the sets of red, green and blue color component sub-pixels appear to an observer as a single full-color image, thereby providing a dot sequential color display.

Description:
SUMMARY OF INVENTION  
         [0001]    There are many ways to produce a full color matrix-addressed display, but almost all methods require 3 independent elements of Red, Green and Blue coloration, so as to be able to mix (in the additive color method) each primary in variable ratios to be able to cover the entire color gamut.  
           [0002]    One method that does not need 3 simultaneous primary color elements is referred to as field-sequential color. In this method an imaging device is illuminated with just one color primary at a time. One can envision a color-wheel that filters the white light and allows passage of first red, then green, then blue light, to be directed onto the imager, and if the sequencing is fast enough the human eye will integrate these R, G, B subimages into one full color image. In the early days of television it was proposed to use field-color sequential display, with a rotating color wheel in front of a white (broadband) emissive display (e.g. CRT) and cycle between red, green, and blue color subimages, and let the human eye-brain synthesize these 3 primary color images to one full-color image.)Very recently the use of field-sequential color has been revived, with the Texas Instruments Digital Micromirror Device (DMD), wherein a single proprietary (expensive) DMD chip can be used with a quickly rotating color wheel to create 180 color fields per second and therefore 60 full-color frames per second. In order to alleviate the flickering effect of this sequential color display, the color wheel might rotate still faster, to provide 120 full-color frames per second.  
           [0003]    The major drawback of this type of display is that one-third of the light emanating from the display (in the case of a single-chip DMD, transmitted through the color wheel and reflected off of the micromirror elements) is used at any one time, and due to a “dead-band” required between the color segments (to prevent any color cross-talk) this may be further reduced to around 30% light utilization efficiency.  
           [0004]    Whereas all displays appear to be improved with increased brightness, this potential 30% efficiency is a serious detriment.  
           [0005]    Another recent attempt at producing a color-sequential device concentrates three bands of light (R, G, and B stripes) onto a single display and “scrolls” these colors dynamically to produce a higher efficiency. The drawback of this “scrolling color” method is the bulkiness of a scanner to scroll the illumination and awkwardness of addressing the imager in a somewhat arbitrary (random access) manner as opposed to simple progressive line manner.  
           [0006]    Shimizu of N. Am. Philips Labs has presented papers at display conferences on this idea that promises to use nearly 100% of the light, but requires that three sub-bands of R, G and B light be incident on the single imaging device and (barber-pole-wise) scrolled so that just as the B (e.g.) light segment finishes lighting the bottom of the display, it is starting to light up the top of the display and G and R (e.g.) are right behind in continuous sequence. This is a very difficult optical task, to “scroll” the three separate light source color bands onto the LCD.  
           [0007]    This invention utilizes all of the light all of the time, i.e. the light utilization efficiency may be nearly 100% (disregarding typical light collection losses which all systems have to some extent).  
           [0008]    In the present invention includes a color separation means so that R, G and B wavelength bands are directed to distinct color component sub-pixels (sometimes called dots) that are arranged in a dot-matrix, color triad arrangement (e.g., stripe or delta) to form individual picture elements (pixels). The entire picture is optically shifted from one set of color component sub-pixels to another in a 3-field sequence. As a result, the sets of R, G, and B color component sub-pixels appear to an observer as a single full-color image. This invention is sometimes called a dot sequential color display.  
           [0009]    This small shifting of the picture must take place rapidly (i.e. at a fast field rate) but will be much less noticeable than the complete change of color that accompanies the typical field sequential color display. The dot sequential display of this invention differs from a field sequential display in that the former uses different sub-pixels or dots for each color component, whereas the latter successively uses the same pixels for each color component. The field sequential display suffers from a macro-color flicker effect that is very noticeable unless the field rate is much higher than 180 Hz.  
           [0010]    In accordance with a preferred embodiment of the invention, we use a pixellated display (typically a liquid crystal display device, LCD) at a particular resolution, for example 900×600 dots and from this “monochrome” device we create a full color display.  
           [0011]    First we create a color display, with one-third the resolution, e.g. 300 (×3)×600 full-color dots and then “sequentially”, on a field by field basis, we displace these 900 dots left and right (for instance) so that R, G and B overlap in space to create a 900 full-color dot image. In conventional display applications, such as electronic (e.g., LCD) display projectors, the pixellated display will have a resolution that corresponds to the desired final overall display resolution (e.g., 640×480, 800×600, 1024×768, 2048×1536, etc,) Dot-sequential Color uses a single LCD (or DMD, etc.) imaging device at the desired final resolution, but creates 3 slightly displaced images over time, to make the equivalent of a full-color image with very low cost. We therefore exploit the ability of the human eye to synthesize three displaced color images into an equivalent higher resolution color image. Such a displacement may also be called “dithering” or “dot dither”.  
