Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • 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
  • G02F1/01—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 
  • 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
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/133509—Filters, e.g. light shielding masks
  • G02F1/133514—Colour filters
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • 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
  • G02F1/01—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 
  • 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
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/133509—Filters, e.g. light shielding masks
  • G02F1/133512—Light shielding layers, e.g. black matrix
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • 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
  • G02F1/01—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 
  • 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
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/133528—Polarisers
  • G02F1/133536—Reflective polarizers
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • 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
  • G02F1/01—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 
  • 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
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/136—Liquid crystal cells structurally associated with a semi-conducting layer or substrate, e.g. cells forming part of an integrated circuit
  • G02F1/1362—Active matrix addressed cells
  • G02F1/1368—Active matrix addressed cells in which the switching element is a three-electrode device
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2203/00—Function characteristic
  • G02F2203/02—Function characteristic reflective

Definitions

• This invention relates to liquid crystal displays (LCD) and, more particularly, to a method for dramatically improving the saturation of the primary colors in an LCD with minimal impact on transmittance.
• a prior art color LCD locates an appropriately-colored absorbing filter at each picture element of the panel.
• Each of these absorbing color filters is placed, as a part of a mosaic array of such elements, inside the panel, on the inner face of the LCD glass nearest the viewer.
• the depth of color and the saturation achievable with these filters is limited by a number of factors, primarily driven by the need to trade off display luminance against color saturation.
• the LCD display chromaticity is determined primarily by the LCD filter transmittance and the backlight spectral emissions.
• the spectral emissions of the phosphors used in avionic LCD fluorescent lamps are typically narrow band. However, if the LCD color filter for a given color has a bandwidth which overlaps an adjacent color, the advantage of the narrow band phosphors is lost.
• the red filter overlaps into the green spectrum sufficiently to desaturate the red emission.
• the invention provides dichroic filters, located in cooperation with absorbing filters on an LCD display.
• the dichroic filters improve the saturation of the transmitted colors by acting with the absorbing filters to narrow the spectrum of the light emitted by each pixel.
• a display made with this combination of filters maintains good color even when viewed from angles other than normal to the display surface.
• the added filters cause less of the incident light to be transmitted through the display.
• this effect is small because the absorbing color filters can be thinner (less absorbance in band) when this performance out of band is augmented by the dichroic filters.
• this reduction in the transmitted light is accompanied by a decrease in unwanted light. Light of the unwanted color is reflected back into the light source cavity. This reflected light is available to offset the transmittance loss due to the increased saturation.
• Figure 1 shows a schematic diagram of the color liquid crystal display of the invention.
• Figure 2 shows a color liquid crystal display utilizing the apparatus of the invention to provide dichroic filters opposite absorptive filters on a liquid crystal display.
• Figure 3 shows a color liquid crystal display wherein the absorptive filters and the dichroic filters are on the same side of the liquid crystal display.
• Figure 4A shows a schematic cross section of a color liquid crystal display where absorptive filters and dichroic filters are on the backlight side of the liquid crystal display.
• Figure 4B shows a schematic of the specular reflective material used in the apparatus of the invention.
• Figure 5 A shows a diagram of the various components of the active matrix liquid crystal display of the invention.
• Figure 5B shows a detailed schematic diagram of the thin film transistor.
• Figure 6 graphically shows a measured reduction in light as a function of exposed surface area by a particular absorber as commonly found in LCD displays.
• Figure 7 illustrates one example of a reflecting backlight cavity as contemplated for use in the invention.
• Figure 8 shows the placement of color filters made from gels in an assembly, the filters being substantially absorbing and made from photographic tape having 4% reflectivity placed on a .060" soda line glass substrate.
• Figure 9 illustrates an alternative front glass for one embodiment of the invention that was constructed using color separation filters placed in a film matrix.
• Figure 10 shows the special characteristics of Kodak Wratten gel filters.
• Figure 11 shows the spectral characteristics of dichroic filters used in one aspect of the invention to reflect out-of-band light into the backlight assembly while transmitting light which falls within the pass band of the filter.
• Figure 12 schematically illustrates the liquid crystal display utilizing a reflective polarizer in accordance with one aspect of the invention.
