USRE36654E - Stacked LCD color display - Google Patents

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  • G02F1/13473—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells in which all the liquid crystal cells or layers remain transparent, e.g. FLC, ECB, DAP, HAN, TN, STN, SBE-LC cells for wavelength filtering or for colour display without the use of colour mosaic filters
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  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
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  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
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Definitions

• the present invention relates to systems for displaying color images, and more particularly relates to such systems wherein the image is formed by passing light through a plurality of birefringent optical elements.
• differently dyed polarizers (yellow, magenta and cyan) are interposed in a series of twisted nematic cells.
• the twist angle of the liquid crystal molecules changes, imparting a variable rotation to the light exiting the cell.
• the colored polarizers cooperate with this controllably twisted light to select desired colors.
• the second approach uses only a single LCD panel, but uses it in conjunction with a mosaic color filter.
• the mosaic filter typically has a plurality of red, green and blue filter elements, each aligned with a pixel in the LCD panel. By controlling the transmissivity of pixels in the LCD panel, the display can pass light through selected areas of the color mosaic filter.
• pixel density must be increased by a factor of three to obtain the same resolution as the stacked cell approach. That is, to provide a color display with a horizontal resolution of 640 colored pixels, for example, the LCD panels must have 1920 pixels, 640 for each of the red, green and blue filter elements. This introduces fabrication problems that decrease yields and increase panel costs. Further, the finite width of the gap between pixels must remain, even though the pitch has decreased, so the actual pixel "aperture ratio" can be decreased dramatically. (Some small format thin film transistor (TFT) displays have a total open aperture area of only 45% of the total display surface due to row and column lines and transistor area, etc.)
• TFT thin film transistor
• the third approach is birefringence color.
• the birefringent operating mode of certain material is exploited to produce color, as opposed to reliance on colored dyes in guest-host type cells or reliance on rotation of light through known twist angles in twisted nematic cells.
• Birefringent color systems typically take two forms: those relying on passive birefringent layers to impart a birefringent effect to a liquid crystal cell (as shown in U.S. Pat. No. 4,232,948), and those in which the liquid crystal material itself exhibits a birefringent effect (sometimes called “electrically controlled birefringence” or “tunable birefringence”).
• the degree of birefringence is a function of the voltage applied to the liquid crystal material. By switching the applied voltage to different values, different colors can be produced. Color displays relying on this principle are shown in U.S. Pat. Nos. 3,785,721, 3,915,554 and 4,044,546.
• STN cells generally function in a birefringent mode. However, unlike earlier birefringent cells, STN cells exhibit a bistable behavior wherein they switch rapidly from a deselect state to a select state and back again as the excitation (RMS) voltage crosses a switching threshold.
• the select and deselect voltage regions can be made quite close to one another, such as 1.20 volts and 1.28 volts, permitting the cells to be multiplexed at high rates.
• FIG. 1 shows the transmission of a representative STN cell (with a particular polarizer orientation) as a function of applied voltage, illustrating the steepness of the switching function. Note that this curve shows the overall photopic "brightness" and does not reveal any coloration of the liquid crystal in the select and deselect states.
• STN provides an inexpensive direct-multiplexed display type requiring only M+N drivers to operate a display comprised of M ⁇ N pixels.
• the main drawback to STN is the optical operating mode--birefringence. That is, the only way to distinguish pixels driven by the "on” voltage from those driven by the “off” voltage is the difference in birefringence between the two pixels. (As noted, for high information content displays, the difference in driving voltages is minute and decreases rapidly with an increase in the number of display lines that must be driven.)
• polarizers are used--one to polarize the entering light to a known polarization, and one to select only one polarization of exiting light for examination. Depending on the state of the pixels, the light oriented to pass through the exit polarizer will be one of two colors.
• the polarizers are usually arranged so that these two colors are yellow and blue. (Actually, only one color can be selected by orientation of the polarizers--and this color can only be selected from a relatively small spectrum of colors. There is very little design freedom in varying the color in the second state--it is essentially a function of the first color.)
• FIG. 2 shows the transmission characteristics of a representative yellow/blue mode STN cell (with associated polarizers) when the cell is in its select and deselect states.
• the transmission spectrum when the cell is "selected" (by applying an excitation voltage of 1.56 volts), the transmission spectrum has a maximum at 400 nanometers, a minimum at 600 nanometers, and an intermediate value at 500 nanometers.
• the transmission spectrum when the cell is "deselected” (by reducing the excitation voltage to 1.41 volts), the transmission spectrum includes a null at 400 nanometers, a maximum at 500 nanometers, and an intermediate value at 600 nanometers.
• Light exiting the cell/polarizer combination in the select state is thus principally blue, and light exiting in the deselect state is green plus yellow plus red, which appears as yellow to the human observer.
• a birefringent STN cell cannot be operated in a black/white mode.
• black requires all wavelengths of light to be linearly cross-polarized with the exit polarizer to effect complete light blockage
• true white requires all wavelengths of light to be linearly polarized parallel with the exit polarizer to effect complete light passage.
• the birefringent operating mode by definition, prevents such results since different wavelengths of light are polarized differently during passage through the material.
• STN cells are unavoidably colored.
• this drawback has been tolerated in order to achieve the high multiplexibility that STN provides.
• compensation layers In order to eliminate the birefringence color, some manufacturers have incorporated various compensation layers in display assemblies.
• One such compensation layer is a second birefringent cell of opposite twist than the first to counteract the wavelength dependence in the cell's behavior.
• Another type of compensation layer sometimes used in conjunction with the above-mentioned blue/yellow mode STN LCDs, is a polarizer that has been dyed to pass cross-polarized light in the blue and red portions of the spectrum in order to make the yellow state of the LCD "whiter.” This still yields a blue/white LCD, instead of the desired black/white.
• this color limitation is usually accepted in order to achieve the high multiplex ratio.
• a color display system is formed by operating a plurality of birefringent cells in cooperation with one or more colored polarizers, thereby complementing and correcting the birefringence colors and yielding a brighter display.