           [0012]    Other prior art projection display systems use 3 LCDs in an optical system that separates the projection illumination into R, G and B paths, and then after illuminating the 3 independent full-resolution devices, these R, G and B images are superimposed at the viewing screen or observer&#39;s eye.  
           [0013]    Another popular display system uses color filters within the display but does not separate the illumination into the separate pixels but rather crudely forces the white light through the red filter, thus losing {fraction (2/3)}rds of the incident light, with a similar 30% light efficiency as field-sequential color displays.  
           [0014]    U.S. Pat. No. 5,969,832 (Nakanishi et al) proposes a display with high efficiency usage of light, but which moves the illumination into the RGB subpixels instead of keeping the colors constant as our invention requires. One problem that may be encountered when using the method of the &#39;832 patent with a liquid crystal can be described briefly as follows. Imagine a red ball moving across a black background: When the red ball moves and a red pixel is turned on to pass light, and now we switch the illumination into this same pixel so that it must pass green light, until such time as the red liquid crystal element takes to “turn off” the light, there will be some green light passing through, which will have produce an artifact of some slight amount of green light in locations throughout the area of the red ball, where there should be NO green light. Likewise there will be blue edges also appearing on the red ball which will be increasingly obvious as it moves around.  
           [0015]    Furthermore the system of the &#39;832 patent does not have a realistic and inexpensive means of shifting the illumination on a field rate (i.e. 60 Hz or faster) basis. The &#39;832 patent requires that R, G and B wavebands be separated and directed into RG and B subpixels, and a display of {fraction (1/3)} resolution is recommended, but the illumination is shifted into these pixel groups, instead of our invention which shifts the image after the illumination is steadily applied. Note that in our invention the red light is steadily applied to a “red” pixel and the error that accrues due to non-instantaneous LC response time causes incorrect amplitudes of red light instead of cross-color contamination (e.g. some green or blue light, when the image calls for only red and black). In our method there could be an imperfect red ball edge amplitude (in fact the edges of the ball may be softened as is desirable for reducing jaggedness of the pixellated image).  
           [0016]    The preferred method of shifting the image of our device uses a simple tilted plate with a piezoelectric actuator. Another means of obtaining such a small image displacement uses a double-birefringence crystal and liquid crystal retarder which is switched between two polarization states to make a “solid state” and reliable and fast-switching displacement control device.  
           [0017]    In U.S. Pat. No. 5,537,256 (for example) Fergason describes a means of dynamically displacing an image, by means of an LC switch and doubly-birefringent device. U.S. Pat. No. 5,161,042 (Hamada) discloses a method which increases the light efficiency similar to the here-in described invention, but fails to show how to use a lower resolution LCD with sequential displacement to increase the effective resolution.  
           [0018]    In U.S. Pat. No. 5,467,206 there is disclosed a holographic color separation system which shows the same intention as Hamada, i.e. to channel the RGB wavelength bands into three sets of pixels, but using holographic dispersion and a holographically formed microlens. While such a system is relatively easy to implement, it suffers the same problem as Hamada&#39;s tilted dichroic and microlens array approach, i.e. the light for the outside two channels becomes widely diverging and the projection lens must be large and the overall image quality suffers as a direct result.  
           [0019]    Our chromatic separation also preferably uses a holographic grating, but as a compensating dispersion, and not as a microlens. We propose a better solution to putting RG and B wavelengths into spatially distinct pixel regions (RGB subpixels) and we furthermore propose a more economical method of displacing the image in time. Our display system and method provides high-efficiency illumination and a high-resolution image to the human eye.  
           [0020]    Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    [0021]FIG. 1 is an optical schematic illustration of a dot sequential color display system according to the present invention.  
         [0022]    [0022]FIG. 2 is a diagrammatic illustration of color component pixel addressing in a conventional prior art field sequential display system.  
         [0023]    [0023]FIG. 3 is a diagrammatic illustration of color component pixel addressing in a dot sequential display system according to the present invention.  
         [0024]    [0024]FIG. 4 is a diagrammatic time-sequential illustration of one implementation by which the portions of a color component plane are delivered to a display device at successive times.  
         [0025]    [0025]FIG. 5 is a diagrammatic illustration of cumulative successive times during which portions of all three color component planes are delivered to a display device during one image frame.  
         [0026]    [0026]FIGS. 6 and 7 are optical schematic illustrations of alternative implementations of a dot sequential color display system with an angular color separation system.  
         [0027]    [0027]FIG. 8 is an enlarged side view illustrating light propagating through a prism array.  
         [0028]    [0028]FIG. 9 illustrates renormalization of RGB light channels as facilitated by the prism array of FIG. 8.  
         [0029]    [0029]FIG. 10 is an optical schematic illustration of a diffractive color filter that includes a holographic grating.  
         [0030]    [0030]FIG. 11 is an optical schematic illustration of another dot sequential color display system according to the present invention.  