• Figure 13 schematically illustrates increased contrast for traditional LCDs under high ambient illumination.
• Figure 14 is a simplified illustration of loss of an LCD's traditional advantage under high ambient illumination.
• Figure 15 graphically shows the opportunity for increased display luminance based upon changing the elements of the display so that they accomplish their function in a manner which conserves radiant energy.
• FIG. 1 shows a schematic of the liquid crystal display of the invention.
• Ambient illumination 37 is composed of a red radiation 32, green radiation 34, and blue radiation 36.
• Each color set is transmitted through front polarizer 2 which polarizes the incoming light along the page.
• the light then is transmitted to absorbing red filter 121, absorbing green filter 123, and absorbing blue filter 125.
• Each of these filters are provided on top of corresponding pixels.
• the red pixel 112 receives light polarizing in the direction of the paper 62.
• the absorbing filter transmits a small amount of non-red light.
• the absorbing filter 121 will absorb all but the red band of radiation.
• the example of Figure 1 shows an "off' pixel 112 which provides for a liquid crystal reflection of the light.
• the pixel 112 polarizes the light perpendicular to the plane of the page and the rear polarizer 4 absorbs it. Reflecting radiation 64 and the non-red wavelength that is green and blue is transmitted back and is reabsorbed by the absorbing filter 121.
• the dichroic or reflecting filter 122 provides for reflection of non-red frequencies and transmittance of red frequencies.
• Dichroic filter 124 will then reflect the non-green components 68 and the absorbing filter 123 on the front end will absorb the reflected light.
• the apparatus of the invention provides for a black display when the pixel is off.
• the black display is a result of the polarization nature of the liquid crystal and the inclusion of a rear-polarizer.
• Backlight 12 provides illumination of the display wherein white light is propagated in a similar manner from the backlight through the filters. The detailed description of this illumination technique will be described with reference to Figures 2, 3 and 4.
• Figure 2 shows a cross section of a liquid crystal display that is configured to provide a composite color signal.
• the color liquid crystal display 10 is composed of a number of planar components.
• the front glass 16 covers the surface of the liquid crystal display 10.
• the front glass 16 is placed over an absorptive filter 241 which provides a method of absorbing a frequency band of radiation.
• the absorptive filter 241 is composed of a number of pixel sized color filters.
• the absorptive filter 241 covers a liquid crystal 18 which is controlled by thin film transistor pixel controllers 261.
• the thin film transistor pixels are addressed with lines 251.
• the thin film transistor pixels 261 are planarized using a planarization technique well known in the art.
• the next layer is a dichroic filter layer 221 which provides a patterned dichroic filter for each pixel tuned to the color of the pixel.
• red light 32 is provided by dichroic filter 222, absorptive filter 241, and thin film pixels 261.
• the color liquid crystal display 10 uses the backlight 12 to provide white light to the back glass 14.
• the white light from the back glass 14 enters a dichroic filter layer 221 and, depending upon the color tuning of the dichroic filter, will either transmit red, green or blue. In the example of pixel 291, red will be transmitted.
• the dichroic filter reflects frequencies other than the tuned frequency, in this case red 32.
• the red 32 radiation enters the thin film liquid crystal pixel 291 and is either transmitted or absorbed back, depending upon the state of the shutter in the pixel 291. If the pixel 291 is open, the red light enters the front glass and is projected to the viewer.
• FIG. 3 shows a cross section of a color liquid crystal display of the invention.
• the liquid crystal display 20 is comprised of a number of layers.
• the front glass 16 covers an absorptive filter 242 which is patterned in a predetermined pattern to cover the liquid crystal display pixels.
• the absorptive filters 242 immediately contact dichroic filters 222.
• the absorptive filters absorb light incident from the front glass and transmit only the light of a desired wavelength to enhance the display.
• the dichroic filters in conjunction with the absorbing filters allow only the transmission of the desired frequencies out of the front glass.
• Figure 3 provides a liquid crystal display 20 which is comprised of thin film transistor pixels 262.
• the liquid crystal display of Figure 3 also includes a backlight by providing a white light source filtered by the dichroic and absorptive filters.