• a plurality of STN birefringent panels are tuned to different subtractive primary colors (i.e. yellow, cyan and magenta) and stacked. Interposed between the panels, and sandwiched about the stack, are polarizers, at least one of which is colored. In some embodiments, this assembly is illuminated by a collimated light source and the resulting image is focused onto a projection screen for viewing. In other embodiments, optics are provided to permit direct wide angle viewing of the display without parallax effects.
• FIG. 1 shows the transmission characteristics of a representative STN cell as a function of applied voltage.
• FIG. 2 shows the transmission spectrum of a representative STN cell when operated in its select state (with an excitation voltage of 1.56 volts) and in its deselect state (with an excitation voltage of 1.41 volts).
• FIG. 3 is a schematic diagram of a display subassembly according to one embodiment of the present invention.
• FIGS. 4-6 are spectral photometer plots showing ideal light transmission characteristics for three liquid crystal panels used in the display subassembly of FIG. 3 when in their selected and deselected states.
• FIGS. 7-9 are spectral photometer plots showing the actual light transmission characteristics of three Kyocera liquid crystal panels used in the display subassembly of FIG. 3 when in their selected and deselected states.
• FIG. 10 is a chromaticity diagram illustrating the performance of the Kyocera panels when in their selected and deselected states.
• FIG. 11 is a diagram showing the eight basic colors achieved by operating yellow, cyan and magenta panels in their various combinations.
• FIG. 12 details the construction of a display assembly incorporating three panels according to the present invention.
• FIG. 13 shows a first projection system according to the present invention.
• FIG. 14 is a perspective view of an integrated assembly including a display assembly and associated optics to facilitate use with an overhead projector.
• FIG. 15 shows a second projection system according to the present invention.
• FIG. 16 shows a self contained color display using a display subassembly according to the present invention with associated projection optics.
• FIG. 17 shows a first direct view display system according to the present invention.
• FIG. 18 shows the spectral distribution of a backlight that may be used with the display system of FIG. 17.
• FIG. 19 shows a second direct view display system according to the present invention.
• FIG. 20 shows a third direct view display system according to the present invention.
• FIGS. 21 and 22 show a fourth direct view display system according to the present invention.
• FIG. 23 shows a portable computer employing a direct view display according to the present invention.
• FIG. 24 shows a laptop computer employing a direct view display according to the present invention.
• FIG. 25 is a view of a portable computer including a direct view display according to one embodiment of the present invention.
• FIG. 26 is a perspective view of the portable computer of FIG. 25.
• FIG. 27 is a view of a portable computer including a direct view display according to another embodiment of the present invention.
• FIG. 28 is a perspective view of the portable computer of FIG. 27.
• FIG. 29 is a view of a portable computer including a direct view display according to yet another embodiment of the present invention.
• FIG. 30 is a perspective view of the portable computer of FIG. 29.
• FIG. 31 shows a display stand that permits a display subassembly to be backlit for direct viewing.
• FIG. 32 shows a display system employing two light sources and two optical paths according to the present invention.
• FIG. 33 shows a display system employing one light source and two optical paths according to the present invention.
• FIG. 34 shows a computer with a roll-up screen according to the present invention.
• FIG. 35 shows a display subassembly using a thin film transistor (TFT) LCD panel in conjunction with an STN panel.
• TFT thin film transistor
• FIG. 36 shows a display subassembly using a TFT panel, an STN panel, and a color shutter.
• FIG. 37 shows a display subassembly using two panels.
• FIG. 38 illustrates a possible color gamut produced by one of the panels of FIG. 37.
• FIG. 39 illustrates the gamut of FIG. 38 after being analyzed with a blue polarizer.
• FIG. 40 illustrates the spectral characteristics of two possible polarizers used in the display subassembly of FIG. 37.
• the primary light colors are generally considered to be red, green and blue.
• White light is composed of all three primaries.
• White light with red filtered therefrom (i.e. removed) is termed cyan;
• white light with green filtered therefrom is termed magenta;
• white light with blue filtered therefrom is termed yellow.
• These latter colors, cyan, magenta and yellow are sometimes termed subtractive primary colors, since they denote the absence of one of the primary colors.
• Filters selectively attenuate (or "absorb") light of certain colors and pass light of other colors relatively unattenuated.
• a red filter for example, attenuates blue and green light and lets red light pass.
• a blue filter attenuates red and green light and lets blue light pass.
• a green filter attenuates red and blue light and lets green light pass. Filters of the primary colors thus subtract two primary colors and let the third pass.
• Filters of the subtractive primary colors subtract one primary color and let the two others pass. For example, a cyan filter attenuates red light and lets blue and green light pass. Similarly, a magenta filter attenuates green light and lets blue and red light pass. Finally, a yellow filter attenuates blue light and lets green and red light pass.
• a blue filter does not absorb blue light. It passes blue light and blocks light of other colors.
• the human eye is more sensitive to certain wavelengths of light than to others.
• the eye's normal daytime response typically peaks at about 554 nanometers and diminishes to near negligible values around 400 and 700 nanometers.
• the optical spectrum is generally segregated into the red, green and blue portions by dividing lines at 500 and 600 nanometers. (For physiological reasons, a precise dividing line cannot be defined.) Using these boundaries, the human eye perceives 55% of the energy in white light from the green portion of the spectrum (500 to 600 nm.), 30% from the red portion (above 600 nm.), and only 15% from the blue portion (below 500 nm.). Perfect green, red and blue filters thus transmit 55%, 30% and 15% of white light, respectively (photopically). Since yellow, cyan and magenta are combinations of these colors, it can be seen that perfect yellow, cyan and magenta filters transmit 85%, 70% and 45% of white light, respectively.
• the LCD panels used in the illustrated embodiments are supertwisted nematic LCD panels that are controllably colored by exploitation of the birefringence effect.
• birefringence is an optical phenomenon in which light oriented along one axis of the material propagates at a different speed than light oriented along another axis. This asymmetry results in different wavelengths of light having different polarizations when they exit the material.