         [0031]    [0031]FIG. 12 is an alternative optical schematic illustration of a diffractive color filter with a holographic optical element grating.  
         [0032]    [0032]FIG. 13 is an exploded functional illustration of a holographic optical element grating functioning as a uniform telecentric color separator.  
         [0033]    [0033]FIG. 14 is an optical schematic illustration of a resolution enhancing dot sequential color display system according to the present invention.  
         [0034]    [0034]FIG. 15 illustrates resolution enhancement provided by the display system of FIG. 14.  
         [0035]    [0035]FIG. 17 further illustrates resolution enhancement provided by the display system of FIG. 14.  
         [0036]    [0036]FIG. 18 is a diagrammatic illustration of a dynamic post-display pixel element alignment system or “wobbler” that includes two prism arrays with piezoelectric actuator stacks therebetween.  
         [0037]    [0037]FIG. 19 illustrates a dynamic post-display pixel element alignment system or “wobbler” that includes a flat doubly birefringent crystal.  
         [0038]    [0038]FIG. 20 is a front view of a dynamic post-display pixel element alignment system or “wobbler” that includes a wheel with four flat refractive segments.  
         [0039]    [0039]FIG. 21 is a side view of the wheel of FIG. 20.  
         [0040]    [0040]FIG. 22 is a top view of the wheel of FIG. 20.  
         [0041]    [0041]FIG. 23 is a diagrammatic illustration of an alternative color mosaic arrangement of sub-pixels in a display device (e.g., LCD).  
         [0042]    [0042]FIG. 24 is a diagrammatic illustration of another alternative color mosaic arrangement of sub-pixels in a display device (e.g., LCD). 
     
    
     DETAILED DESCRIPTION  
       [0043]    The present invention relates to pixelated electronic (e.g., liquid crystal display, digital micromirror device, etc.) projection displays, sometimes referred to as electronic display projectors. The invention includes a dot sequential color display system that may be used in such an electronic display projector. It will be appreciated, however, that the dot sequential color display system of the present invention could alternatively be used in other display applications.  
         [0044]    [0044]FIG. 1 is an optical schematic illustration of a dot sequential color display system  10  according to the present invention. A parabolic reflector  12  collects generally white light from a lamp  14  (e.g., an arc lamp) and directs the light in generally parallel rays to a first grating  16 . Grating  16  disperses or separates color components of the light (e.g., red, green and blue “RGB”) and directs them to a microlens array  18  that focuses the dispersed light onto or toward a pixelated electronic display (e.g., a liquid crystal display)  20 . A second grating  22  re-normalizes the angle of incidence for the color components (e.g., RGB), thereby compensating for the dispersion imparted by grating  16 .  
         [0045]    LCD  20  includes triads of color component sub-pixels for controlling the intensity of each color component of light (e.g., RGB). A wobbler or “dot-shifter,” such as a dynamically tilted plate  24 , is tilted at a fast field rate to form a three-frame sequence. Plate  24  is dynamically tilted, “wobbled,” or dithered in synchronism with the application of color component image signals to LCD  20  to direct three overlapping images of color component sub-pixels or dots to a projection lens assembly  26 . Projection lens assembly  26  projects the overlapping images of color component sub-pixels or dots onto a display screen  28  that is viewed by one or more observers.  
         [0046]    [0046]FIG. 2 is a diagrammatic illustration of color component pixel addressing in a conventional prior art field sequential display system  30 . Display system  30  includes a frame buffer memory  32  with red, green, and blue color component planes  34  that store at each of multiple addresses or locations  36  a value corresponding to the intensity of a color component for a pixel of a display image. Although color component planes  34  are illustrated as being separated for each of the red, green, and blue color components, it will be appreciated that in many implementations the locations  36  of the color component planes  34  are interleaved in the physical memory structure where the color component values are stored.  
         [0047]    At successive times t1 , t2, and t3, the color component information for a corresponding single color component plane  34  is delivered to and a corresponding image is rendered by a pixelated display device  38 , such as a liquid crystal display or a digital micromirror device. The pixels  40  of display device  38  have a one-to-one correspondence with locations  36  in frame buffer memory  32 . For example, frame buffer memory  32  with color component planes  34  having j-by-k (columns-by-rows) arrays of addresses  36  will correspond to display device  38  having an x-by-y array of pixels  40 . In one common display format, the j-by-k arrays of addresses  36  and the x-by-y array of pixels  40  may correspond to 1024-by-768. It will be appreciated that the arrays of addresses in some frame buffer memories may be of a size different than (e.g., typically larger than) the array of pixels in the display device. In these situations, the above description is directed to the matching portions of the frame buffer memory and display device arrays.  
         [0048]    For purposes of illustration, display device 38 is illustrated as receiving and rendering the red, green, and blue color components at successive times t1, t2, and t3, respectively. At each time t1, t2, or t3, display device  38  functions to render monochrome red, green, or blue image information. The red, green, and blue color components rendered at successive times t1, t2, and t3 by display device  38  are superimposed on a display screen to form an image.  