• the embodiment of Figure 3 also includes reflective specular structures 97 over both the thin film transistors and the array addressing apparatus in order to provide as much of a reflective environment as possible.
• Figure 4A shows a cross section of the liquid crystal display of the invention in an alternate embodiment.
• the absorptive filters 243 are in direct contact with the dichroic color filters 223.
• the dichroic color filters are attached to the back glass 14 instead of the front glass 16 as in the configuration of Figure 3.
• the backlight 12 provides white light to the dichroic and absorptive filters which only permit a desired pre-determined color to be transmitted through each pixel.
• the absorptive filters prevent the reflection of unwanted radiation from the cell due to the ambient environment. This enhances the color of the liquid crystal display.
• the invention can be used as a display in an aircraft. Since avionic displays are subjected to full daylight operation, the effect of having a reflective filter in the display must be considered. As seen in Figure 4 A, ambient light 39 will enter the display 30, passing through the front polarizer and absorptive filter 243 before striking the dichroic reflective filter 223. The front filter 243 absorbs most of the energy reflected from the dichroic filter 24. The red filter absorbs blue and green which will be reflected by the red dichroic filter 243. Any energy reflected from the dichroic filter 223 must pass through the front filter 243 again, where even more of the unwanted color is absorbed.
• the red cell 131 in Figure 4A illustrates an "off' pixel in a normally black LCD construction.
• the greatly reduced blue and green spectrums pass through the liquid crystal 18 material twice and experience a 180 polarization change due to the dichroic filter 223 reflection.
• the red spectrum will pass through the dichroic filter 223.
• the polarization of the red light which passes through the dichroic filter is crossed relative to the rear polarizer 134 and is absorbed.
• the green cell 133 in Figure 4A is shown to be "on” or transmitting.
• the liquid crystal 18 in the green cell does not change the polarization and allows the green light to pass into the lighting cavity, where it can be redirected toward the viewer.
• the red and blue spectrums are reflected with a 180 degree polarization rotation, but are greatly reduced by the front green filter 243 absorption.
• the end result is that the absorptive filter 243 provides a high contrast display when exposed to full daylight operations.
• the front filter absorbs the unwanted ambient photopic spectrum reflected from the dichroic filter. Light of the desired chromaticity is allowed to pass into the lighting cavity 100, where it may be redirected toward the observer.
• the dichroic/absorptive filter LCD 30 provides a significant advantage over prior art LCDs using traditional filter techniques. The invention minimizes absorption and therefore reduces power consumption for equivalent luminance outputs.
• the patterned dichroic filters are created in a matrix aligned with the absorptive filters.
• the patterned dichroic filter plate is placed into the TFT substrate.
• the TFTs are built directly on top of the dichroic filters 223.
• the basic matrix of dichroic filters is constructed so that there is no space between the dichroic filters.
• the dichroic filters 223 will cover the rear of the LCD active area.
• An alternative approach is to place specular reflective material 97 between each dichroic color filter. This increases the specular reflections from the rear of the LCD. Since 40-50% of the active display area is non-transmissive, this 40-50% can be covered with specular reflective material to decrease the amount of absorption in the LCD glass assembly. This approach has a slight advantage over covering the entire surface with dichroic filter material.
• the dichroic filters will reflect 50-60% of the light in the non- transmissive area versus >90% from a specular reflective area.
• the Reflective/Absorptive color filter LCD can be constructed with the following methods: 1. Dichroic filters under the active pixel area, with or without specular reflective material between the active color pixel areas as shown in Figure 2.
• Figure 4B shows the apparatus of 4A speculative reflective coating.
• the speculative reflective coating 908 is provided to enhance the radiant energy conservation in the backlight area.
• the red pixel 131 is surrounded by specular reflective material.
• the blue pixel 132 is also surrounded by specular reflective material 908.
• the green pixel 133 is also surrounded by specular reflective material 908.
• the theoretical basis for creating the radiant energy conservation apparatus is discussed below.
• the various pixels are arranged in a predetermined fashion to arrive at a red, green, blue color display 30.