• Polarizers can be used to analyze the elliptically polarized light exiting the panel to select colors.
• Prior art uses of birefringence to control color in LCD panels are discussed in U.S. Pat. Nos. 3,876,287, 4,097,128, 4,127,322, 4,394,069, 4,759,612 and 4,786,146, the disclosures of which are incorporated by reference.
• FIG. 3 there is shown a basic display subassembly 10 according to one embodiment of the present invention.
• the illustrated subassembly includes four LCD panels 12, 14, 16, 18 sandwiched alternately between five polarizers 20, 22, 24, 26 and 28.
• An optional retardation film layer 30 is also shown.
• the birefringent properties of the panels 12-18 are "tuned” (by choosing the thickness (d) of the liquid crystal layer and its optical refractive index anistropy ( ⁇ n)) to yield a desired coloration.
• the birefringent properties of the first panel 12 are tuned so that incoming green light (which has been polarized by the entrance polarizer 20) propagates through the liquid crystal material in such a manner that the orientation of its principal axis upon leaving the cell is orthogonal to the exiting polarizer 22 when the panel 12 is in its deselected (i.e. deenergized) state.
• the panel 12 and polarizers 20 and 22 thus act as a magenta filter when the panel is deselected.
• the tuning of panel 12, and the orientations of the associated polarizers, are also selected so that, when the panel is in its selected (i.e. energized) state, green light is passed, together with red and blue light, to yield a substantially "white” color.
• panel 12 is sometimes called the "magenta” panel and is said to controllably absorb green light. It will be recognized, however, that this and the other panels must be operated in conjunction with associated front and back polarizers to achieve the desired coloring effect.
• the illustrated second panel 14 is similarly tuned to operate as a yellow filter (i.e. absorbing blue) when in its deselected state and to pass all wavelengths of light (i.e. white light) when in its selected state. It is sometimes termed the "yellow" panel.
• the illustrated fourth panel 18 is similarly tuned to operate as a cyan filter.
• the illustrated third panel 16 is an optional "black" panel that may be included to increase contrast. Its construction may take any of a number of forms, as discussed below.
• the ⁇ nd/ ⁇ ratios referenced in Table II can be achieved with any number of cell thicknesses.
• the choice of cell thickness is a tradeoff between several factors, including the panel's response time and uniformity.
• the response time of the panel generally increases with the panel thickness. Consequently, to achieve a fast response time, it is desirable to use a thin panel.
• small fabrication errors such as a 1 ⁇ change in cell thickness over the width of a panel, yields a relatively large variation in panel color behavior and switching threshold voltage.
• To insure color uniformity it is desirable to use a thick panel so fabrication errors are kept to a small percentage of the total liquid crystal thickness.
• a cell thickness of 6 to 12 ⁇ may be used.
• Panels suitable for use as panels 12, 14 and 18 are available from Kyocera of Hayato, Japan as part numbers KC-6448ASTP-SC-M, KC-6448ASTP-SC-Y and KC-6448ASTP-SC-C, respectively, or may be fabricated using known techniques.
• Spectral photometer plots showing the actual behavior of the Kyocera panels are provided in FIGS. 7-9. The plot for the magenta panel in FIG. 7 was made with a red entrance polarizer. The plot for the cyan panel in FIG. 9 was made with a blue exit polarizer.
• FIG. 10 A chromaticity diagram illustrating performance of the Kyocera panels in their selected and deselected states is provided in FIG. 10.
• Each of panels 12-18 comprises a plurality of pixels that can be individually energized to change the spectral distribution of the light that is permitted to pass therethrough.
• light of any color can be transmitted through the display subassembly 10.
• a pixel in the yellow panel 14 is deselected to absorb blue light and the correspondingly positioned pixel in the cyan panel 18 is deselected to absorb red light.
• the magenta panel 12 is left selected (i.e. white transmitting) in this example and thus has no relevant filtering effect.)
• the color blue can be similarly achieved by deselecting corresponding pixels in the cyan and magenta panels, and red can be achieved by deselecting corresponding pixels in the yellow and magenta panels. If it is desired to absorb all light and thus produce a black pixel on the image plane, pixels in all three panels are deselected.
• FIG. 11 shows the eight basic colors achieved by operating a yellow/cyan/magenta series of panels in their various combinations.
• Polarizers are needed to analyze the light passing through the liquid crystal panels in order to achieve perceptible contrast.
• the polarizers are typically neutral (i.e., dyed black by iodine).
• colored polarizers (which are "leaky”) can be used in certain positions to pass more light, improving the brightness and allowing color balance improvements.
• the first panel 12 is illustrated as being "magenta.” Light entering it is polarized by the first polarizer 20. Normally, all colors of light orthogonal to the axis of polarizer 20 would be absorbed by the black dye of a conventional, neutral polarizer, resulting in an immediate loss of 50% (theoretical) of the light. (In actual practice, the loss of a neutral polarizer is about 55-58%.) This loss can be cut dramatically if the first polarizer is dyed magenta. Such a polarizer still passes the white light parallel to the polarizer's axis, but additionally passes blue and red light orthogonal to its axis.
• This additional blue and red light is permitted to pass further into the display subassembly and ultimately contributes to the overall brightness of the resulting display, instead of being absorbed by the first polarizer as is normally the case.
• the losses normally associated with this first polarizer are thus cut by about two thirds. Display brightness improves commensurately.
• the entrance polarizer 20 may be dyed red. While theoretically not as advantageous as a magenta polarizer, a red polarizer is easier to realize and still offers a substantial improvement in brightness, passing about 59% of the incident light, as opposed to 45% or less for a neutral polarizer.)
• the same benefit can be achieved at the exiting end of the sandwiched display subassembly 10.
• the last panel 18 in the subassembly is illustrated as being cyan.
• the exit polarizer 28 may be dyed blue instead of cyan.
• a blue polarizer passes about 56% of the incident light, still yielding a significant improvement in brightness over a neutral polarizer.
