US 4,917,464 AGrant
High Brightness/Contrast Color Display System
Issue Date:1990-04-17
•8 Claims
•9 Drawing Sheets
Abstract
A display system is formed by stacking three birefringent LCD panels that are tuned to different substractive primary colors (i.e. yellow, cyan and magenta). Interposed between the panels, and sandwiched about the stack, are polarizers. At least some of the polarizers are colored to improve the resulting brightness. In some embodiments, the assembly is illuminated by a collimated light source and the resulting image is focused onto a projection screen for viewing. In other embodiments, optics on the front and rear surfaces of the assembly permit direct viewing without parallax effects.
Metadata
Assignee
- In Focus Systems, Inc.
Inventor
- Arlie R. Conner
Application Information
Application Number:US 3299384
Filing Date:1989-03-28
Priority Date:1989-03-28
Art Unit:254
Classifications
IPC:
G02F 113G03B 2100
Field of Search:
350353335;339 F;337;334;331 R122
Patent Drawings (9 sheets)
Drawing sheets are not available for this patent.
Description
Field of the Invention
The present invention relates to systems for optically displaying color images, and more particularly relates to such systems wherein the image is formed by projection of light through a series of stacked, light-transmissive panels.
Background and Summary of the Invention
It has often been proposed to use stacked arrays of transmissive cells, such as colored LCD panels, to yield color displays. However, such efforts have heretofore not been satisfactory. In some realizations of the theory, the resulting display contrast is unacceptable. In others, the display brightness suffers. In still others, the gamut of possible colors is limited. In many, parallax limits the viewing angle.
It is an object of the present invention to overcome these disadvantages to provide a high contrast, high brightness color display that can be satisfactorily viewed over a wide viewing angle.
In accordance with one embodiment of the present invention, a display system is formed by stacking three birefringent LCD panels that are tuned to different subtractive primary colors (i.e. yellow, cyan and magenta). Interposed between the panels, and sandwiched about the stack, are polarizers. At least some of the polarizers are colored to improve the resulting brightness. In some embodiments, the assembly is illuminated by a collimated light source and the resulting image is focused onto a projection screen for viewing. In other embodiments, arrays of lens elements on the front and rear surfaces of the assembly permit direct viewing of the image without parallax effects.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
Brief Description of the Drawings
FIG. 1 is a schematic diagram illustrating the basic configuration of a color LCD display system according to the present invention.
FIG. 2 is a simplified schematic diagram illustrating construction of a display subassembly used in the system of FIG. 1.
FIG. 2A illustrates the eight basic colors achieved by operating the three panels of the FIG. 2 display subassembly in their various combinations.
FIGS. 3-5 are spectral photometer plots showing ideal light transmission characteristics for the three liquid crystal panels used in the display subassembly of FIG. 2 when in their selected and nonselected states.
FIGS. 6-8 are spectral photometer plots showing the actual light transmission characteristics of three Kyocera liquid crystal panels used in the display subassembly of FIG. 2 when in their selected and nonselected states.
FIG. 9 is a chromaticity diagram illustrating the performance of the Kyocera panels when in their selected and nonselected states.
FIGS. 10 and 11 schematically illustrate the relative orientations of panels and polarizers in a particular display subassembly.
FIG. 12 is a schematic illustration of a first embodiment of the present invention.
FIG. 13 is a schematic illustration of a second embodiment of the present invention.
FIGS. 14 and 15 are schematic sectional and front plan illustrations, respectively, of a third embodiment of the present invention.
FIG. 16 is a chart illustrating the spectral distribution of light in a florescent backlight used in the third embodiment shown in FIGS. 14 and 15 .
Detailed Description
To provide an enabling disclosure without unduly lengthening this specification, applicant incorporates by reference the disclosures of U.S. Pat. Nos. 4,547,043, 4,549,174, 4,652,101, 4,709,990 and 4,763,993 which teach certain fundamental concepts useful in the construction of a device according to the present invention.
Referring to FIG. 1, a display device 10 according to one embodiment of the present invention includes a light source 12, a first lens 14, a display subassembly 16, a second lens 18 and a third lens 20. White light from source 12 is collimated by the first lens 14, passes through the display subassembly 16 and provides a color image to the second lens 18. Lens 18 focuses the parallel light beams exiting the subassembly into the third lens 20, which in turn projects this image onto a viewing screen 22.
The display subassembly 16 is shown in FIG. 2 and includes three stacked display panels 24, 26 and 28 sandwiched between two polarizers 30 and 36. Two additional polarizers 32 and 34 are provided within the stacked assembly 16, one between the first and second panels and the second between the second and third panels.