         [0049]    [0049]FIG. 3 is a diagrammatic illustration of color component pixel addressing in a dot sequential display system  50  according to the present invention. Display system  50  includes a frame buffer memory  52  with red, green, and blue color component planes  54  that store at each of multiple addresses or locations  56  a value corresponding to the intensity of a color component for a pixel of a display image. Although color component planes  54  are illustrated as being separated for each of the red, green, and blue color components, it will be appreciated that in many implementations the locations  56  of the color component planes  54  are interleaved in the physical memory structure where the color component values are stored.  
         [0050]    At successive times t1, t2, and t3, a portion (e.g., one-third) of the color component information for each of the color component planes  54  is delivered to and a corresponding partial image is rendered by a pixelated display device  58 , such as a liquid crystal display or a digital micromirror device. The pixels  60  of display device  58  have a one-to-one correspondence with a portion (e.g., one-third) of locations  56  in frame buffer memory  52 . For example, frame buffer memory  52  with color component planes  54  having j-by-k (columns-by-rows) arrays of addresses  56  will correspond to display device  58  having an x-by-y array of pixels  60 . In one common display format, the j-by-k arrays of addresses  36  may correspond to 1024-by-768, and the x-by-y array of pixels  40  may correspond to 1024-by-768. Color component information in an array of about 1024/3-by-768 addresses  56  from each color component plane  54  is delivered to display device  58  at each time t. It will be appreciated that the arrays of addresses in some frame buffer memories may be of a size different than (e.g., typically larger than) the array of pixels in the display device. In these situations, the above description is directed to the matching portions of the frame buffer memory and display device arrays.  
         [0051]    At each time t1, t2, or t3, display device 58 functions to render a portion (e.g., one-third) of the full-color image information. These partial full-color images are distinct from the successive monochrome images formed in conventional field sequential system  30 . The partial full-color images rendered at successive times t1, t2, and t3 by display device  58  overlap and are interleaved on a display screen to form an image  62 .  
         [0052]    [0052]FIG. 4 is a diagrammatic time-sequential illustration of one implementation by which the portions of a color component plane  54  (e.g., green) are delivered to display device  58  at successive times t1, t2, and t3. For purposes of simplicity, this illustration shows only a small fraction (e.g., 6 columns) of the typically many more of columns locations  56  and pixels  60  in color component plane  54  and display device  58 , respectively. Moreover, this description of green color component plane is similarly applicable to the red and blue color component planes  54 .  
         [0053]    At a step  70  corresponding to a time t1, color component information in every third column of addresses or locations  56  in green color component plane  54  is delivered to every corresponding third column of pixels  60  in display device  58 . In the illustration of step  70 , for example, columns j and j+3 of locations  56  in green color component plane  54  are delivered to corresponding columns x+1 and x+4 of pixels  60  in display device  58 .  
         [0054]    At a step  72  corresponding to a time t2, color component information in every next successive third column of locations  56  in green color component plane  54  is delivered to every corresponding third column of pixels  60  in display device  58 . In the illustration of step  72 , for example, columns j+1 and j+4 of locations  56  in green color component plane  54  are delivered to corresponding columns x+1 and x+4 of pixels  60  in display device  58 .  
         [0055]    At a step  74  corresponding to a time t3, color component information in every next successive third column of locations  56  in green color component plane  54  is delivered to every corresponding third column of pixels  60  in display device  58 . In the illustration of step  72 , for example, columns j+2 and j+5 of locations  56  in green color component plane  54  are delivered to corresponding columns x+1 and x+4 of pixels  60  in display device  58 .  
         [0056]    The operations described for green color component plane  54  are simultaneously carried out for red and blue color component planes  54 . The color component information in every third column of addresses or locations  56  in green color component plane  54  is successively delivered to every corresponding third column of pixels  60  (e.g, columns x+1 and x+4) in display device  58 . Similarly, the color component information in every third column of addresses or locations  56  in red color component plane  54  is successively delivered to every corresponding third column of pixels  60  (e.g, columns x and x+3) in display device  58 , and the color component information in every third column of addresses or locations  56  in blue color component plane  54  is successively delivered to every corresponding third column of pixels  60  (e.g, columns x+2 and x+5) in display device  58 . Accordingly, each column of pixels consistently receives color component information of only one color component.  
         [0057]    [0057]FIG. 5 is a diagrammatic illustration of the cumulative times t1, t2, and t3 during which portions of all three color component planes  54  are delivered to display device  58  during one image frame. Display device  58  illustrates that at each of times t, pixel columns x and x+3 receive blue color component information, pixel columns x+1 and x+4 of display device  58  receive green color component information, and pixel columns x+2 and x+5 of display device  58  receive red color component information.  
