Abstract:
A reflective display can be configured around a waveguide illuminated with collimated light. A variety of light sources, light valves, light extracting devices, and light redirecting means may be employed to complete the display. The light extracting devices, light valves, and light redirecting means cooperate to selectively extract, attenuate, and redirect selected portions of the light in specific locations within the display.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/037,842 filed on Feb. 7, 1997, pending and U.S. Provisional Application Ser. No. 60/051,546 filed on Jul. 2, 1997, pending. 
    
    
     BACKGROUND OF THE INVENTION 
     The apparatus described herein relates to reflective displays and in particular reflective displays that utilize a variety of light valves. 
     Present displays generally lack one or more of the of the following characteristics attributable to video displays: compact packaging, high color resolution, higher monochrome resolution; high luminance; high color fidelity; wide gray scale dynamic range, high contrast, high degree of multiplexibility, sharpness, wide viewing angles and high ambient light rejection. It is difficult to obtain all of the above characteristics in a single display as a result of design tradeoffs that are inherent using current display technology. 
     For example, loss of display sharpness occurs when a generally poor collimated backlight is combined with a necessary separation gap between LCD pixels and a diffusion viewscreen. Alternatively, the absence of a separate viewscreen element requires the use of uncollimated light to provide an acceptable range of view angles. The use of uncollimated light passing through LCD pixels, however, causes undesirable color inversions and contrast loss at larger view angles. This effect is reduced by any of a wide variety of available compensation films. Such films, however, further reduce the luminance of the display. In many such cases contrast is greatly improved; however, the lower luminance reduces gray scale dynamic range. 
     Accordingly, there exists a need for a display that exhibits most, if not all of the referenced display characteristics. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     A more complete understanding of the present invention, as well as other objects and advantages thereof not enumerated herein, will become apparent upon consideration of the following detailed description and the accompanying drawings, wherein: 
     FIG. 1 is an elevation view of a reflective display; 
     FIG. 2 is a partial perspective drawing of the waveguide of the display of FIG. 1; 
     FIG. 3 is an elevation view of a cell of the waveguide; 
     FIG. 3A is an elevation view of a waveguide having an alternative light extraction feature; 
     FIG. 4 is a cross-sectional diagram of a cell of the waveguide having an alternative reflective element; 
     FIG. 5 is an elevation view of a reflective display having a light valve; 
     FIG. 6 is an elevation view of a reflective display having a second waveguide; 
     FIG. 7 is a partial perspective drawing of the waveguide of a color display; 
     FIG. 8 is an elevation view of a reflective display with volume holographic elements; 
     FIG. 8A is an illustration of the lower output surface of the waveguide of FIG. 8, illustrating an arrangement of the volume holographic elements and the adjacent gradient index lenses; 
     FIG. 9 is a cross-sectional diagram of a waveguide having volume holographic elements and gradient index lenses; 
     FIG. 10 is a perspective view of a U-shaped assembly for redirecting light; 
     FIG. 11 is a cross-sectional diagram of a turning prism for launching light into the waveguide of a reflective display; 
     FIG. 12 is an elevation view of a reflective display utilizing bandpass light filters and volume holographic elements; 
     FIG. 13 is a elevation view of a stair-step reflective element; 
     FIG. 14 is a cross-sectional diagram of a refractive medium with a diffuser; and 
     FIG. 15 is an illustration of the upper output surface of the refractive medium of the display of FIG. 12, illustrating an arrangement of the bandpass filters and their associated holographic elements. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is intended to overcome the deficiencies of the prior art, and at the same time provide a low cost display. The invention is designed to accept precollimated light from a remote high intensity, spatially concentrated light source to replace fluorescent lamps usually disposed to edgelight the display or placed behind the display. Most prior art backlighting means do not employ precollimated light. It has been determined by calculations of Etendue that large area light sources such as fluorescent lamps have a very limited capability to provide high luminance to a desirable degree for cockpit and other sunlight readable displays. Thus the invention is meant to use intense spatially concentrated light sources, such as metal halide HID that, by virtue of their superior Etendue properties, have the potential to provide the desired higher luminance. 
     Lower cost potential is provided by virtue of the fewer piece parts and the fewer fabrication steps required for holographic displays described by the invention. 
