Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
1999-07-30
2001-08-28
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S031000, C353S084000, C348S742000
Reexamination Certificate
active
06280034
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an image projection system, providing efficient use of illumination, and more particularly to such a system employing a primary color imaging panel and a complementary color luminance augmentation panel.
BACKGROUND OF THE INVENTION
Image projection systems such as light valve projection systems use one or more small flat panel electro-optic display devices to generate an image by modulating a light beam generated by a lamp This image is then magnified and projected on a screen for viewing.
Typically, the flat panel display device is an active matrix liquid crystal display (AMLCD) device, having, for example, a resolution of 1280 by 1024 pixels. Since the image is projected, the display device may be relatively small, e.g., less than about 6 cm. Further, the preferred mode of operation is a reflective mode, which allows use of thinner layers of liquid crystal light modulation material and correspondingly faster response times, since the light passes through the liquid crystal twice.
In conventional projectors using white light illumination, the light is split or filtered into three primary color components, red, green and blue, representing “corner” of a color triangle defining a color gamut on the CIE 1931 color chart, so that a complete rendering of the colors within the triangle can be achieved. These three components are separately modulated.
Known LCD projection systems provide one or more LCD panels, up to, for example, six panels, to modulate the spatial and color aspects of an image. Thus, in a single LCD panel display, the LCD is time or space division multiplexed with the respective colors. In a time-multiplexed system, red, green and blue polarized light is sequentially projected onto the panel, spatially modulated, analyzed (by, e.g., a second polarizer) and then projected, with the human eye merging the colors. In a spatially multiplexed system, a tri-color masked LCD panel is illuminated with white polarized light, with the modulated light passing through an analyzer (a second polarizer) and then projected. These single panel systems are known to be inefficient. For example, in a time or space domain multiplexed system, the peak brightness is much less than one-third of the brightness of the polarized white light source, for example utilizing as little as only 1% of the light output for typical images.
Image projection systems of high illumination efficiency allow the use of smaller projection lamps, smaller power supplies, reduced air flow cooling (resulting in reduced noise), greater portability, battery operation, and other advantages. In addition, the availability of higher power lamps may be limited, and thus higher efficiency may result in more available light sources.
In order to provide improved light use efficiency, four-color (red, green, blue, white) techniques were developed, in which the white light increases luminance by introducing white light into the image in proportion to the luminance signal. The amount of white light that may be acceptably added to the red, green, and blue light at an image point is a function of both the color and the luminance of that point. While there are many different algorithms to determine the amount of white to add, they share many common features. For saturated colors, very little white may be added to the red, green, blue. For desaturated colors, increasing amounts of white may be added.
The final luminance is L
R
+L
G
+L
B
+L
W
. Viewing tests have shown that if L
W
is greater than about 40-50% of L
R
+L
G
+L
B
, the colors are unacceptably desaturated and the primaries are unacceptably dim. This limits the overall gain in lumens possible with this technique.
It is also known to use colors other than red, green, blue for forming an image in a projection system. Alternatives include four or more spectral bands, and a six color system including red, green, blue, magenta, cyan and yellow. See, JP 09-230301 and U.S. patent application Ser. No. 08/579,655, expressly incorporated herein by reference.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of one embodiment of the invention to provide an imaging system, having a broadband light source, a selective beamsplitter, adapted to selectively spatially separate light within a controllably selected spectral band from light outside said controllably selected spectral band, and a pair of light modulators, each being respectively disposed along a path of light within said controllably selected spectral band from said light outside said controllably selected spectral band.
It is also an object of one embodiment of the invention to provide an imaging method, comprising the steps of: providing a broadband light source; splitting the output of the broadband light source, to selectively spatially separate light within a controllably selected spectral band from light outside the controllably selected spectral band; and separately spatially modulating light disposed along a path of light within the controllably selected spectral band and light outside the controllably selected spectral band.
It is a still further object of one embodiment of the invention to provide an imaging system, comprising a broadband unpolarized white light source; a polarization converter system for converting polarization axes of unpolarized white light into a substantially single polarization axis, to produce a beam of polarized light; a selective polarization filter, adapted to selectively rotate a polarization axis of a selected spectral band of light of the with respect to remaining polarized light based on a control signal; a polarized beam splitter, for separating light having a the substantially single polarization axis from light having a rotated polarization axis; a set of electro-optic spatial light modulators, disposed along a path of light within said spectral band and a path of remaining light outside said spectral band, respectively, and being adapted to modulate an image therein; and a polarized beam splitter, for recombining modulated light from the set of light modulators.
The light modulator disposed along a path of light within the controllably selected spectral band preferably modulates red, green and blue light while the light modulator disposed along a path of light outside the controllably selected spectral band preferably modulates cyan, magenta and yellow light.
The splitting step may be controlled electrically or mechanically to rotate a polarization axis of a selected color band, and may also be presented in a dynamic scrolling pattern.
The present invention provides a system which seeks to efficiently use substantially all of the light output of a projector illumination source by splitting the light into two components based on color, and separately modulating each split color component. In order to achieve full color range, the wavelength characteristics of the color splitter are varied in time sequential manner, to provide increased coverage of the full color gamut.
In a preferred design, white light is sequentially split into three different primary color components from a respective complementary color component. In practice, the primary color components are available for RGB color space imaging, while the complementary color components, CMY, are simultaneously available for white level augmentation. This results in efficient use of the illumination. It is noted that, in certain applications where complete coverage of the RGB color gamut is not required, the color split may be static, e.g., representing a respective color and its complement, neither of which need represent a primary color.
The present invention allows substantially all of the light output of a projector illumination source by modulating a primary color image signal using illumination within first color space and modulating a luminance augmentation signal using complementary illumination within a second, complementary color space. Therefore, the illumination at any pixel is the sum of the luminance and complementary
Dowling William
Philips Electronics North America Corporation
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