         [0058]    Wobbler  24  illustrates its positions at times t1, t2, and t3 and also the manner in which wobbler  24  superimposes on a display screen  76  red, green, and blue color components rendered at successive times t1, t2, and t3 to form three exemplary pixels  78  (m, m+1, and m+2) of a display image.  
                                                           TABLE 1                           [t1]            Time   Display Device Pixels   Display Screen Pixels                    t1   x + 1   x + 2   x + 3   m   m + 1   m + 2           Green   Red   Blue   Green   Red   Blue       t2   x   x + 1   x + 2   m   m + 1   m + 2           Blue   Green   Red   Blue   Green   Red       t3   x − 1   x   x + 1   m   m + 1   m + 2           Red   Blue   Green   Red   Blue   Green                  
 
         [0059]    Dot sequential color display system  10  includes high efficiency, color-separated, fixed illumination of color-component sub-pixels in LCD  20  and dynamic post-display device alignment of the color-component sub-pixels. The color-separated, fixed illumination of color-component sub-pixels in LCD  20  includes splitting the generally white projection light into  3  (or  4 ) primary color wavelength bands and directing each separate band to separate sub-pixel elements arranged in a color mosaic or color-stripe pattern on LCD  20 . Various means can be used to separate the generally white light into color components, including: 1. absorptive (lossy) color filter triads (e.g., Fergason U.S. Pat. No. 5,715,029)—very poor efficiency 2. angular color separation (ACS) (e.g., Hamada, U.S. Pat. No. 5,161,042) 3. holographic Acs (e.g., Huignard et al. U.S. Pat. No. 5,467,206) 4. telecentric approach (e.g., Nishihara U.S. Pat. No. 5,764,319) 5. telecentric filter, microlens+HOE (holographic optical element), described hereinbelow.  
         [0060]    The dynamic post-display device re-alignment of the color-component sub-pixels (i.e., wobbling or dithering) includes displacing the image to the eye by a subpixel element in time, over  3  (or  4 ) field time periods so as to superimpose the color dots on top of each other to realize full color dots. Various wobbling or dithering means can be used, including: 1. Liquid crystal switch and birefringent crystal (e.g., Fergason U.S. Pat. No. 5,715,029) 2. piezoelectric actuators between symmetrical prism arrays, as described below 3. solenoid or piezos to tilt a plate, as described below 4. other mechanical means (e.g., CCD dithering).  
         [0061]    The present system and method include separating the color components into mosaic color primary picture elements (on the illumination side of the display) and then subsequently superimposing these elements to the eye (on the viewing side of the display). It may be seen that each of these two processes can be accomplished by a variety of means.  
         [0062]    Prior displays described by Fergason (U.S. Pat. No. 5,715,029) and by Nakanishi (U.S. Pat. No. 5,969,832) do not realize the present invention. Fergason suffers significant inefficiency; throwing away {fraction (2/3)} of the illumination light by using absorptive color filters. Nakanishi requires that the illumination be shifted, which is particularly difficult because the illuminator typically has a significant mass that can be difficult to shift or displace at an adequate frequency, particularly in comparison to the significantly lower mass of a wobbler  24  of the present invention. Moreover, shifting or displacement of the illuminator would make it particularly difficult, if not impossible, to precisely fill or illuminate the pixel apertures of the separate color component light channels. Finally, cross-color contamination could be introduced by such illumination shifting due to display devices (e.g., LCDs) having less than idealized response times. The present invention is much more practical, shifting the display image but statically placing color illumination light into separate dedicated color subpixel elements.  
         [0063]    [0063]FIGS. 6 and 7 are optical schematic illustrations of alternative implementations of this invention in which grating  16  of dot sequential color display system  10  is replaced with an angular color separation system  90  of the type described in U.S. Pat. No. 5,161,042 of Hamada. The implementation of FIG. 7 further includes a prism array  92  that functions as a total internal reflection (TIR) ‘deflector’ that receives normal incident light and deflects or angles the light to some desired direction so as to be appropriate for the next stage. FIG. 8 is an enlarged side view illustrating light propagating through prism array  92 .  
         [0064]    In the implementations of FIGS. 6 and 7, grating  22 , whether holographic or not, is positioned between microlens array  18  and the pixel apertures of LCD  20 . This allows the incident RGB light channels to be renormalized (as illustrated in FIG. 9) and go through the RGB pixel apertures at approximately the same (0 degree) angle. For example, a holographic grating  22  may be positioned midway between microlens array  18  and the pixel elements of LCD  20 . The renormalization that this provides is the same as that provided by two layers of microlenses or a microlens and a microprism (see for example U.S. Pat. No. 5,764,319 of Nishihara), but with a larger angular acceptance angle and an easier, more exacting renormalization.  