Panels 24-28 used in the illustrated embodiment are supertwisted nematic LCD panels that are controllably colored by exploitation of the birefringence effect. The use of birefringence to control color in LCD panels is 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. Briefly, in birefringence color systems, light of different wavelengths is rotated differing amounts as it passes through the liquid crystal material. When the exiting light is analyzed by a polarizer, certain color components are oriented in a direction that passes through the polarizer and other color components are oriented in directions that are attenuated or blocked. This selective filtering of the optical spectrum by the exiting polarizer produces the color effect.
Before proceeding further, it may be helpful to also review certain principles of the subtractive color theory employed by the present invention. The subtractive color primaries are usually considered to be yellow, cyan and magenta. A "yellow" filter is said to pass yellow light. More accurately, a yellow filter absorbs (i.e. blocks) blue light and passes green and red light. Similarly, while a cyan filter passes cyan light, it more accurately absorbs red light and passes blue and green light. Finally, a magenta filter absorbs green light and passes blue and red light. These properties are summarized in the following table.
In the illustrated embodiment of the present invention, the birefringent properties of the first panel 24 are "tuned" (by choosing the thickness (d) of the liquid crystal layer and its optical refractive index anistropy (.DELTA.n)) to rotate the polarization of incoming blue light to a direction in which it is absorbed by the exiting polarizer 3 when the panel is in its nonselected (i.e. deenergized) state. The panel 24 and polarizers 30 and 32 thus act as a yellow filter when the panel is nonselected. When the panel is in its selected (i.e. energized) state, the liquid crystal molecules nearly align with the electric field, thereby reducing the effect of the twist so that all light passes through nearly unaffected, i.e. still white. (For expository convenience, panel 24 is sometimes called the "yellow" panel and is said to absorb blue 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 26 is similarly tuned to operate as a cyan filter (i.e. absorbing red light) when in its nonselected state and to pass all wavelengths of light when in its selected state. Finally, the illustrated third panel 28 is tuned to operate as a magenta filter (i.e. absorbing green light) when in its nonselected state and to pass all wavelengths of light when in its selected state.
Spectral photometer plots showing the light transmission qualities of ideal panels 24, 26 and 28 (again, considered in conjunction with their associated polarizers) are provided in Figs. 3, 4 and 5, respectively. Panels suitable for use as panels 24-28 are available from Kyocera of Hayato Japan or may be fabricated using known techniques. Spectral photometer plots showing the actual behavior of the Kyocera panels are provided in FIG. 6-8. (As can be seen from these curves, neither the passage of light of the desired color nor the attenuation of light of undesired colors is perfect, but the resulting effect is more than adequate to provide saturated colors throughout the human visual area.) A chromaticity diagram illustrating performance of the Kyocera panels in their selected and nonselected states is provided in FIG. 9.
Each of panels 24-28 comprises a plurality of pixels that can be individually energized to change the spectral distribution of the light that is permitted to pass therethrough. By selecting corresponding pixels in the three panels, light of any color can be transmitted through the display subassembly 16 and projected onto display plane 22. To transmit a pixel of green light, for example, a pixel in the yellow panel 24 is nonselected to absorb blue light and the correspondingly positioned pixel in the cyan panel 26 is nonselected to absorb red light. By superimposing the spectral transmission curves of these two pixels, it will be recognized that the remaining, transmitted light has a peak in the region of the spectrum the eye perceives as green. (The magenta panel 28 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. 2A shows the eight basic colors achieved by operating the three panels in their various combinations.
Polarizers are needed to analyze the light passing through the liquid crystal panels in order to achieve perceptible contrast. In prior art projection systems, the polarizers are typically neutral (i.e., dyed black by iodine). In the present invention, 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 24 is illustrated as being "yellow." Light entering it is polarized by the first polarizer 30. Normally, all colors of light orthogonal to the axis of polarizer 30 would be absorbed by the black dye of a conventional, neutral polarizer, resulting in an immediate loss of 50% of the light. This loss can be cut dramatically if the first polarizer is dyed yellow. Such a polarizer still passes the white light parallel to the polarizer's axis, but additionally passes green and red light orthogonal to its axis. This additional green and red light is permitted to pass into the display subassembly 16 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 two thirds. Display brightness improves commensurately.
The same benefit can be achieved at the exiting end of the sandwiched display subassembly 16. The last panel 28 in the assembly is illustrated as being magenta. By dying the polarizer 36 adjacent thereto magenta, the blue and red light that would normally be absorbed thereby is allowed to leak through and pass to the display plane 22, again improving display brightness.