• neutral polarizers can be used at the positions (22, 24, 26) intermediate the liquid crystal panels and a significant improvement in display brightness is still achieved by virtue of the two colored polarizers described above.
• the use of neutral intermediate polarizers also assures that there is no birefringence interaction between panels (i.e. the deselected or selected nature of the ⁇ nd of the center panel makes no difference to the passage of light and total birefringence of the adjacent panels).
• the polarizers at the intermediate positions in the subassembly may be colored. Care must be taken, however, not to interfere with the color-selective properties of the birefringent panels. For example, if a yellow colored polarizer is interposed between the magenta and yellow panels 12, 14, it will interfere with the color-selective properties of the magenta panel. As noted, the magenta panel itself does not absorb the undesired green light. Instead, its birefringence is tuned so that light propagating through the panel exits with the axis of its principal green component oriented orthogonally to the polarizer 22, causing it to be blocked.
• this polarizer 22 is colored yellow, it will leak green and red light, including the green light that is meant to be blocked. Consequently, use of a yellow polarizer between the magenta and yellow panels defeats the careful tuning of the first panel's birefringence.
• magenta An equally poor color choice for the first intermediate polarizer 22 is magenta.
• a magenta polarizer would permit blue and red light to enter the yellow panel 14 at an unexpected orientation.
• the yellow panel was tuned so that blue light entering at a known polarization would propagate and exit with a principal polarization that would be blocked by the exiting polarizer 24. If the blue light enters the yellow panel 14 at an unexpected orientation, it will exit at an unexpected orientation and will not be blocked by the exiting polarizer. Consequently, use of a magenta colored polarizer 22 defeats the careful tuning of the yellow panel's birefringence.
• Polarizer 22 should be colored, if at all, a color that both of the adjoining panels are intended to pass. In this case, since the magenta panel is intended to pass blue and red, and the yellow panel 14 is intended to pass green and red, the polarizer 22 should be colored the common color: red.
• black panel 16 is omitted (together with associated retardation film 30 and polarizer 26), similar logic would dictate that the polarizer 24 between the remaining yellow and cyan panels should be colored, if anything, green.
• the polarizers positioned adjacent thereto should be neutral (i.e. not colored) since any polarizer coloring would permit the black panel to leak light--an undesired effect.
• the dyed polarizers should exhibit a high degree of transmissivity to cross-polarized light in their "leaky" portion of the spectrum.
• the polarizers each comprise a dyed 5 mil sheet of stretched polyvinyl alcohol.
• Table III specifies suitable dichroic dyes, which are available under various brand names from Crompton & Knowles, Atlantic, Ciba-Geigy and a variety of other dye suppliers.
• FIG. 12 illustrates in greater detail a display subassembly using just the magenta, yellow and cyan panels.
• the polarizers are magenta, black, black and cyan, respectively. Included in FIG. 12 are details of the relative alignment of the component panels and polarizers in an implementation using the Kyocera panels. The alignment angles are typically specified by the manufacturer and depend, inter alia, on the rubbing angles of the front and rear panel plates, the twist of the LCD molecules, and on various boundary layer phenomena associated with the liquid crystal material.
• such a three panel subassembly can produce the color "black” (the absence of light) by deselecting each panel. Since the light passing through the subassembly is progressively stripped of its green, blue and red components, theoretically no light exits the subassembly. As a practical matter, however, the imperfect responses of the three panels permit some light of various colors to leak through at an attenuated level. The net result is a dark brown or grey color. While such an arrangement yields a contrast ratio of approximately 10:1--more than adequate for many applications--some applications require contrast ratios on the order of 100:1. To achieve such ratios, a fourth panel, such as the "black" panel 16 illustrated in FIG. 3, may be included in the subassembly. The characteristics of the black panel may be optimized for the intended application.
• an "intensity" signal is used to differentiate each of the eight basic colors used in RGB systems into two colors, yielding a total of 16 colors.
• the black cell is optimized for maximum transmission when in the selected state.
• the contrast provided by the cell is of lesser importance. That is, a contrast range of 2:1; or even 1.5:1, will suffice to distinguish the 16 colors of the RGBI system.
• the black panel 16 is a supertwisted nematic cell operated in conjunction with a retardation film 30 that tunes the cell for maximum contrast.
• a double supertwisted nematic cell or even a twisted nematic cell may be used.
• the "black" cell need not be black.
• a birefringent cell tuned to the blue end of the spectrum may be used since the human eye is relatively insensitive to blue light, yielding a relatively high photopic contrast ratio.
• One advantage of the display subassembly of the present invention is the flexibility it affords in possible panel/polarizer sequences. If one sequence seems unworkable, a design can be optimized about another one. For example, if it is found that a good quality magenta polarizer cannot be obtained, then a design that does not require a magenta polarizer can be adopted.
• a display subassembly 10 can be used in a variety of applications, such as color projection systems and in direct view displays. A variety of such applications are detailed below.
• a display subassembly 10 is positioned on the transparent projection surface 34 of a conventional overhead projector 36.
• Such projectors typically include an illumination bulb 38 and a Fresnel lens 40 under the projection surface to produce light beams that pass through a transparency and converge onto a projection lens assembly 42. (Due to the short focal length and high power required of lens 40, it is often formed by cementing two or more lower powered Fresnel lenses together.)
• a Fresnel lens 44 When display subassembly 10 is used in such an embodiment, it is desirable to provide a Fresnel lens 44 to collimate the converging light from the projection surface 34 prior to illumination of the display subassembly.
• the light exiting the subassembly is then focused by a lens 46 (which is also desirably in Fresnel form) onto the projection lens assembly 42.
• Lens 46 here serves the same purpose as the Fresnel lens 40 provided under the projection surface of the projector in the projector's normal operation, namely to focus light towards the projection lens assembly 42.
• An integrated assembly 47 including both the display subassembly. 10 and the Fresnel lenses 44, 46 is shown in FIG. 14.
• a second projection system embodiment 48 of the invention a portion of which is shown in FIG. 15, the collimating and focusing Fresnel lenses 44, 46 used in the FIG. 13 embodiment are omitted.
