Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
1999-05-14
2001-02-06
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S033000, C349S009000
Reexamination Certificate
active
06183091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to color imaging systems and methods. In particular, this invention is related to systems and methods of separating different light into two or more spectrums, modulating the separated spectrums, and re-combining the modulated spectrums to create color images.
2. Background of the Related Art
Some prior art color image projector systems separate input light into three primary colors, separately modulate the colors, and recombine the modulated color images to create a consolidated, modulated color image. Prior art systems typically separate input white light into multiple colors using multiple dichroic beamsplitters. Usually, a first dichroic beamsplitter will separate the white light into a first primary color, and a complementary color. The complementary color light is then further separated into second and third primary colors by a second dichroic beamsplitter. Once the white light has been separated into three primary color beams, the light beams are separately modulated. Additional dichroic elements are then used to re-combine the modulated beams.
The dichroic beamsplitters used in these prior art systems typically use a thin film of a dichroic material that is deposited on a transparent substrate, or sandwiched between two shaped transparent elements. Unfortunately, the performance of the dichroic thin film material is poor off-axis. Thus, the performance of devices using the dichroic beamsplitters is usually poor off-axis.
The dichroic thin films used in the prior art systems are intended to act on light based only on wavelength. The polarization orientation of the light passing through the dichroic elements should not matter, and the dichroic elements should not change the polarization orientation of the light. In practice, however, the polarization orientation of the light does have an effect on how the dichroic thin films affect the light.
If a prior art system utilizes liquid crystal modulators, the light may be polarized into specific orientations to allow the modulators to perform their function. This polarization orientation can affect how the dichroic thin films perform the color separation function. In addition, if the liquid crystal modulators are reflective panels, the colored light will usually pass through a dichroic thin film a first time with a first polarization orientation, and then a second time with a different polarization orientation. Because the polarization orientation changes, the dichroic thin film will operate on the light differently during each pass. All these effects combine to deteriorate the performance of the prior art systems.
In addition, to improve color saturation, many of the prior art devices that utilize dichroic beamsplitters to separate light into three primary colors also utilize notch or band-pass filters to remove light having wavelengths between the three primary colors. For instance, notch filters are often used to remove cyan and yellow “pastel” colors. The removal of the pastel colored light reduces the brightness of the resulting color images.
SUMMARY OF THE INVENTION
The present invention is directed to systems and methods that utilize one or more polarizing beamsplitters to separate light into different color spectrums.
In a system embodying the present invention, at least one retarder stack is used to condition input light so that different spectrums of the light have different polarization orientations. One or more polarizing beamsplitters are then used to separate the different spectrums based on their respective polarization orientations. The separated color spectrums are then modulated with respective modulators, and the modulated spectrums are re-combined to create a consolidated, modulated color image.
In preferred embodiments of the invention, the polarizing beamsplitters are also used to accomplish the recombination of the different modulated color spectrums. Also, preferred embodiments may utilize reflective liquid crystal modulator panels that selectively modulate the polarization orientation of the different spectrums. The polarization change imparted by such modulators can also be used to control the direction of progression of the light through the system.
A system that separates input light into different spectrums, separately modulates the spectrums, and then re-combines the spectrums, must operate so that the re-combined image is properly re-integrated. If projection lenses are used to project the image, the modulators that perform the modulation function for each color must all be located at substantially the same distance from the projections lenses to ensure that the magnification is the same for each of the separate color images. If this does not occur, the image may be blurred or distorted. Embodiments of the present invention have their beam splitting and recombining elements arranged so that the same effective distance exists between the projection lenses and each modulator.
Systems embodying the invention that utilize reflective liquid crystal modulator panels may also utilize light doublers, as described later in the application. Although ferroelectric liquid crystal modulators require DC balancing, which results in alternating inverse images, the light doublers allow the inverse images to be converted to normal images. Thus, every frame of the image data can be output by the system, and brightness is maximized.
Systems embodying the present invention can also utilize output retarder stacks that orient all the different modulated spectrums into the same polarization orientation. If projection lenses are used to project the resulting consolidated, modulated color image, the unitary polarization orientation can improve the efficiency of the lens system.
Preferred embodiments of the system may also include both a polarizing beamsplitter and a dichroic beamsplitter. In this type of embodiment, a retarder stack and the polarizing beamsplitter can be used to separate the input light into two spectrums, and the dichroic beamsplitter can be used to separate one of the spectrums into two additional spectrums. Each of the resulting three spectrums can then be separately modulated with respective modulators. The modulated spectrums can then be re-combined using the same dichroic beamsplitter and polarizing beamsplitter.
Because polarizing beamsplitters do not have the same off-axis problems described above for dichroic thin films, systems embodying the present invention have much better off-axis performance. Also, because systems embodying the present invention do not rely exclusively on dichroic thin films to separate the light into different spectrums, the use of reflective liquid crystal modulators does not negatively impact system performance.
The retarder stacks that condition the light so that it will have certain polarization orientations can also be designed to block or reflect the “pastel” wavelengths between the primary colors. This eliminates the need for separate notch filters, and may help to improve the overall efficiency or brightness of the system.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
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Johnson Kristina M.
Sharp Gary D.
ColorLink, Inc.
Dowling William
Fleshner & Kim LLP
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