     A reflective display apparatus is illustrated in FIG.  1 . The display has an optical waveguide  10  illuminated by a light source  20  generating collimated or nearly collimated light. For example, white light, precollimated to within a conical half angle of 6° (in air) can be utilized. Preferably, the light is plane polarized and injected into the waveguide  10  from two opposing input port edges  12  to improve the uniformity of light density across the length of the waveguide  10 . This can be accomplished with either two separate sources or a single source and proper channeling of the light to both input port edges  12  of the waveguide  10 . Electrodeless high intensity discharge and other high intensity discharge lamps, such as xenon and mercury xenon, and tungsten filament lamps, and other lamps capable of emitting light from a small volumetric region may be used for the source. 
     An electrically controlled light valve assembly  30 , adjacent to the lower output surface  14  of the waveguide  10 , accepts light extracted from the lower output surface  14  and attenuates and redirects the light. The light valve assembly  30  can be an absorptive, scattering, polarizing, variable refractive index mechanism operated to partially or fully pass light, or prevent the passage of light altogether. Regardless of the method of operation, the light valve assembly  30  redirects light back into the waveguide  10  and into an optional diffuser  40 . 
     The waveguide  10  can be fabricated from a clear, refractive material such as glass or a plastic such as acrylic or polycarbonate. Total internal reflection will confine light within waveguide  10  until light is extracted through an output surface  14  and  16 . The lower and upper output surfaces  14  and  16  of the waveguide  10  can be parallel or at an angle with respect to each other to form a taper. In the latter case, light would be injected into only end of the waveguide  10 . 
     As illustrated in FIG. 2, the waveguide  10  is subdivided into a matrix of individual resolution cells  50  of rectangular cross-section. The cross-sectional dimensions of the resolution cells  50  can be selected to correspond to a single pixel in a display. For example, in a 6×8 inch display, one might select a resolution cell having cross-sectional dimensions of 66μ×200μ, but of course other dimensions could be employed. If the resolution cells  50  are discrete elements, they could be optically isolated utilizing a material having a refractive index less than that of the material of the resolution cells  50 . The resolution cells  50  can be arranged in a rectangular array of rows and columns, or staggered within either the rows or columns. 
     As shown in FIG. 3, there is a V-groove or divot light extraction feature  52  embossed or molded into the upper output surface  16  of the waveguide  10 . For example, the V-groove may have a depth of approximately 2μ. One or more light extraction features  52  are provided for each resolution cell  50 . In the embodiment shown, the faces  54  of the V-groove can be normal to each other and form 45° angles with respect to the surface  16 , but it should be understood that other angles could be selected. For purposes of illustration and explanation, the V-grooves are drawn in the figure much larger than they would physically appear relative to the thickness of the actual device. 
     The light enters through the input edges  12  of the waveguide  10  and propagates simultaneously in opposite directions through the waveguide  10 . When light hits one of the faces  54  of a V-groove light extraction feature  52  at greater than the critical angle, it will be reflected off of the face  54  at an angle equal to the angle of incidence and directed towards the lower output surface  14  of the waveguide  10 . If any such reflected light hits a side  56  of the resolution cell  50 , the lesser index of refraction between resolution cells  50  and/or total internal reflection will confine the light energy to the resolution cell  50 . 
     The thickness of the waveguide  10  and the depth of the V-groove light extraction features  52  are selected to optimize light extraction, efficiency, and uniformity of the display. As the thickness of the waveguide  10  increases and/or the depth of the V-groove light extraction features  52  decreases, display uniformity (the ratio of the difference of luminance variance to the sum of luminance variance, measured across the display face) will increase but waveguide extraction efficiency (the ratio of the waveguide lower output surface flux output to the waveguide flux input) will decrease. 
     Instead of a V-groove or divot light extraction feature  52 , the trapezoidal light extraction feature  60  of FIG. 3A may be employed. When light traveling through the waveguide  10  impinges on one of the angled opposing sides  62  of the trapezoidal light extraction feature  60 , it is reflected downwards towards and through the lower output surface  14  of the waveguide  10 . 
     Alternatively, the mechanism for extracting light from the waveguide  10  could be located on the lower output surface  14 . Such a structure may use refractive, diffractive, or reflective elements or a combination thereof to redirect the light. For example, as illustrated in FIG. 4, opposing angular cuts  70  could be made on the lower output surface  14 , where each cut presents a surface  72  exposed to air that totally internally reflects any light impinging on that surface  72 . Alternatively, the surfaces  72  can be coated with a reflective material such as aluminum to reflect intercepted light. If the cuts were at 45° angles, the reflected light would propagate in a direction normal to the lower output surface  14  and pass to the light valve assembly  30 . 