• the panels comprising the display subassembly are fabricated with different pixel spacings. The spacings on the various panels are selected so that corresponding pixels in the various panels are aligned with the converging light exiting the projection surface of the projector. By this arrangement, no accessory optics are required. Parallax effects are avoided since the internal optics of the display subassembly are designed to cooperate with the focused light used by the projector.
• Projection technology may also be used to provide a self contained display in which an image is projected onto the rear of a viewing screen.
• a color monitor for a computer may be realized in this fashion.
• One such arrangement 50 is shown in FIG. 16.
• a field lens 52 is used to collimate the light from bulb 54 prior to its passage through the display subassembly 10.
• the resulting image is projected by a second lens 56 onto a translucent medium 58 which can then be viewed from the opposite side by a user.
• a display subassembly 10 according to the present invention can also be incorporated into a number of direct view display systems, such as color graphics displays for portable or laptop computers.
• the display subassembly 10 is backlit from a diffused light source, such as a fluorescent light panel 62.
• a diffused light source such as a fluorescent light panel 62.
• entrance and exit optic elements 64, 66 generally collimate the diffuse light prior to entrance into the display subassembly and scatter the approximately-collimated light exiting the display.
• Each of optic elements 64, 66 may comprise a plate having formed thereon a plurality of microlenses 68, one aligned to each pixel. Light incident on one of the microlenses on element 64 is directed substantially normal to the plane of the subassembly and thus passes through the pixels of the component panels in the proper alignment, regardless of its initial orientation.
• Collimated light exiting the subassembly 10 is dispersed by the microlenses on the exit optic element 66, thereby permitting the color image to be viewed from a wide range of angles without parallax effects.
• the interstitial areas 69 between the lenses on exit optic 66 may be colored black to minimize stray light and to improve perceived contrast.
• the arrays of microlenses can be replaced by arrays of fiber optic collimator faceplates or lenticular lenses.
• FIG. 18 shows the spectral distribution of a representative fluorescent backlight 62 that may be employed in the embodiment of FIG. 17.
• the spectrum has characteristic peaks corresponding to certain chemical components used in the light. These peaks (and the nulls) can be tailored to specific applications by changing the chemistry of the lamp.
• the backlit illumination can be collimated by a novel arrangement employing a parabolic mirror 72 (desirably in Fresnel form).
• a pair of linear light sources such as fluorescent bulbs 74, illuminate a generally flat mirrored surface 76 that has facets arranged to provide one axis of collimation. The angles of the facets vary with placement on the surface to simulate a sectioned parabolic reflector. Light reflected from this mirrored surface is substantially collimated.
• a microvenetian blind material 78 such as Light Control Film marketed by 3M Corp, is desirably placed between the mirror and the display subassembly. This material is a thin plastic film containing closely spaced black microlouvers to absorb light misaligned with respect to the louvers. Substantial collimation of the illuminating light is thus achieved.
• a translucent light dispersing material 80 such as a ground glass plate or a commercially available diffusion material (i.e. Rolux film manufactured by Rosco of Port Chester N.Y.) is mounted adjacent the exit side of the display subassembly 10 to display the resulting color image.
• a translucent light dispersing material 80 such as a ground glass plate or a commercially available diffusion material (i.e. Rolux film manufactured by Rosco of Port Chester N.Y.) is mounted adjacent the exit side of the display subassembly 10 to display the resulting color image.
• FIG. 20 shows a third direct view embodiment 82 of the invention.
• the display subassembly 10 is illuminated by a tungsten-halogen lamp 84 that operates in conjunction with a curved reflector 86.
• the reflector is computer designed (using well known optical modeling programs or ray tracing techniques) to provide equal energy illumination to all regions of the display subassembly.
• a corrector plate 88, mounted adjacent the display subassembly, provides a normalization of illumination angle, i.e. perpendicular to the assembly.
• the lamp 84 in the FIG. 20 embodiment is desirably part of a removable module that also includes a shield 90 for preventing direct illumination of the display subassembly by the lamp.
• a diffuser material 92 is mounted adjacent the exit side of the display subassembly to permit direct, wide angle viewing.
• a fourth direct view embodiment 94 of the invention is shown in FIGS. 21 and 22 and includes fiber optic backlighting of the display subassembly.
• a tungsten-halogen lamp 96 is again used, but this time is optically coupled to a bundle of optical fibers 98.
• Each fiber terminates at a microlens 100 on a plate 102 of such microlenses.
• These microlenses can be arrayed in a rectangular pattern on the plate 102, or can be arranged in a hexagonal pattern for higher density. In either event, the microlenses are matched to the dispersion patterns of the fiber so that light exiting the fibers is substantially collimated by the lenses.
• a diffuser optic 104 is desirably positioned adjacent the exit side of the display subassembly.
• FIG. 23 shows a portable computer 106 including a direct view display 108 according to the present invention.
• the case 112 is opened and the display is positioned for viewing.
• the display is coupled to the computer by a coiled cable and can be positioned where desired.
• the display is packed into the case, secure against abuse.
• FIG. 24 shows a laptop computer 114 including a direct view display 108 according to the present invention. As can be seen, the display is coupled to the remainder of the computer by a hinge arrangement 115. The laptop's internal rechargeable battery 105 powers both the computer and the display.
• the hinged display 108 is lifted, exposing it for viewing.
• the hinged display is secured in its collapsed position, protecting it from abuse.
• FIGS. 25-30 illustrate a variety of other portable computer designs that are adapted for use with a display subassembly according to the present invention.
• a computer 200 includes a display subassembly 10 mounted by a hinge 202 to the front top edge of a computer case 204.
• the display subassembly 10 is illuminated by light reflected off a mirrored surface 206 from a lamp 208.
• the lamp 208 is a point source (i.e. it has a relatively small physical extent, such as a small filament) and is fixedly attached to the body of the computer case 204.
• the diverging light from this point source is collimated by a flat lens (not particularly shown in the figures) mounted adjacent the display subassembly.