Conventional neutral polarizers can be used at the two positions (32,34) intermediate the display panels and a significant improvement in display brightness can still be achieved by virtue of the two colored polarizers described above.
To further optimize display brightness and contrast, the polarizers at the two positions intermediate the display panels can be colored too. Care must be taken, however, not to interfere with the color-selective properties of the birefringent panels. For example, if a cyan colored polarizer is interposed between the yellow and cyan panels 24,26, it will interfere with the color-selective properties of the yellow panel. As noted, the yellow panel itself does not absorb the undesired blue light. Instead, the panel orients the blue light in a direction that permits polarizer 32 to absorb it. If the polarizer is colored cyan, it will leak blue and green light, including the blue light oriented by the yellow panel so that it could be absorbed. Consequently, use of a cyan polarizer between the yellow and cyan panels renders the yellow panel effectively inoperative.
An equally poor color choice for the first intermediate polarizer 32 is yellow. A yellow polarizer would permit green and red light to enter the cyan panel 26 at an unexpected orientation. The purpose of the cyan panel is to rotate the orientation of the red light exiting the panel to a direction in which it can be absorbed by its exiting polarizer 34. If the red light enters the cyan panel 26 at an unexpected orientation, it will exit at an unexpected orientation and will not be absorbed by the exiting polarizer. Consequently, use of a yellow colored polarizer 32 interferes with the color-selective properties of the cyan panel.
Polarizer 32 should be colored, if at all, a color that both of the adjoining panels are intended to pass. In this case, since the yellow panel 24 is intended to pass green and red, and the cyan panel 26 is intended to pass blue and green, the polarizer 32 should be colored the common color: green.
Following similar logic, it will be recognized that the polarizer 34 interposed between the cyan and magenta panels should be colored blue.
The foregoing discussion has described only one of many possible sequences of polarizers and panels. Others can be devised. For example, while the first polarizer 30 in the above example has been described as being yellow in order to achieve an improvement in brightness, an alternative embodiment with the same sequence of LCDs can here use a red or green polarizer instead. A red or green polarizer still provides some improvement in brightness since it leaks light that would be absorbed by a black polarizer. Of course, a black polarizer can also be used if desired. The basic LCD sequence itself can also be varied with corresponding changes in the associated polarizers. The basic sequences are set forth in the following table:
where Y is yellow, BLK is black, G is green, C is cyan, BLU is blue, M is magenta and R is red. It will be recognized that the above three sequences can all be reversed to yield three more sequences.
One advantage of the present invention is the flexibility it affords in possible 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.
Figs. 10 and 11 illustrate in greater detail a display subassembly using one of the alternative sequences. In this sequence, the first LCD panel is cyan, followed by a yellow panel and finally a magenta panel. The polarizers are cyan, black, black and red, respectively.
Included in FIGS. 10 and 11 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.
In a first particular embodiment 38 of the invention, shown in FIG. 12, the display subassembly 16 is positioned on the transparent projection surface 40 of a conventional overhead projector 42. Such projectors typically include an illumination bulb 12 and a Fresnel lens 44 under the projection surface to produce light beams that pass through a transparency and converge onto a projection lens assembly 48. (Due to the short focal length and high power required of lens 44, it is often formed by cementing two or more lower powered Fresnel lenses together.) When display subassembly 16 is used in such an embodiment, it is desirable to provide a Fresnel lens 50 to collimate the converging light from the projection surface prior to illumination of the display subassembly. The light exiting the subassembly is then focused by a lens 52 (which is also desirably in Fresnel form) onto the projection lens assembly. (Lens 5 here serves the same purpose as the Fresnel lens provided under the projection surface of the projector in the projector's normal operation, namely to focus light towards the projection lens assembly 48.)
In a second embodiment 58 of the invention, shown in FIG. 13, the display apparatus is self-contained and may be incorporated in a cabinet 62 to serve as a color monitor for use with a computer or the like. In such embodiment, a field lens 64 is used to collimate the light from bulb 12 prior to its passage through the display subassembly 16. The resulting image is projected by a second lens 65 onto a translucent medium 66 which can then be viewed from the opposite side by a user.