• the active matrix liquid crystal display structure of the invention is shown in a three dimensional schematic in Figure 5 A.
• the front glass 16 is shown at the top.
• the front glass 16 covers the absorbing filters 24.
• the absorbing filters then cover a liquid crystal 18.
• the liquid crystal 18 covers a planarization layer 72, which planarizes the thin film transistor pixels underneath.
• the thin film transistor pixels are shown better in Figure 5A.
• the dichroic color filters 22 are shown beneath the thin film transistor array 50.
• the rear glass 14 is shown beneath the dichroic color filters. This configuration corresponds closely with the color liquid crystal display disclosed in Figure 1.
• Figure 5B shows a detailed schematic of the thin film pixel structure used in the liquid crystal displays of the invention.
• the thin film pixel 50 is comprised of a source bus 54 and a gate bus 52.
• the source bus and gate bus control a thin film transistor 58.
• the thin film transistor 58 controls the indium tin oxide pixel 26.
• a thin film pixel 50 is provided at each pattern location in the liquid crystal display.
• Each thin film pixel 50 is provided with a tuned color absorbing filter and a tuned color dichroic color filter.
• Fluorescent lamp phosphors which are capable of producing saturated primary chromaticities when used in conjunction with sharp cut-off LCD filters, are employed. However, the filters do not have extremely sharp cut-off. Some lamps have unwanted emissions between 570 and 600 nanometers which desaturate both the red and green colors. This unwanted emission is primarily a side band emission of the green phosphor.
• Tri-band absorptive filters commonly used on avionic CRT color displays, are also an alternative. However, the overall transmission of absorptive filters must be low if saturated colors are to be achieved.
• band rejection filters to modify the LCD primary chromaticity is dependent on the basic performance of the LCD filters and the background leakage.
• the filters must have very low transmissions outside of the color of interest, for example, no blue leak in the red filter. Dark area transmittance (blue and green off) must be minimized for high contrast and good color saturation.
• the background emission has two predominant factors. First is the polarizer efficiency.
• the polarizers which are being considered are very high efficiency (extinction ratios greater than 500:1).
• the second factor, cell spacing is particularly sensitive in the normally black displays.
• normally black construction the black intensity is controlled by insuring that the light passing through the LCD is rotated 90 degrees to the front polarizer orientation. The 90 degree rotation is controlled by the cell spacing of the LCD and the birefringence of the material. If this is not carefully controlled, the LCD will exhibit low contrast and chromaticity desaturation.
• Dichroic filters can provide fairly sharp cut-off lowpass, highpass, bandpass and band-rejection filters. These filters work very well at the viewing angle for which the filter is designed. Typically, the filter is designed for viewing normal to the filter surface. The viewing angle can be optimized for off-axis viewing at the expense of performance at the normal viewing angle.
• Absorption filters have the advantage of being very angle-insensitive.
• the primary type of filters used by the prior art are dye-based.
• the filter materials include organic materials, polyimides and photo-resists.
• the general movement in industry is away from the organic materials and dyes toward more stable pigments and deposition materials such as polyimides.
• the photo-resist materials have the advantages of well known processing requirements and equipment availability.
• the result of increased color saturation is higher power dissipation due to increased absorption of the LCD or backlight filters.
• the invention reduces absorption in the LCD while increasing color saturation by exploiting advantages of both dichroic and absorptive filters.
• the dichroic filter that passes only the appropriate band of wavelengths is placed behind each pixel.
• red dichroic filters 223 are located behind each red pixel 131 filter (toward the backlight).
• the red dichroic filter 223 cutoff frequency can be selected to remove the unwanted orange lamp emission and ensure that there is no blue leak to desaturate the red.
• the cut-off frequency of the dichroic filter will shift to the blue spectrum as the display 30 is viewed from off-axis. The result is a slight red desaturation at off-axis viewing angles.
• the desaturation is limited to the absorbing filter transmission since it is not angular sensitive.
• the red dichroic filter 223 is only over the red pixel 131, this does not adversely affect the green or blue chromaticity, as would be the case with a dichroic filter over the entire backlight assembly.