• the display subassembly 10 on computer 200 pivots rearwardly into the body of the computer case, and the panel 210 to which the mirrored surface is attached folds down over the display, protecting it from abuse.
• the computer keyboard 212 slides into a recess 214 in the front portion of the computer case and a door 216 closes to secure the keyboard in place.
• FIGS. 27 and 28 show a portable computer 218 in which the display subassembly 10 is illuminated by light reflected from a mirror 220 that slides out the back of the computer case 222.
• the illumination is provided by a point source, such as a tungsten-halogen bulb 224 that is mounted to the computer case 222 rather than to display subassembly itself.
• the display subassembly is positioned in a substantially vertical orientation on a hinge 226 at the rear top portion of the case.
• the display subassembly folds forwardly and latches in place over the keyboard 228.
• the mirror 220 is slid towards the case and locks with the mirrored surface adjacent the case's back side.
• the mirrored surface is small enough to be positioned entirely within the computer case.
• the illustrated mirror is hinged at point 230, permitting it to be folded flat and slid entirely within the computer case.
• a flat correction lens is desirably mounted on the rear of the display subassembly to collimate the light reflected from the mirror 220.
• FIGS. 29 and 30 show a portable computer 232 in which the display subassembly is directly illuminated from a point source 234, without an intervening mirror.
• the display subassembly 10 is again attached by a hinge 236, this one in a cavity 240 in the front portion of the computer case 238.
• the display subassembly 10 is positioned substantially vertically and is illuminated by the point source 234.
• the display subassembly 10 folds rearwardly into the cavity and is held secure by the keyboard 242, which is inverted and latched into place to serve as a top cover.
• FIG. 34 shows yet another computer 250 according to the present invention.
• a point light source 252 is disposed within a case 254 and illuminates a display subassembly 256, which fills an aperture formed in the case.
• An image formed by projection of light through this display subassembly is projected on a screen 258 positioned at the back of the computer case 254.
• the projection screen is flexible and is rolled for storage about a spring-loaded roller 260 disposed at the bottom rear portion of the computer case.
• a screen support 262 is pivoted upwardly from its collapsed storage position to an upright position at the back of the computer. The unrolled screen can then be fastened to the screen support by one or more clips, or like means (not shown).
• the pixel pitches on the various panels may be made different (as shown in FIG. 15) to align the pixels with the orientation of the incoming light.
• it is not necessary to collimate or otherwise process the light prior to illumination of the stacked subassembly.
• the invention can be practiced by simply illuminating a stack of uniformly pitched panels with uncollimated 55 light, although parallax effects may cause improper pixel registration, blur and false color edges.
• FIG. 31 shows a final embodiment 116 illustrating use of a display subassembly 10 according to the present invention in a direct view display.
• the display subassembly is removably positioned on an illumination stand 118 for direct viewing.
• the illumination stand 118 has a light-transmitting surface 120 against which the display subassembly can rest, and an internal light source 122 for directing illumination therethrough.
• a small shelf 124 on which the display can be positioned is provided on the exterior of the stand.
• the stand 118 is desirably collapsible to permit ready portability. This can be achieved with a hinge and bellows arrangement 126. Small size can be maintained by using folded optics that include mirroring on the inside back wall 128 of the stand.
• the stand may be provided with optics that emulate the optics of a conventional overhead projector. That is, these optics may focus light incident on the display 10 so that it converges on a point a short distance away.
• these optics may comprise a Fresnel plate lens 130.
• the Fresnel entrance optic 44 used in the FIG. 13 projection system embodiment may be used to collimate the focused light prior to its illumination of the display subassembly.
• the exit optic 132 is again a simple translucent dispersion medium to permit wide angle viewing of the collimated image.
• the viewing stand 118 advantageously permits an LCD display to be used either as a projection device for large audiences (i.e. as an "electronic transparency"), or as a single-user computer screen.
• the stacked panels are split into two sub-stacks to permit illumination by two different light sources.
• the first light source 136 is a tungsten-halogen incandescent lamp, which produces a spectrum that is strong in red, especially when the lamp's operating voltage is decreased, which may be desired to increase the lamp's life.
• the second light source 138 is a mercury arc-lamp, which produces a spectrum rich in deep blue light (430 nm), with a large amount of energy also in the mid-green (540 nm) portion of the spectrum.
• the complementary spectrums produced by these two light sources are advantageously combined in the embodiment of FIG. 32 to achieve good brightness, long lamp life and high color temperature "white.”
• light from the tungsten-halogen lamp 136 follows a first optical path that includes a holographic or dichroic mirror 139.
• This mirror may be designed to pass all of the spectrum except a narrow notch [20 or 30 nm] at 540 nm.
• This filtered light continues on to illuminate a stacked assembly 140 that includes red- and green-controlling panels (i.e. "cyan” and "magenta”).
• red- and green-controlling panels i.e. "cyan” and "magenta”
• the polarizers, collimator, and other optical elements used in this stack and elsewhere in the FIG. 32 embodiment are not illustrated.
• the entrance polarizer on the magenta panel may be red
• the exit polarizer on the cyan panel may be green
• the intermediate polarizer may be neutral.
• the light exiting the stacked assembly 140 is reflected off mirrors 142 and 144 and is directed to exit optics for projection or direct viewing.
• the tungsten-halogen light 136 thus provides illumination at the red and green portions of the spectrum, and the stacked assembly 140 controls these colors.
• red- and green-controlling panels can be tuned without regard to their blue performance [since they encounter no blue light] and the blue-controlling panel can be similarly tuned without regard to its red and green performance.
• a black/white panel may be included in either the first or second optical paths.
• an additional magenta (i.e. green controlling) cell may be included in the stack 140 since green is the dominant contributor to photopic brightness.
• FIG. 32 embodiment provides different optical paths for different portions of the optical spectrum
• the different optical paths can be dedicated to different polarizations of light.
• Such split-by-polarization systems offer improved brightness since the cross-polarized light that is filtered from single path systems is instead directed to a second path where it is utilized.
