Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2001-06-02
2003-01-28
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C349S009000
Reexamination Certificate
active
06511183
ABSTRACT:
FIELD OF THE INVENTION
The present invention broadly concerns digital image projectors and more particularly concerns digital image projectors based on reflective digital-image polarization modulators such as reflective liquid-crystal display polarization modulators.
BACKGROUND ART
Digital image projectors are widely used to project color images generated from digital signals encoding the images onto the front of a reflective display screen for a conference-room presentation or the like or onto the rear of a semi-transparent diffusive screen of a rear-projection display monitor or a projection television.
A conventional digital-image projector employing reflective liquid-crystal polarization modulators is disclosed in U.S. Pat. No. 5,777,789 to Chiu et al. The projector of the '789 patent has a metal-halide arc lamp as a source of unpolarized “white” light for the projector. Light from the arc lamp passes through illumination optics which function to form a generally parallel, visible white-light illumination beam of generally uniform intensity spatially with respect to polarization-modulator faces of the liquid-crystal polarization modulators employed in the projector. The unpolarized illumination beam is directed into a polarizing beamsplitter cube, which according to the '789 patent is designed for use over a range of wavelengths of the visible light spectrum and over a range of angular divergence of the beam. The polarizing beamsplitter cube splits the unpolarized beam into two beams of substantially—but, conventionally, not perfectly—polarized light, with the respective polarizations of the two beams being substantially orthogonal. One of the two light beams so produced in the polarizing beamsplitter cube of the digital-image projector of the '789 patent serves as a substantially polarized source beam and is directed from the polarizing beamsplitter cube into a color splitting/combining prism assembly. The color splitting/combining prism assembly is comprised of three prisms with certain of the faces of the prisms bearing dichroic coatings for sequentially separating red, blue, and green light components from the visible white light of the substantially polarized source beam and directing each substantially polarized color-component light beam onto a corresponding reflective liquid-crystal polarization modulator—referred to as a “light valve” in the '789 patent.
Each of the three polarization modulators of the digital-image projector of the '789 patent is positioned with a reflective polarization-modulator face perpendicular to a component-beam optical path defined with respect to the corresponding substantially polarized color-component light beam exiting the color splitting/combining prism assembly. In general, a reflective polarization modulator serves to modulate the polarization of the corresponding color-component light beam spatially by means of selective rotation of the polarization of the light of the beam on a pixel-by-pixel basis over the polarization-modulator face in accordance with a signal applied to the polarization modulator which encodes a component color image of a desired composite color image. In particular, for each pixel of the projected image which is to be illuminated in a given color, the polarization of the substantially polarized color-component light beam of that color is rotated by the reflective liquid-crystal polarization modulator at a location on the polarization-modulator face corresponding to the location of the pixel in the projected image. Such illuminated pixels are referred to as “light” pixels. Conversely, for each pixel of the projected image which is not to be illuminated in a given color, the substantially polarized color-component light beam of that color is reflected with the polarization of the beam nominally unchanged by the reflective liquid-crystal polarization modulator at the location on the polarization-modulator face corresponding to the location of the pixel in the projected image. Such non-illuminated pixels are referred to as “dark” pixels. The ratio of the intensity of a maximally illuminated light pixel to the intensity of a minimally illuminated dark pixel defines a contrast ratio, which is a significant figure of merit for digital-image projectors. Generally, the higher the light-to-dark contrast ratio for a projector, the more clearly an image produced by the projector may be discerned by a viewer.
The color-component light beam thus spatially selectively polarization modulated by a liquid-crystal polarization modulator of the digital-image projector of the '789 patent is reflected from the reflective polarization-modulator face of the polarization modulator substantially back along the corresponding component-beam optical path through the color splitting/combining prism assembly. Each of the three reflected color component light beams substantially retraces its original path through the prism assembly and recombines with the other two color component light beams to form one composite spatially selectively polarization-modulated light beam. The composite light beam emerges from the color splitting/combining prism assembly and passes into the polarizing beamsplitter cube. The polarizing beamsplitter cube splits the composite light beam into a nominally polarization-modulated light-pixel component beam which carries the composite color image made up of light pixels and a nominally non-polarization-modulated dark-pixel component beam which carries a color-negative image made up of dark pixels. Since the polarization of the dark-pixel component beam was nominally unchanged by the reflective liquid-crystal modulators, the dark-pixel component beam at least ideally retraces an optical path through the projector back towards the arc lamp which was the source of illumination. The polarization-modulated light-pixel component beam is directed from the polarizing beamsplitter into a projection lens of the digital-image projector of the '789 patent, which serves to project the desired composite color image onto a projection screen.
A difficulty with conventional digital-image projectors that are based on reflective polarization modulators arises because a conventional MacNeille-type multilayer dielectric film polarizing beamsplitter of the type heretofore typically used in such projectors generally treats two light rays impinging upon the polarizing beamsplitter differently with respect to polarization properties if the directions of incidence of the two rays differ with respect to the polarizing beamsplitter. Light beams which impinge upon the polarizing beamsplitter of a conventional digital-image projector are typically made up of light rays which have angles of incidence with respect to the beamsplitter which range over several degrees, since, in order to obtain sufficient illumination intensity for an adequately bright projected image from economically feasible light sources, conventional digital-image projectors typically employ illumination beams having a numerical aperture on the order of 0.1 or greater. See A. E. Rosenbluth et al.,
IBM Journal of Research and Development,
volume 42, pages 359-386 (May/July 1998). A conical light beam with a numerical aperture of 0.1 passing through air subtends an angle of about ±6°. As discussed below in connection with
FIGS. 1 through 3
, light rays impinging upon a conventional MacNeille-type multilayer dielectric film polarizing beamsplitter in directions which differ from the direction of a principal axis defined with respect to the beamsplitter and the associated reflective polarization modulators give rise to leakage of light onto dark-pixel areas of the projected image. Consequently, variation in the directions of light rays around the pupil of the illumination beam in a conventional digital image projector employing beams of numerical aperture of 0.1 or so generally leads to an overall reduction in the light-to-dark contrast ratio relative to the light-to-dark contrast ratio which would be expected for a hypothetical projector
Janssen Peter J. M.
McClain Stephen
Shimizu Jeffrey A.
Dowling William
Koninklijke Philips Electronics , N.V.
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