     The light valve assembly  30  can be any electrically controlled device that can selectively pass or, fully or partially prevent the passage of light. It may operate on an absorptive, scattering, polarizing, or variable refractive index basis. One such device is a scattering-type light valve such as a polymer-dispersed liquid crystal (PDLC). PDLCs are described in  Liquid Crystal Displays , pp. 85-90, Stanford Resources, Inc. (1995), incorporated herein by reference. Although the PDLC is generally a monolithic sheet, it can be viewed as being subdivided into “resolution cells” or “pixels” corresponding in size and location to the resolution cells  50  of the waveguide  10 . 
     Another form of light valve is a suspended-particle device (SPD). SPDs are described in Saxe and Thompson, “Suspended-Particle Devices,”  Information Display , April-May 1996, incorporated herein by reference. Ferroelectric and anti-ferroelectric liquid crystal displays, twisted nematic (TN) and supertwisted nematic (STN) active-matrix LCDs, electrophoretic devices, and optical phase shifters that alter the refractive index may also serve as light valves. 
     As illustrated in the cross-sectional diagram of FIG. 5, the light valve assembly  30  may comprise an electrode layer  100 , such as a thin layer of ITO (indium tin oxide) or some other electrode material, adjacent to the lower output surface  14  of the waveguide  10 , a light valve layer  110 , a reflective layer  120  comprising a series of reflective pixel electrodes  122 , electrically insulated from one another, and an underlying substrate  130  such as a printed circuit board (PCB). The PCB can be rigid or flexible to a greater or lesser degree as required by the application. The PCB can be manufactured using a variety of substrates, using either subtractive (e.g., etching) or additive processes, or a combination of those processes, to create electrical conductors. Additionally, the PCB can be used to integrate other electronic and optical devices, including, but not limited to light valve drive devices (not shown), other system electronics, and the light source  20  and its associated electronics. 
     The size and shape of the reflective pixel electrodes  122  in the reflective layer  120  would normally conform to the size and shape of the resolution cells  50  but could assume other configurations to suit the application. If the light valve assembly  30  is bonded to the lower output surface  14  of the waveguide  10 , the adhesive used should have a refractive index less than that of the waveguide  10  in order to ensure that the precollimated light within waveguide  10  remains captive within the waveguide (owing to total internal reflections) until extracted by face  54  of a V-groove  52 . 
     When a voltage is applied between the electrode layer  100  and a pixel electrode  122 , an electric field is created between the two, altering the crystalline microstructure of the portion of the light valve  110  adjacent to the pixel electrode  122 . As the applied voltage is changed, the structure will vary from a medium that nearly fully blocks the passage of light to one that partially attenuates the light rays to a clear device through which the light rays will freely pass. Alternatively, instead of static operation, a fast-acting on/off, i.e., binary, light valve can be operated with a varying duty cycle to achieve a desired light throughput over time. 
     When the light valve  110  is clear or nearly clear, light exiting the lower output surface  14  of the waveguide  10  will pass through the light valve  110  until it reaches a reflective pixel electrode  122  and is reflected back through the light valve  110 . Ultimately, the light will pass out of the waveguide  10  through the upper output surface  16 , unless it is reflected off of a face  54  of a V-groove feature  52 . It should be understood that the light reflected by the V-groove face  54  is a relatively small percentage of the light reflected by the pixel electrode  122  and in any event will be recycled within the waveguide  10 . 
     To further increase the optical isolation between adjacent resolution cells, a second waveguide assembly  200  of corresponding resolution cells  202  can be placed between the first waveguide  10  and the electrode layer  100  of the light valve assembly  30 , as shown in FIG.  6 . The walls  204  of the resolution cells  202  in the second waveguide  200  can be coated with a low refractive index transparent layer followed by a light-absorptive material such as black resin to totally internally reflect the unscattered collimated rays while it absorbs the scattered uncollimated rays, thus preventing the latter from entering an adjacent resolution cell  202 . Alternatively, a low refractive index black resin could be employed alone. The depth of the resolution cells  202  in the second waveguide  200  can be increased to limit the passage of less closely collimated light rays. 