• FIG. 33 shows a system 150 similar to that of FIG. 32, except the FIG. 33 system uses a single light source 152. Blue light from this light source is stripped off by a dichroic mirror 154, reflected off a mirror 156, collimated by a collimator 158, and directed into a blue controlling LCD assembly 160. Light exiting this LCD assembly is focused by a lens 162 through a blue-passing mirror 164 and into a lens 166 for projection onto a viewing screen.
• the red/green light from lamp 152 passes through mirror 154, is collimated by a collimator 168, and illuminates a stack 170 that includes cyan and magenta panels (which control red and green light, respectively).
• the light exiting the stack 170 is again focused by a lens 172, reflected off the mirror 164 and directed into the projection lens 166.
• TFT thin film transistor
• EBC electrically controllable birefringent
• a limited birefringence mode liquid crystal effect may be given to the TFT panel by adding a retardation film to the (90°) TN of a standard TFT-LCD and by selecting the polarization orientation appropriately.
• the TFT is optimized for whitest white (instead of blackest black).
• the TFT is also tuned to broaden the dip in the spectral transmissivity curve and place it at the appropriate wavelength required by the stacked combination of panels.
• a double dip in this curve may be obtained by use of a retardation film. By providing several layers of retardation film, ideal "notch filter" performance may more nearly be achieved.
• the ECB element may be a supertwisted nematic panel detailed earlier, or may comprise a conventional twisted nematic cell, or a great variety of other elements, such as electro-optic or electro-acoustic crystals.
• STN cells have generally not been used in a classical electrically controlled birefringence mode due to the very restricted range of operating voltages dictated by multiplexibility requirements. Rather, they have been operated in a bistable mode, operating in either the select or non-select states, not in between.
• the present invention exploits the voltage-dependent birefringence exhibited by STN cells within the narrow R.M.S. operating range between V select and V non-select to achieve a broad range of intermediate birefringent colors.
• the TFT panel 302 and ECB panel 304 are sandwiched between three polarizers 306, 308 and 310.
• the subtractive coloration provided by each pixel in the ECB display element 304 is a function of the signal driving that pixel.
• the range of colors produced by this variable birefringence is augmented by one or more additional colors attainable by use of the TFT panel 302 to produce a full color display.
• a display subassembly 320 comprises a white/yellow mode STN (or DSTN) panel 322, a thin film transistor panel 324 and a color shutter assembly 326 in stacked arrangement with associated polarizers.
• Color shutter assemblies are known in the art and are described, inter alia, in U.S. Pat. Nos. 4,758,818, 4,726,663, 4,652,087, 4,635,051, 4,611,889 and 4,582,396, the disclosures of which are incorporated herein by reference.
• the color shutter is operated in alternate frames to block red and green light, respectively (thus giving the appearance of cyan and magenta).
• the TFT provides a very fast switching speed.
• the STN cell 322 is relatively slower than the TFT, but the blue light it controls is relatively less perceptible to the human eye, so the slower response speed is of little significance.
• the FIG. 36 embodiment is intended to lower the costs associated with making a high-information content full color LCD based display system.
• This embodiment requires only one (monochrome) TFT panel, as opposed to three that may otherwise be used to control red, green and blue.
• the STN 322 may be grey-scaled, using a single bit plane of RAM, to 8 or 16 levels. It can provide sufficiently fast response time for moving images.
• the TFT 324 is operated at twice the normal frame rate (i.e. greater than or equal to 120 Hz) and, along with the color shutter 326, controls the red and green image fields sequentially. Both the color shutter and the TFT (with their associated polarizers) are tuned to leak blue all the time, thereby improving the color balance, especially when a tungsten-halogen lamp is used.
• two LCD panels may be stacked and operated independently to produce a full gamut of colors.
• a display assembly 330 according to this construction is shown in FIG. 37.
• the panels will be referenced as STN panels 332, 334, although again, a variety of other technologies can be used.
• STN panels 332 and 334 are fabricated with a higher value of ⁇ nd than those STN panels illustrated earlier.
• An exemplary panel may have a 240 degree twist angle, with a ⁇ nd value of 1.4. The larger ⁇ nd value produces a wider variation in the voltage variable coloration effect.
• multiplex-addressed LCD panel is grey-scaled (either by PWM or multiple frame averaging), intermediate voltages (between V select and V non-select ) can be attained on each pixel, despite the nearly-bistable switching behavior that characterizes STN cells.
• the first panel 332 can obtain, for example, the color gamut shown in FIG. 38.
• the gamut is as illustrated in FIG. 39, instead.
• the color gamut with the blue polarizer is shifted towards the blue, with the result that "yellow” cannot be obtained, but “white” is obtained instead.
• the "blue” may not be as pure as desired ideally, but human-factors experts suggest that a desaturated blue is better for communicating visual information.
• the second panel 334 needs only to be able to make yellow and white to make the full gamut of saturated primary and secondary colors.
• the leakages of the first and third polarizers 336, 338 must not overlap, so rather than choose a yellow polarizer for polarizer 338, it is somewhat preferable to choose an orange polarizer instead (FIG. 40).
• an orange polarizer instead (FIG. 40).
• the display subassembly can be optimized for various purposes and with different performances for different needs.
• polarizers 336 and 338 are cyan and red, green and red, green and violet, green and magenta, and perhaps green and blue.
• the magenta formed by electrically controlled birefringence is generally poor, because the red edge is too soft and must be supplemented.
• a red polarizer is desirable, which provides excellent sharpness (i.e. steepness of edge between yellowish-green and red).
• the gamut of the first LCD 332 may thus be, inter alia, one of the following:
• the second panel 334 should subtract red, at least, so cyan is its chief color and it may attain one of the following:
• Preferred combinations include a+f, b+g or c+d, above. (a+e requires alternating red and green to make yellow; b+e is undesirable because it requires alternating red plus blue pixels to make magenta and cannot make blue; a+d cannot make green, b+d cannot make blue, etc.)