• the cut-off frequency of the dichroic filter is approximately 40 nanometers greater (toward the red spectrum) than the main spectral emissions of the lamp 12. This is done intentionally to ensure that the angular effects of the dichroic filter do not cause luminance loss at extreme viewing angles.
• LCD liquid crystal display
• the backlight assembly for an LCD may be regarded as an integrating sphere with a huge exit aperture.
• the backlight assembly of an LCD contains materials with less than ideal reflective properties.
• a fluorescent bulb used as the LCD's light source cannot accurately be represented as a point source within the lighting cavity. These two factors make the amount of energy absorbed a function of material placement and shape. These effects can be reduced by using diffuse reflecting surfaces instead of specular reflecting surfaces and by paying some attention to the size and distribution of the fluorescent light source within the reflecting cavity. This description details how to use an integrating sphere to measure the absorbance of materials.
• Honeywell light absorption experiments measured the decrease in light as the amount of absorptive material was increased within a luminance integrating sphere purchased from Hoffinan Engineering model number LS-65-8C.
• Light absorbing material was made in two different shapes: spheres and disks. Each shape was fabricated from metal and made in various diameters which were sandblasted to achieve a rough texture and painted flat black to achieve approximately 100% absorption of incident visible light. Each sphere or disk was individually placed inside of the integrating sphere, and the light was measured through the exit aperture of the integrating sphere using a Pritchard 1980A photometer. Figure 6 shows the measured reduction in light intensity as a function of the exposed surface area of the particular absorber.
• the vertical axis 90 of Figure 6 is the measured luminance plotted as a percentage of the integrating sphere's light output, normalized to 100% for the condition where no additional absorptive materials are added to the integrating sphere.
• the horizontal axis 92 of Figure 6 is the surface area of the added absorber divided by the internal surface area of the integrating sphere expressed as a percentage. The horizontal axis 92 was selected to allow for extrapolation of the measured data to LCD lighting cavity applications.
• the squares 94 on Figure 6 represent the measured data for sphere-shaped absorbers, and the circles 96 represent the data for the disk shaped absorbers.
• a first curve 98 was plotted for the measured sphere data and a second curve 1100 was plotted for the measured disk data. The difference between the first and second curves indicates the effect of absorber shape within the experimental apparatus.
• the continuous curve 98 underlying the measured disk data shown in Figure 6 is predicted data for the integrating sphere with an average internal reflectance of 96.5%.
• the basis for calculations used in the experiment is found in two references: Illumination Engineering. J. B. Murdoch, 1985, pp 45-48, and "Applied Optics", Volume I, L. Levi, 1968, pp 31-32.
• Murdoch presents an equation which calculates the exitance at the integrating sphere's exit aperture.
• the exitance having units of lumens per unit area, and the luminance, having units of candela per unit area, are directly proportional at the integrating sphere's exit aperture.
• Murdoch's equation is
• E (p ⁇ )/(A t (l-p)) where E is the exitance, p is the average reflectance of the sphere, ⁇ is the luminous flux of the sphere's light source, and A t is the internal surface area of the integrating sphere.
• the average reflectance (p) of the sphere is decreased.
• the reflectance of the sphere is dependent upon two factors:
• the reflectance of the sphere with no absorbers added can be estimated using the following equation,
• variable, (A a / A,) for the horizontal axis of Figure 6 is in both the numerator and denominator of the above expression.
• the predicted data of Figure 6 was calculated using this expression with p m equal to 0.965. Applica ion of P
• Figure 15 graphically exhibits the extension of this energy loss model to the conditions expected for a typical avionics display.
• the bottom curve 310 reflects the operating condition for a conventional display.
• the other curves 302 - 308 represent the potential gains associated with the addition of the proposed perfect reflective optical components.
• curve 308 represents the addition of a reflective aperture matrix.
• Curve 306 represents the addition of reflective polarizers to the display of curve 308.
• Curve 304 represents the addition of reflective color filters to the display of curve 308.
• Curve 302 represents a display having reflective aperture masks, reflective color filters and a reflecting polarizer such as a wire grid polarizer.
• an effective exit aperture is defined as a component wherein light which is completely absorbed can be regarded as having passed out of an exit aperture.