         [0065]    In contrast, angular color separation systems of the type described in U.S. Pat. No. 5,161,042 of Hamada have output angles that are widely diverging with a center channel on-axis but two outer channels emanating to extreme directions left and right from that. In such prior systems, the dispersive (e.g. holographic) element is positioned first, then a microlens near the LCD. U.S. Pat. No. 5,467,206 also contemplates making the hologram perform as a microlens. The present invention places the holographic diffractive element between the microlens and the pixel apertures, in contradistinction to the &#39;206 patent in which the microlens is placed between the HOE and the pixel.  
         [0066]    [0066]FIG. 10 is an optical schematic illustration another implementation in which a color-dispersing element (e.g., a grating or ACS of Hamada, not shown) separates the light into 3 different angular channels for three distinct wavelength ranges—red, green and blue. These three channels pass in sequence through a color filter  93  that includes a refractive (i.e., not holographic) lens array  94  (which may preferably be an array of cylindrical lenses, i.e. a lenticular) and a holographic grating  96  to an imaging device  98  (e.g., LCD), such that the average angle for all three exiting channels (i.e. R, G and B) is made to be substantially normal to the imaging plane of imaging device  98 . As illustrated in FIG. 10, this provides a telecentric configuration in which the pixel apertures of imaging device  98  are located at the front focus, resulting in the chief rays being parallel to the optical axis in the image space (i.e., normal to the plane of imaging device  98 ).  
         [0067]    This arrangement of dispersive (color-separating) element plus (refractive) lens array element  94  plus holographic (counter-dispersive) element  96  is unique and has the important feature that final grating  96  is continuous and without any optical power. As a result, final (holographic) grating  96  need not be aligned to the pixels of imaging device  98  other than to ensure that the grating axis is parallel with the columns of the display pixels. The micro-lenticular  94  is carefully aligned, but this may be added onto the built and tested imaging device  98  as a secondary process step, which would increase the manufacturing yield rate for both imaging device  98  and lenticular  94 . The “dispersion-compensating” element  96  is preferably a volume hologram so as to be able to be immersed and which has a high diffraction efficiency over the narrow angles that it is designed to accept. In contrast, a surface grating cannot be glued between glass or plastic optical layers.  
         [0068]    In one embodiment, the first color dispersive element is identical to the final counter-dispersive element  96  and both are volume holographic transmission gratings. From the standard simplified grating equation:lambda divided by “d”=sin(input angle)+sin(output angle), in which lambda is the wavelength of light and d is the grating spacing. The ideal input angle can be selected based on an arbitrary color channel separation angle. In the case of the output angle being normal to the holographic plane, and for lambda for green light of approximately 0.55 microns wavelength, the formula may also reveal a nominal grating spacing, “d”:“d”=0.55 divided by sin(input angle)or likewise input angle=arcsin (d/lambda) Table 2 below shows various channel spacings as a function of incident (input) green light.)  
                                     TABLE 2                           [t2]            Channel   Red incident   Green   Blue   Grating       separation   angle   incident   incident   spacing       (delta R-G)   (degrees)   angle   angle   (microns)               8   49   41   33   0.83       6   39   33   27   1.0        4   28   24   20   1.35       3   21   18   15   1.75       2     14.7     12.7     10.7   2.5                   
 
         [0069]    Note that spatial frequency is inverse of “d”. Multiply by 1000 to get cycles/mm. Therefore a grating spacing of 1.75 microns corresponds to 571 cycles/mm.  
         [0070]    In a specific application, a suitable LCD is selected, with a specific pixel spacing “p” and glass substrate thickness “h”: The holographic element is desirably a volume grating, as opposed to surface (replicated, etc) structure. This volume grating suppresses the higher diffraction orders so that principally one channel for each R, G and B bundle is obtained, not multiple random paths. The off-axis higher orders can probably be eliminated within the projection lens, since it would tend to vignette such extreme angles.  
         [0071]    The central angle (i.e. for 550 nm green light) may be adjusted as required to match the LCD pixel pitch (spacing between R, G, B subpixels) and the thickness of LCD substrate glass and the HOE thickness. The grating functions to straighten out each of the wavelength bands, so the input angles are designed arbitrarily to get maximum efficiency for red (e.g., 632 nm), green (e.g., 546 nm) and blue (e.g., 480 nm). A specific LCD of interest has 0.7 mm glass and a subpixel pitch of 42 microns. A simple ray trace gives a nominal design with 2 degrees between each color primary, so the HOE can be made to do a ‘normalization’ for green light. The central illumination angle for green is then designed so that red and are blue diffracted on either side at about 2 degrees. In this case the nominal input angle is 15 degrees, and the spatial frequency of the transmission (holographic) grating is about 1000 cycles per mm.  
         [0072]    In one implementation, there may be some refinement since the diffraction is not linear. The photopic response of the eye makes it less important to worry about blue light below 450 nm and red light above 660 nm, and the lamp has its own unique spectral signature. Generally the yellowish peak (Hg has strong 579 line) is attenuated since this light contaminates both red and green color primary channels.  