In a third embodiment 68 of the invention, shown in FIGS. 14 and 15, the display subassembly 16 is backlit from a diffused light source, such as a fluorescent light panel 70. In such embodiment, the display subassembly 16 is fabricated with end plates 71,73 on which are formed a plurality of microlenses 72, 74, one pair aligned to each pixel. Light incident on one of microlenses 72, regardless of its orientation, is directed normal to the plane of the subassembly and thus passes through the pixels of the three component panels in the proper alignment. Collimated light exiting the subassembly 16 is dispersed by microlenses 74 which permit the color image to be viewed from a wide range of angles without parallax effects. The interstitial areas between the lenses may be colored black to minimize stray light and to improve perceived contrast.
In other embodiments of the FIG. 14 device 68, the arrays of microlenses can be replaced by arrays of fiber optic collimator faceplates or lenticular lenses.
FIG. 16 shows the spectral distribution of a representative florescent backlight 70 that may be employed in the embodiment of FIGS. 14 and 15. As is characteristic of florescent lighting, the spectrum has characteristic peaks corresponding to certain chemical components used in the light. These peaks (and the nulls) can be tailored to a specific application by changing the chemistry of the light.
Having described and illustrated the principles of my invention with reference to a preferred embodiment and several variations thereon, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. For example, while the display subassembly has been described as including single supertwisted liquid crystal panels, other types, such as double supertwisted panels or single panels embodying other technologies, can alternatively be used. Similarly, while the display subassembly has been described as including four polarizers and three LCD panels, in other embodiments of the invention, additional optical components can also be used. For example, it is sometimes desirable to include a retardation film between the first polarizer and the first panel, and between the last polarizer and the last panel, in order to tune the range of birefringence effects to desired frequencies. In other embodiments, if any of the LCD birefringence colors are not ideal, some attenuation of specific light frequencies might enhance the color gamut and overall contrast. Thus, two polarizers might be used together or a weak color filter compensator (i.e. a conventional gelatin filter) might be added.
In view of these and the wide variety of other embodiments to which the principles of my invention may be applied, it should be recognized that the illustrated embodiments are to be considered illustrative only and not as limiting the scope of the invention. Instead, I claim as my invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.
Claims
I claim:
1. An accessory for use with a conventional overhead projector to project electronically generated color images therefrom, the projector having a bulb, a projection lens, and a projection surface having a Fresnel lens thereunder, the Fresnel lens focusing light from the bulb into the projection lens, the accessory being adapted to rest on the projection surface and comprising: first lens means for receiving converging white light form the projection surface of the projector and collimating said light; second lens means or receiving collimated light and focusing said light into the projection lens; a display subassembly positioned between the first and second lens means and illuminated by the collimated light, said subassembly including first, second, third and fourth polarizers and further including first, second and third transmissive liquid crystal birefringent display panels, the birefringence of said panels being tuned, respectively, to first, second and third subtractive complementary colors, each of said panels having a plurality of electronically operable pixels, said panels being stacked so that corresponding pixel in each of said panels are aligned with the direction of the collimated light, the first panel being positioned between the first and second polarizers, the second panel being positioned between the second and third polarizers and the third panel being positioned between the third and fourth polarizers.
2. The accessory of claim 1 in which the first polarizer is colored the first color.
3. The accessory of claim 2 in which the fourth polarizer is colored the third color.
4. The accessory of claim 1 in which the second polarizer is colored a color other than the first or second color or black.
5. The accessory of claim 1 in which: the first polarizer is colored the first color; the fourth polarizer is colored the third color; the second polarizer is colored a color passed by both the first and second panels; and the third polarizer is colored a color passed by both the second and third panels.
6. The accessory of claim 1 in which: one of the panels is yellow; one of the panels is cyan; one of the panels is magenta; the colors of the first and fourth polarizers are selected from the list: yellow, cyan, magenta and black; and the colors of the second and third polarizers are selected from the list: red, green, blue and black; wherein each of the polarizers is colored differently.
7. A method of projecting a colored image, comprising the steps: providing first, second and third birefringent liquid crystal color panels that include a plurality of pixels, said panels being tuned to first, second and third subtractive color primaries, respectively; stacking said first, second and third panels so that corresponding pixels in each of said panels are aligned along an axis orthogonal thereto, said stacked panels forming a subassembly; providing a first polarizer between the first and second panels; providing a second polarizer between the second and third panels; and sandwiching the subassembly between third and fourth polarizers; passing light through the subassembly in a direction parallel to the axis of alignment; focusing the light exiting the subassembly parallel to the axis of alignment into a projection lens; and projecting the light exiting the projection lens onto a display plane.
8. The method of claim 7 which further includes the step of collimating the light parallel to the axis of alignment of the panels prior to passage into the subassembly.
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