• Substituted materials which reflect absorbent light can be modeled by reducing exit aperture area.
• the lighting cavity's size was assumed to be 6 x 8 x 2.3 inches to calculate the internal surface area of the integrating cavity (A,).
• p m was assumed to be 1.0, and the horizontal axis variable, (A a / AJ, was expressed as a percentage ranging from 0% to 10%.
• the bottom curve 310 was calculated with * .
• Curve 306 reflects the potential gain associated with adding a wire grid polarizer to an LCD module using a reflective row and column address structure.
• the effective aperture is decreased to 13.2 square inches based upon the assumption that the light previously lost in the rear polarizer is now recovered by reflection within the lighting cavity. Previously the lost light amounted to about 50% of the total light produced by the backlights.
• the absorption of each component can be quantitatively estimated. This can be accomplished by repeating the integrating sphere experiment using the components or representative pieces of the components to measure the light loss. From this measured data an equivalent black disk surface area can be determined. This equivalent surface area can then be scaled to the ratio of the proper size for the material which will be present within the lighting cavity divided by the size of the measured piece.
• Table 1 hereinbelow shows the light loss for sections of an ATSD (Honeywell Air
• Dashed line 312 at 2.88% represents the amount of relative absorption associated with a fluorescent lamp having a 15 mm diameter, and 1226.6 mm length.
• a section of a milky white diffiiser was placed inside of the integrating sphere and the light loss was measured.
• the diffiiser piece had an exposed surface area of 1.565 square inches.
• the measured loss was 5.96% for this piece.
• a black disk with a surface area of 0.3219 square inches will exhibit this percentage of loss from the Hoffman Engineering integrating sphere.
• Scaling the loss for a diffiiser which is 48 square inches and placed inside of the lighting cavity gives 6.15% of absorptive material relative to the lighting cavity. Taken together, the fluorescent bulb and the diffiiser result in 9.03% of absorptive material within the lighting cavity.
• the effective aperture is reduced by a factor of 2 because the loss associated with the rear polarizer may advantageously be approximately
• a reflecting backlight cavity 700 was constructed using Spectralon (TM) material to coat inside wall 706.
• Spectralon (TM) was selected because of its excellent diffuse reflectance properties across the visible spectrum.
• a fluorescent bulb 720 was installed to provide a representative source of illumination.
• a front glass 710 was constructed with 1 by 1 inch pixels in a 6 x 8 inch active area 702.
• Figure 8 shows the placement of color filters 402 made from Kodak Wratten gels in a front glass assembly 400 that was substantially absorbing made from photographic tape placed on a 0.060 inch soda lime glass substrate.
• the photographic tape had 4% reflectivity. This is intended to simulate the approximate aperture ratio (51 %) and absorbance of matrix displays with RGB color filter structure and about 170 pixels per inch aperture density.
• Such a front glass surface represents the conventional approach to manufacturing flat panel displays, in this case with a flat field white image displayed.
• Figure 10 shows the spectral characteristics of the Kodak Wratten gel filters used in the construction of the front glass assembly 400. Note that the vertical axis 500 represents transmittance and the horizontal axis 502 represents wave length in nanometers.
• Curves 504, 506 and 508 respectively represent blue, green and red light transmittance characteristics. Note the substantial overlap area at point 510 between the blue and green filters. Such an overlap blurs color distinctions and is undesirable because of the resultant de-saturated primary colors.
• an alternative front glass 800 was constructed using color separation filters 802, available from OCLI of California, placed in a commercially available 3M brand Silverlux (TM) film matrix 806 (not shown) having greater than 95% reflectivity. All surfaces, generally designated 804, which are not within RGB squares are reflective of incident light arriving from the back lighted side of the glass 800.
• Figure 11 shows the spectral characteristics of the OCLI filters 802.
• the vertical axis 600 represents transmittance and the horizontal axis 602 represents wave length in nanometers.
• Curves 604, 606 and 608 respectively represent blue, green and red light transmittance characteristics.
• the overlap area at point 610 between the blue and green filters is substantially less than for the absorbing glass assembly 400 as graphically illustrated in Figure 15 at point 510.