     The diffuser  40  (FIG. 1) can be any device that will accept light rays from the upper output surface  16  of the waveguide  10  and diffuse them to the degree desired for the particular application. Suitable diffusers are described in copending U.S. patent application entitled “Optical Structures for Diffusing Light” by Beeson et al., filed Dec. 2, 1996 as U.S. patent application Ser. No. 08/759,338; U.S. Pat. No. 5,462,700, issued Oct. 31, 1995, to Beeson et al., for a Process for Making an Array of Tapered Photopolymerized Waveguides; U.S. Pat. No. 5,481,385, issued Jan. 2, 1996, to Zimmerman et al., for a Direct View Display with Array of Tapered Waveguides; and U.S. Pat. No. 5,696,865 to Beeson et al. for an Optical Waveguide Having Two or More Refractive Indices and Method of Manufacturing Same, all foregoing are assigned to the same assignee as the present patent application and are incorporated herein by reference. In particular, the tapered waveguides can be surrounded by light-absorptive black particulate material or black absorptive coating to reduce glare from ambient light and improve contrast. 
     The reflective display can be configured to provide color output. The light from the light source  20  can be spectrally divided into three visible light primary color bands (e.g., red, green, blue). The individual colors could then be routed along separate paths to individual alternating rows of red, blue, and green, as shown in FIG.  7 . The individual colors would be selected on a pixel-by-pixel basis to have a resolution cell pass or scatter the colored light as necessary, using multiplexing techniques, well known to those skilled in the art, to control the light valve assembly. By alternating between color and monochrome inputs, the device can offer both color and monochrome output and varying degrees of resolution. 
     Another alternative display apparatus uses volume or surface hologram elements to extract and direct the light. A color-selective transmitting separate phase-only volume or surface hologram element is provided for each color and each pixel to extract light from the waveguide. Holographic elements are described in Caulfield, H. J., and Lu, S.,  The Applications of Holography , New York: John Wiley &amp; Sons, Inc.: 1970, pp. 43-49, incorporated herein by reference. As shown in FIG. 8, a holographic element  300  is located on the lower output surface  14  of the waveguide  10 . The holographic element  300  is designed to pass a specific wavelength or a range of wavelengths of light, for example, corresponding to a red, green, or blue primary color that arrives at the holographic element  300  in a predetermined range of propagation directions. 
     The lower output surface  14  of the waveguide  10  is shown in FIG. 8A, illustrating an arrangement of holographic elements  300  and adjacent gradient index lenses  330 . Each holographic element  300  cooperates with an adjacent gradient index lens  330  to create a resolution cell  50 . To accommodate a holographic element and a gradient index lens, a resolution cell can have an aspect ratio of 3:1 and dimensions of 66 μm×200 μm, although other ratios and dimensions may be selected to suit the application. Alternatively, the gradient index lenses  330  may extend beyond the boundaries of the resolution cells  50  provided there is no overlap of adjacent lenses. 
     Referring again to FIG. 8, the light of a predetermined range of wavelengths and incidence angles will pass through the holographic element  300 . The holographic element  300  focuses and directs the light across an air gap or other low refractive index material  302 , through an optional refractive medium  304 , and through a light valve  310 , such as a PDLC, an SPD, or some other suitable light attenuating mechanism, to regulate the amount of light in a respective resolution cell  50  of FIG. 8 a , until the light finally reaches a reflective pixel electrode  320 . 
     The reflective pixel electrode  320  reflects the light back through light valve  310  towards the lower output surface  14  of the waveguide  10  to a point on the lower output surface  14  of the waveguide  10 , at the focal point of the holographic element  300 , where there is a first gradient index lens  330 . Depending on the degree of attenuation of light valve  310 , nearly all, a portion, or almost no light will be transmitted to lens  330  by light valve  310 . 
     It should be understood that the first gradient index lens  330  may be fabricated immediately above or beneath the surface of the waveguide  10  or within the waveguide itself using techniques well known in the art or on the surface of the optional refractive medium  304 . The first gradient index lens  330  focuses and directs the light towards the upper output surface  16 , in a direction normal to the surface  16 . A volume or surface holographic element may used in lieu of the first gradient index lens  330 . 
     From the first gradient index lens  330  on the lower output surface  14 , a series of internal gradient index lenses  340   a-c  refocus and direct the light upwards, as shown in FIG.  9 . The focal length of the individual lenses  340  and the vertical spacing therebetween are selected to insure that the bundles of light traveling to the upper output surface  16  of the waveguide  10  do not overlap with one another thus avoiding cross-talk between adjacent resolution cells. 
     A shorter focal length can be selected for the last gradient index lens  340   c  to make its focused image area  341  at the upper output surface  16  smaller. Preferably, output surface  16  is covered with a black matrix with the exception of the surface area occupied by the image area  341 . The smaller image will allow the black matrix material on the waveguide top face to occupy a greater area to reduce ambient light reflection or scattering from the display. Additionally, the axes of the converging light bundles from the top lenses, corresponding to the red, green, and blue primary colors, can be deflected to superimpose their respective focal points and create a tri-color pixel or resolution cell location on the waveguide top face. 