         [0073]    The microlens may be refractive and needs to be precisely aligned with the LCD. The holographic grating should be relatively thick to suppress higher orders and yet structurally thin so that the spacing from microlens to LCD pixel can be minimized to increase the angular acceptance for the illumination. The hologram not need have any lens function, counter to the teaching of U.S. Pat. No. 5,467,206.  
         [0074]    [0074]FIG. 11 is an optical schematic illustration of another dot sequential color display system  100  according to the present invention. A parabolic reflector  112  collects white light from a lamp  114  (e.g., an arc lamp) and directs it in parallel rays to an angular color separation (ACS) system  116 . ACS system  116  disperses or separates color components of the light and directs them to a microlens array  118  that focuses the dispersed light onto a pixellated electronic display (e.g., a liquid crystal display)  120 . A holographic optical element  122  re-normalizes the angle of incidence for the color components (e.g., RGB), thereby compensating for the dispersion imparted by ACS system  116 .  
         [0075]    LCD  120  includes triads of color component sub-pixels for controlling the intensity of each color component of light (e.g., RGB). A “dot-shifter,” such as a dynamically tilted plate  124 , is tilted at a fast field rate to form a three-frame sequence. Plate  124  is dynamically tilted, “wobbled,” or dithered in synchronism with the application of color component image signals to LCD  120  to direct three overlapping images of color component sub-pixels or dots to a projection lens assembly  126 . Projection lens assembly  126  projects the overlapping images of color component sub-pixels or dots  128  onto a display screen (not shown) that is viewed by one or more observers.  
         [0076]    [0076]FIG. 12 is an optical schematic illustration of an alternative implementation of dot sequential color display system  100  in which a holographic optical element  130  (e.g., grating) that disperses or separates color components of the light and directs them to a microlens array  118  is substituted for ACS system  116 .  
         [0077]    Holographic optical element  130  cooperates with color filter  93  to function as a uniform telecentric color separator, which as illustrated in FIG. 10 means that the pixel apertures of imaging device  98  are located at the front focus, resulting in the chief rays being parallel to the optical axis in the image space (i.e., normal to the plane of imaging device  98  as illustrated). In one implementation, holographic optical element  130  and holographic optical element  122  are identical and microlens array  118  is of the lenticular type.  
         [0078]    [0078]FIG. 13 is an exploded functional illustration of holographic optical element  130 . Holographic optical element  130  includes three holographic lens grating layers, each for diffracting a different wavelength range. In the illustrated implementation, Δ n and thickness are controlled to give each layer a bandwidth of 60-75 nm, for example.  
         [0079]    [0079]FIG. 14 is an optical schematic illustration of a resolution enhancing dot sequential color display system  200  according to the present invention. Dot sequential color display system  200  illustrates a conventional three-panel color projector configuration that further includes a “dot-shifter,” such as a dynamically tilted plate  202 , which is tilted at a fast field rate to form a four-frame sequence. For example, dot sequential color display system  200  includes a pair of color separating dichroic mirrors  204  and  206  that reflect respective blue and green light and transmit other light. Fold mirrors  208 ,  210 , and  212  redirect the color separated light components toward monochrome display devices (e.g., LCDs)  214 ,  216 , and  218 . An X-cube prism combination  220  combines the color component images, which pass through dynamically tilted plate or wobbler  202  to a projection lens assembly  222 .  
         [0080]    [0080]FIG. 15 illustrates the resolution enhancement provided by display system  200 . Display image pixel  230  illustrates the overlapping color component sub-pixels provided by a conventional three-panel color projector configuration (The color component sub-pixels are shown with slight offset for purposes of illustration). Display image pixel  232  illustrates one implementation of four non-overlapping color component sub-pixels  234  provided by dynamically tilted plate  202  of display system  200 .  
         [0081]    [0081]FIG. 16 shows an expanded non-overlapping pixel  236  with an exemplary sub-pixel displacement sequence  238  (sometimes referred to as a “bowtie” sequence) that includes alternating diagonal and vertical sub-pixel displacements. Sequence  238  is preferable over circumferential sequence (e.g., sub-pixels A, D, B, and C) because the alternating rows of sequence  238  are compatible with interlaced display formats, such as standard television. FIG. 17 further illustrates the resolution enhancement provided by display system  200 . Pixels  240  illustrate a conventional original four full-color pixels without displacement. Pixels  242  illustrate 16 apparent full-color pixels that are provided by sub-pixel displacement. The non-overlapping arrangement of color component sub-pixels  242  provides an enhanced image resolution. (The 16 pixels are shown with slight offsets for purposes of illustration).  
         [0082]    FIGS.  18 - 22  are diagrammatic illustrations of dynamic post-display pixel element alignment systems or “wobblers.” FIG. 18 shows two prism arrays  250  and  252  with two piezoelectric actuator stacks  254  and  256  and a voltage waveform that is applied to piezoelectric actuator stacks  254  and  256 . The voltage waveform cooperates with piezoelectric actuator stacks  254  and  256  to separate prism arrays  250  and  252  by different distances so as to shift color component dots into alignment to get the desired R, G, B superposition.  