• These filters are of dichroic construction, so they reflect out-of-band light into the backlight assembly while transmitting light which falls within the pass band of the color filter.
• Another front glass assembly was constructed to hold an absorbing polarizer Sanritsu model 9218. Each of the front glass assemblies 400, 800 were placed in front of the backlight assembly as shown in Figure 7.
• a macro scale model was constructed to provide a means of measuring the increased display luminance for a constant luminous input such as the lamp 720.
• the model is considered to be a "macro scale" model because the glass plates 710 were made having approximately 1/170 times the aperture density of an LCD designed for a typical avionic application.
• the macro scale model had individual color apertures which were approximately one inch square distributed on a 6 x 8 inch glass substrate in a RGGB mosaic.
• the lighting cavity 700 itself was the same size as the lighting cavity planned for use in an avionics application.
• the first method of measurement used a calibrated 1980A Pritchard photometer located normal to the display surface and focused on a single green aperture of the glass plate under measurement. This method is referred to as the "green pixel” method hereinbelow.
• the second method utilized a 40 inch diameter integrating sphere to collect the radiant energy emitted by the display surface in all directions. This was accomplished by affixing the face of the display to the sphere's large aperture such that all of the radiant energy from the display surface was emitted into the integrating sphere's cavity, which is coated with a diffuse, highly reflective material such as Spectralon (TM). Another much smaller aperture on the integrating sphere was used to measure the radiant energy emitted by the display surface. A calibrated Pritchard 1980A photometer was focused onto the small aperture to measure the emitted radiation. This method of measurement eliminates the issue of direction from radiant energy measurements, and will be referred to as the
• the black photographic tape was used to represent the opaque row and column interconnection area on an LCD.
• the glass plate is referred to hereinbelow as the absorptive glass plate.
• a second glass plate was constructed from reflective color filters, namely, OCLI's color separation filters, and reflective tape, namely, 3M Silverlux (TM) tape.
• the second glass plate was utilized to simulate the construction of a display using the concepts described in this patent application.
• the second glass plate included reflective color filters and a reflective aperture mask.
• Abs is defined to be the absorptive glass plate described above.
• Ref is defined to be the reflective glass plate described above.
• Pol is defined to be the absorptive polarizer (Sanritsu model 9218) glass plate described above.
• Abs + Ref indicates that the absorptive plate was placed between the reflective glass plate and the light source within the box (i.e., the viewer saw the reflective plate's surface).
• Wire Grid Polarizer Any polarizer which has minimal absorption of the incident radiant energy can be used to increase AMLCD luminous efficacy if it is used as described herein.
• a polarizer may be constructed from a grid of thin conductive wires which are aligned parallel to one another. The approximate width of the conductors needed to polarize the visible spectrum is 0.1 micrometers. These conductive wires must be aligned parallel to one another with a spacing of approximately 0.3 micrometers between wires.
• the reflective polarizer described in this patent application will offer the desirable feature of having a good extinction ratio for a broad angle of incidence of radiance.
• the most desirable feature is reflection of the incident radiation which is not aligned with the reflective polarizer's transmissive polarization axis. This presents the opportunity of returning this energy to the system by redirecting and "re-polarizing" the radiation such that it may pass through the reflective polarizer and the traditional polarizers to the display's observer.
• wire grid polarizer has been constructed for operation in the visible spectrum.
• the wire grid polarizer is only one type of reflective polarizer which conserves radiant energy by returning untransmitted energy through reflection, and other equivalent devices may be employed to accomplish the objectives of the instant invention.
• Wire grids have traditionally functioned as polarizers where their application has migrated upward along the electromagnetic spectrum as the technological improvements enabled devices with progressively smaller dimensions. This progression has continued for two reasons. The first reason is driven by the need for finer and finer detailed metallic structures for use within the semiconductor industry. Secondly, the high conductivity of metals has been maintained at the wavelengths of radiation previously considered. The conductivity of commonly available metals will decrease in the visible spectrum making this more of a limiting characteristic.
