Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2000-03-10
2002-01-22
Dowling, William (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S020000, C349S018000
Reexamination Certificate
active
06340230
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention is directed to systems, methods and apparatus for achieving enhanced contrast in reflective imaging systems, such as those utilizing reflective liquid crystal display imagers and color splitting devices, such as Philips prisms. The invention maximizes the transmission of light polarized in a certain direction while minimizing the transmission of light polarized in another direction, thereby achieving a high contrast ratio which significantly improves the final image quality.
2. The Relevant Technology
Liquid crystal displays are commonly used as spatial light modulators in projection imaging systems. A reflecting type of liquid crystal panel (which is also known as a liquid crystal light valve) comprises an array of pixels, which when activated works by reflecting incident light while simultaneously rotating the polarization vector of the light by 90 degrees, typically when a voltage or signal is applied to an individual pixel. Thus the signal or image information is contained in the light which is of a particular polarization. If the liquid crystal imager is not activated, then those particular pixels of the liquid crystal imager are in the “off” state, and the light which is reflected from them will have no rotation of the polarization state. The signals from these “off” pixels should correspond to dark spots in the final image. One aspect of the quality of an image in such a system is measured through a parameter known as the contrast ratio, which is defined as the ratio of the light transmitted through the system in the “on” state divided by the amount of light transmitted in the “off” state. The higher the contrast ratio, the better the overall quality of the image.
Loss of contrast through a non-polarizing color splitting device such as a Philips prism results from a combination of the geometrical effect from skew rays as well as diattenuation and phase differences in the coatings of reflective and total internal reflection surfaces.
The geometrical effects of a polarizing beam splitter have been described in detail in Ootaki (U.S. Pat. No. 5,459,593) and Miyatake (U.S. Pat. No. 5,327,270), the disclosures of which are hereby incorporated by reference, as follows below. These geometrical effects are a pure rotation of the input linearly polarized light by a polarizing beam splitter. Rear projection imaging systems typically have a contrast ratios of not less than 50:1 as suggested in Ootaki, in the plots showing a 2% dark level (100%/2%=50:1).
In the Ootaki patent, white light from a halide or xenon lamp is incident at an angle of approximately 45 degrees onto a polarizing cubic beam splitter. The polarizing cubic beam splitter reflects light which is of s-polarization and transmits light which is of p-polarization (where s-polarization refers to light which has its polarization vector perpendicular to the direction of propagation, while p-polarization refers to light which has its polarization vector lying in the plane of propagation). The light which is of s-polarization is reflected by the polarizing beam splitter towards a dichroic mirror. The dichroic mirror in the Ootaki patent is designed in such a way as to reflect the s-polarized light which is of one color while transmitting the other color components of the beam. The use of more than one dichroic mirror results in a separation of the incident white light into various color channels. In a typical imaging system, two dichroic mirrors are sufficient to separate incident white light into red, green, and blue color channels. The color selectivity of the dichroic mirror is achieved by the placement of specific optical coatings upon the mirror, which is a well known technique in the art for color separation.
A limitation in the quality of the performance of this system originates from the rotation of the plane of polarization in the polarizing prism for incident light rays which are not in an eigenstate. Since this rotation is independent of the state of the image generating pixels it results in a leakage of light in the “off” state pixels which degrades the image contrast. The optical coatings on the dichroic mirror were designed to compensate for rotation of the polarization state.
Miyatake discloses a similar approach to compensate for the polarizing beam splitter. The approach disclosed in the Miyatake patent is to compensate for the polarizing beam splitter with a quarter waveplate in the optical path between the reflecting type liquid crystal device and the polarizing beam splitter. In Miyatake the quarter waveplate retarder was aligned with its plane perpendicular to the optical axis and is laminated to the terminal surface of polarizing beam splitters to reduce the Fresnel reflection at its interface with the prism. However this patent does not teach or consider phase differences that may occur in a color splitting device, such as a tilted dichroic mirror or a Philips prism.
In U.S. Pat. No. 5,594,591 which issued to Yamamoto et al.; the disclosure of which is hereby incorporated by reference, the inventor has attempted to solve the same problem in a projection display wherein the color separation element is a Philips prism. The Philips prism disclosed in the Yamamoto et al. patent employs optical coatings upon the faces of the Philips prism for color separation and an anti-reflection coating on the incident prism faces, which also form a total internal reflection (TIR)surfaces. Yamamoto et al. also assert that the optical coatings on the TIR surfaces, which comprise alternating layers of SiO
2
and TiO
2
, have a phase control function. The dichroic optical coatings used for color separation cooperate with the anti-reflection coating layers at the TIR surface in achieving this phase control function; which combined the 90 degree phase difference at the TIR surface corrects for the image degradation contributed by the polarizing beam splitter.
When utilizing a quarterwave compensating plate , or quarter waveplate retarder, in the optical path between each of the three liquid crystal light valves and the Philips prism in such a reflective imaging systems, the contrast ratio is improved by ensuring that the black level is closer to being completely black. While use of quarter waveplates in such a system proposes a means for the correction of rotations in the polarization vector due to the polarizing beam splitter, it does not address the undesired ellipticity and additional rotation added by the color splitter.
A quarter wave compensation plate , or waveplate retarder is also used in U.S. Pat. No. 5,576,854 issued to Schmidt et al., the disclosure of which is hereby incorporated by reference. The Schmidt et al. patent was developed for monochromatic systems and does not address the issue of color imaging. The system disclosed in Schmidt et al. works in a manner similar to the system disclosed in Miyatake, as previously described, namely by the reduction of off-axis depolarization induced by geometric effects when the light encounters the polarizing beam splitter. Schmidt et al. specifically mentions using a waveplate with a value of retardance equal to 0.25 to compensate for the off-axis polarization components generated by the polarizing beam splitter. However, Schmidt et al. additionally suggests that an additional retardance of 0.02 be included to compensate for the unwanted polarization shifts generated by the thermally induced birefringence of the liquid crystal light valve, an effect which results in the dark state being lighter than desired. Accordingly, Schmidt et al. suggests that in monochromatic imaging systems the waveplate compensator have a total retardance value equal to 0.27 to compensate for the additional retardance, or phase delays between components due to the thermally induced birefringence in the LCLV.
In commonly assigned U.S. Pat. No. 5,986,815,incorporated herein by reference, Bryars teaches methods and apparatus for correcting undesired depolarization of color splitters through the use of uniquely d
Bryars Brett J.
Greenberg Michael R.
Sullivan Sean
Dowling William
Optical Coating Laboratory, Inc.
Sherma Edward S.
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