     In lieu of the light valve  310  and the reflective pixel electrode  320  of FIG. 8, a U-shaped channel assembly  350 , illustrated in FIG. 10, can be employed to channel light from the light extraction hologram to grin lens  330  of FIG.  9 . The channel assembly  350  has an input face  352 , an output face  354 , a light valve layer  356 , bounded by ITO or some other electrode material layers  358 , and two angle surfaces  360 . The light valve layer  356  can be a PDLC, an SPD, or some other suitable light-attenuating device. The angle surfaces  360  act as reflectors operating by TIR. Alternatively, if TIR is partial or absent, surfaces  360  may have a reflective coating such as aluminum. Light enters channel assembly  350  through input face  352  and exits through output face  354 . Light valve layer  356  is electrically controlled to vary the light throughput of the channel assembly  350 . When a scattering-type light valve, such as a PDLC, is employed for the light valve layer  356 , a black film or particulate coating can be applied to all surfaces of the channel assembly  350  except the input, output and angle surfaces  352 ,  354 , and  360  to absorb scattered light within the assembly  350  and thereby enhance display contrast. 
     Referring again to FIG. 1, to minimize space requirements around the display edges, a turning prism  400  can be used to channel light to the waveguide  10 . This turning prism  400 , shown in FIG. 11, accepts light from a light pipe  410  and redirects the light to the waveguide  10 . To deflect the direction of the precollimated light entering the waveguide  10  and increase the number of total internal reflections within the waveguide, an optional serrated input surface  420 , or a diffraction grating, a volume or surface holographic coating, or a binary optic surface can be employed on the input port edges  12  of the waveguide  10  to alternately deflect the collimated rays upwardly and downwardly as they enter the waveguide  10 . For example, the serrated edges can form a 90° angle with respect to each other and have a pitch of  50  serrations per inch. It should be understood that the serrated surface could be located on the waveguide  10  proper or on a separate element adjacent to the waveguide  10 . The separate element could be attached by an optical adhesive or there may be a small air gap between the element and the waveguide  10 . 
     Color-selective light extraction can be achieved with a bandpass light filter, as illustrated in FIG.  12 . Light enters the waveguide  500  through an optionally serrated edge  510 . Light travels through the waveguide, reflecting off the surfaces of the waveguide  500  until light reaches the bandpass filter  520  on lower surface  504 . Bandpass filter  520  can be a dichroic filter, a transmitting, phase-only volume holographic element, or some other wavelength-selective device that will pass a specific range of wavelengths while almost completely reflecting all others. 
     The filter  520  will transmit a selected band of light wavelengths into refractive medium layer  530  while reflecting wavelengths outside the selected band. The transmitted light passes through valve  540  and is reflected by electrode assembly  550  in a functionally similar manner as the corresponding elements of FIG.  8 . The redirected light then passes back into the waveguide  500  through a holographic transmissive directional diffuser element  560  and air gap  570  between the element  560  and the lower surface  504  of the waveguide  500 . Holographic element  560  alters the propagation direction of light rays and also diffuses the light. As a result, the light rays travel upwardly towards top surface  508  of the waveguide  500 . 
     Instead of a reflective mirror electrode  550 , a stair-step mirror  600 , shown in FIG. 13, located below the light valve  540  can be used to reflect the light. Additionally, as shown in FIG. 14, a diffuser structure  620 , such as, for example, an array of microlenses, can be placed on the upper surface  532  (FIG. 12) of the refractive medium layer  530  in place of holographic elements. The microlenses can be fashioned in the manner described in U.S. Pat. No. 5,598,281, issued Jan. 28, 1997 to Zimmerman et al. for a Backlight Assembly for Improved Illumination Employing Tapered Waveguides, incorporated herein by reference. 
     A view normal to the upper output surface  532  of the refractive medium  530  is illustrated in FIG.  15 . The figure shows an arrangement of the bandpass filters  520  and their associated holographic elements  560 . A bandpass filter  520  and two holographic elements  560  comprise a resolution cell  570 . The solid-line, dotted line, and dashed-line renditions, respectfully, reflect resolution cells  570  of different color bands, for example, red, green, and blue. By staggering resolution cells  570  as shown, all of the constituent elements can be tightly packed, increasing the planar density of the resolution cells  570  therefore enhancing the resolution of the display. 
     While there has been described what is believed to be the preferred embodiment of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such embodiments that fall within the true scope of the invention.