• Wire grid polarizers manufactured in accordance with the present invention exhibit good extinction ratios over a broad spectrum as is shown, for example, in Figure 15 of "The Wire Grid as a Near-Infrared Polarizer" by G. Bird and M. Parrish in Journal of the
• the extinction ratio is controlled by the conductivity of the metal used to create the wire grid. The more conductive the wire material is, the broader the angle over which the polarizer's extinction ratio is acceptable.
• the first choice of common metals based upon this selection criterion is silver, however, those skilled in the art will recognize that any metal which maintains a high fairly constant conductivity over the visible spectrum, aluminum for example, could be used.
• a wire grid polarizer made in accordance with the invention may be manufactured using well-known lithographic techniques such as employing, for example, an electron beam process for etching.
• Typical pitch between conductors for a reflective wire grid polarizer which will work in the visible spectrum should be approximately l A the wavelength of the radiation ( ⁇ .2 ⁇ m for green light).
• the apparatus shown in Figure 12 includes an LCD display 2100, including a conventional LCD assembly 102, an absorbing rear polarizer 104, a reflecting wire grid polarizer 106, a backlight cavity 108, and a fluorescent light source 110 of known serpentine construction.
• the conventional LCD assembly may include liquid crystal cells, a polarizer and a display glass.
• Ray 1112 which extends from the fluorescent bulb source to surface 120 of the reflecting wire grid polarizer 106, represents emitted light from the fluorescent light source 110 having both p and s polarization.
• Ray 1114, which extends from surface 120 to back plate 1122, represents reflected light of s polarization only.
• Ray 116 which extends from the wire grid polarizer 106 to outside of the view surface 1102, represents transmitted light of p polarization only.
• the backlight cavity is advantageously light tight and coated with diffusely reflective material.
• the reflected light of ray 1114 is scrambled by the diffusely reflective material 107 which coats the backlight cavity
• the polarization axes of the wire grid polarizer 106 are aligned with the polarizing axes of the absorbing polarizer 104.
• FIG. 13 a simplified illustration of increased contrast for traditional LCDs under high ambient illumination is schematically illustrated. Shown is a reflective backlight cavity 108, a first absorbing polarizer 130, a transmissive liquid crystal cell 132, a non-transmissive liquid crystal cell 134, and a second absorbing polarizer 136.
• the liquid crystal cells are advantageously polarization-twisted in a well known manner.
• the first absorbing polarizer 130 transmits p polarized light.
• the second absorbing polarizer 136 transmits s polarized light.
• Ray 140 represents ambient illumination which enters the second absorbing polarizer and is reflected out along ray 144 after being reflected off of the backlight cavity 108.
• Ray 142 represents ambient illumination absorbed in the first polarizer 130.
• FIG 14 a simplified illustration of loss of an LCD's traditional advantage under high ambient illumination is shown.
• a reflective polarizer 150 is used here between the backlight cavity and the liquid crystal cells. Shown in combination are a reflective backlight cavity 108, a reflective polarizer 150, a transmissive liquid crystal cell 132, a non-transmissive liquid crystal cell 134, and an absorbing polarizer 136.
• the liquid crystal cells are polarization-twisted in a well known manner.
• the reflective polarizer 150 transmits p polarized light.
• the absorbing polarizer 136 transmits s polarized light.
• Ray 152 represents ambient illumination which enters the absorbing polarizer and is reflected out along ray 154 after being reflected off of the backlight cavity 108.
• Ray 156 represents ambient illumination reflected by the reflecting polarizer 150 to an observer along ray 158, thus eliminating a traditional LCD's increased contrast under high ambient illumination.
• the reflecting polarizer is aligned with the traditional absorbing rear polarizer to maintain an LCD's inherent increased contrast in ambient illumination. Reflective metal used in the reflecting wire grid polarizer must maintain high conductivity over the visible spectrum to ensure performance independent of wavelength. It is advantageous to vary the duty cycle of the reflecting polarizer by mini-mizing the width of the reflective portion to provide high transmission.

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• 1994
  • 1994-03-01 IL IL10880794A patent/IL108807A0/xx unknown
  • 1994-03-04 WO PCT/US1994/002668 patent/WO1994020879A1/en active Application Filing
  • 1994-03-05 TW TW083101915A patent/TW253049B/zh active

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