         [0083]    [0083]FIG. 19 illustrates a dynamic post-display pixel element alignment system or “wobbler” that includes a flat doubly birefringent crystal  260  (e.g., calcite) with a crystal polarization direction  262 . Crystal  260  and its polarization direction are rotated about a central axis  264 . Crystal  260  is illustrated in combined front and side views at successive times t1, t2, and t3.  
         [0084]    In the illustrated implementation, light has a horizontal polarization so that it passes through crystal  260  without displacement whenever crystal polarization direction  262  is horizontal, as at time t1. Whenever crystal polarization direction  262  is vertically upward, as at time t2, light passes through crystal  260  with an upward displacement. Whenever crystal polarization direction  262  is vertically downward, as at time t3, light passes through crystal  260  with a downward displacement. As crystal  260  rotates, the light will in sequence be displaced in one direction, pass straight through, be displaced in the opposite direction, pass straight through, etc. As a result, image resolution can be tripled to form a pixel pattern  264 .  
         [0085]    [0085]FIG. 20 is a front view of a dynamic post-display pixel element alignment system or “wobbler” that includes a wheel  270  with four flat refractive segments A, B, C, and D with different angled orientations to displace the light in four different directions. Segmented wheel  270  is rotated about a central axis  272  so that the light propagating along a path  274  passes through the angled segments successively. FIG. 21 is a side view of wheel  270  showing segment A displacing light in a downward direction. It will be appreciated by the orientation of segment B that it will displace light in an upward direction when segment B is positioned across path  274 . FIG. 22 is a top view of wheel  270  showing segment C displacing light in a rightward direction. It will be appreciated by the orientation of segment D that it will displace light in a leftward direction when segment D is positioned across path  274 .  
         [0086]    Although illustrated in a four-segment implementation, it will be appreciated that wheel  270  could alternatively be implemented as three segments, rather than four. In one three segment implementation, for example, a first and a third segment could have the orientations of segments C and D, and an intervening second segment could be oriented with no angular tilt (i.e., perpendicular to path  274 ).  
         [0087]    [0087]FIG. 23 is a diagrammatic illustration of an alternative color mosaic arrangement  300  of sub-pixels  302  in a display device (e.g., LCD). Implementations described above refer to arrangements in which the sub-pixels of each color component are arranged in distinct vertical columns. Color mosaic arrangement  300  positions sub-pixels  302  in a denser, closer-packed arrangement that provides improved image characteristics because the human eye sees the staggered, offset pixel arrangement as having a higher spatial resolution, particularly in the horizontal direction. In addition, television signals may also be sampled with offsets by using alternating clock edges (e.g., chrominance signals).  
         [0088]    As with implementations described above, a reflector  12  collects generally white light from a lamp  14  and directs the light through an angular color separation system  304  that provides regular angle color separation in which red, green and blue color components are separated across one axis (e.g., horizontal). In this implementation, the light is shown passing through a microlens  306  having an elongated, close-packed (e.g., hexagonal) configuration in which each microlens  306  is aligned with a full-color triplet of sub-pixels  302 . As a result, the microlenses  306  at the display device (e.g., LCD) turn the horizontal angular separation of color into LCD pixel separation. Linear (e.g., horizontal) dot sequential modulation  308  displaces light horizontally during three successive times in each frame to provide a complete display image in accordance with the present invention.  
         [0089]    [0089]FIG. 24 is a diagrammatic illustration of an alternative color mosaic arrangement  310  of sub-pixels  312  in a display device (e.g., LCD). Color mosaic arrangement  310  positions sub-pixels  312  in another dense, closer-packed arrangement that differs from arrangement  300  in that the former includes one sub-pixel  312 ′ (e.g., a center pixel, illustrated as receiving green light) that is offset from alignment with the other two sub-pixels  312 .  
         [0090]    As with implementations described above, a reflector  12  collects generally white light from a lamp  14  and directs the light through an angular color separation system  314  that provides angle color separation in which red and blue color components are separated from each other across one axis (e.g., horizontal) and from green across two axes (e.g., horizontal and vertical). In this implementation, the light is shown passing through a microlens  316  having a regular, close-packed (e.g., hexagonal) configuration in which each microlens  316  is aligned with a full-color triplet of sub-pixels  312 . As a result, the microlenses  316  at the display device (e.g., LCD) turn the angular separation of color into LCD pixel separation. Triangular or circular dot sequential modulation  318  displaces light during three successive times during each frame to provide a complete display image in accordance with the present invention.  
         [0091]    In view of the many possible embodiments to which the principles of our invention may be applied, it should be recognized that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Rather, the invention includes all such embodiments as may come within the scope and spirit of the following claims and equivalents thereto.