• the display subassembly 330 has been described as operating in a color subtractive mode, it will be recognized that the display may also be operated in an additive mode, either with adjacent pixels being operated together to add spatially, or with a single pixel in the stacked assembly being operated with alternate colors to add temporally.
• liquid crystal panels 332, 334 are tuned to produce four distinct shades of color, they can be operated co-jointly to product sixteen colors without the need for grey scaling.
• the presently preferred is to tune panel 332 to produce a color gamut extending from magenta, through yellow to white, and dye polarizer 336 red.
• Panel 334 can be tuned to produce a color gamut extending from cyan, through green and yellow to white, and polarizer 338 can be dyed green. Again, partial compensation can be used to optimize the various colors produced.
• magenta-dyed polarizer causes the cell to pass blue and red regardless of the characteristics of the cell. With the importance of these factors minimized, the design of the cell can focus on just one factor--high contrast between the select and deselect states for green--and no compromises need be made.
• a birefringent cell operated in conjunction with a neutral polarizer, exhibits a sinusoidal transmissivity versus wavelength curve, as was shown in FIG. 2.
• the cell's ⁇ nd is selected so the minimum of its sinusoidal curve falls somewhere in the green portion of the spectrum. This minimum, however, may be relatively narrow, permitting relatively large amounts of higher and lower wavelength green light to pass through the cell/polarizer combination.
• a retardation film may be employed. While retardation films are generally used to tune the cell's characteristics (i.e.
• the film's action in reversing part of the cell's twist also serves to broaden the dip somewhat.
• the transmissivity curve of the magenta cell in its deselect state may be made to more nearly approximate the ideal (i.e. a rectangular notch that encompasses all of green--500 to 600 nm.).
• display subassembly has been described as including single supertwisted liquid crystal panels, other types of birefringent optics, such as double supertwisted panels or single panels embodying other technologies (such as electro-optic [i.e. lithium tantalum niobate], acoustic-optic, or PET cells) can alternatively be used.
• a higher resolution display can be achieved by stacking two or more cells for each color, with the active lines on one cell overlapping active lines on the other, similar to the basic technique shown in U.S. Pat. No. 4,448,490, the disclosure of which is incorporated by reference.
• Faster switching times can be achieved by stacking several thin panels for each color, as disclosed in U.S. Pat. No. 4,547,043, the disclosure of which is incorporated by reference.
• the basic principles of the invention are also applicable to other display technologies, such as interference color systems.
• certain birefringent panels may be stacked without intervening polarizers.
• two panels may be stacked without an intermediate polarizer to produce white, yellow, green and cyan in the four combinations of select states.
• a green polarizer can be used on the outside layer, since green is common to all these colors.
• Such an embodiment is especially valuable for a white, magenta, cyan and blue combination, since overhead projection needs more blue throughput, which may be obtained by use of a "pure blue" polarizer.
• any of the LCD birefringence colors are not ideal, some attenuation of specific light wavelengths might enhance the color gamut and overall contrast.
• two polarizers might be used together, or a weak color filter compensator (i.e. a conventional gelatin filter) might be added.
• grey scaling techniques can readily be applied to the present invention to provide the full gamut of possible colors.
• grey scaling is applied to each of the three colored cells.
• grey scaling is applied simply to a fourth (typically black) cell included in the stack.
• U.S. Pat. Nos. 4,840,462, 4,840,460, 4,818,078, 4,766,430, 4,743,096, 4,709,995, 4,560,982, 4,508,427, 4,427,978 and 4,043,640 teach various grey scaling techniques and are incorporated herein by reference.
• the invention can be practiced with more or less panels than the three or four illustrated.
• laboratory instruments such as oscilloscopes and analyzers
• displays formed by stacking two supertwisted birefringent panels with one or more colored polarizers.
• the display may take virtually any of the forms discussed above and still be suitable for inclusion in the instrument. While color gamut is compromised somewhat by such a two panel stack, brightness is increased and cost is reduced.
• TFT panels are cojointly operating several TFT panels, either in stacked or split-optic arrangement, rather than including a single TFT panel in a stack with other panels, as particularly discussed above.
• a number of such embodiments may be realized by substituting TFT panels for the STN panels in the illustrated embodiments.
• the neutral polarizers typically provided on a TFT panel by the manufacturer may be removed, any spectral deficiencies of the panel may be compensated for by retardation film [i.e. commercially available panels are particularly deficient in the blue portion of the spectrum, which deficiency can be alleviated by retardation film], and colored polarizers may be added to achieve the benefits discussed earlier.)

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Abstract

Description

              TABLE I                                                     
______________________________________                                    
Filter        Absorbs       Passes                                        
______________________________________                                    
Red           Green, Blue   Red                                           
Green          Red, Blue     Green                                        
Blue           Red, Green    Blue                                         
Yellow         Blue           Green, Red                                  
Cyan           Red            Blue, Green                                 
Magenta       Green           Blue, Red                                   
______________________________________                                    

              TABLE II                                                    
______________________________________                                    
Panel      Δnd/Ψ  Ψ                                         
______________________________________                                    
Magenta    0.19             4.19 (rad.)                                   
Yellow      0.23              3.84                                        
Cyan        0.25              4.19                                        
______________________________________                                    

              TABLE III                                                   
______________________________________                                    
POLARIZER          DYE                                                    
______________________________________                                    
Magenta            Direct Red #81                                         
Yellow               Direct Yellow #18                                    
Cyan                 Direct Blue #1                                       
______________________________________                                    

              TABLE IV                                                    
______________________________________                                    
POL1   LCD1     POL2   LCD2   POL3 LCD3   POL4                            
______________________________________                                    
Y/G/R/K                                                                   
       Y        G/K    C      B/K  M      M/R/B/K                         
M/R/B/K                                                                   
       M        R/K    Y      G/K  C      C/G/B/K                         
Y/G/R/K                                                                   
       Y        R/K    M      B/K  C      C/G/B/K                         
______________________________________                                    

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• 1996
  • 1996-08-05 US US08/692,300 patent/USRE36654E/en not_active Expired - Lifetime

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