Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2001-12-21
2003-02-04
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S639000
Reexamination Certificate
active
06515801
ABSTRACT:
FIELD OF THE INVENTION
The invention relates broadly to color digital-graphics projection displays based on primary color digital-graphic-encoding beam modulator panels and projection optics for such projection displays. More particularly, the invention relates to such color digital graphics projection displays and projection optics providing compensation for a lateral color chromatic aberration.
BACKGROUND ART AND SUMMARY OF THE INVENTION
A typical color digital-graphics projection display arrangement uses three digital-graphic-encoding beam modulator panels, one for each of a red, a green, and a blue component of a color graphic. Examples of beam modulator panels include transmissive polysilicon liquid crystal displays (LCDs), reflective digital micromirror devices (DMDs), and reflective liquid crystal displays (RLCDs)—also known as reflective liquid crystal on silicon displays (LCoS). Often graphic-encoded beams from the three panels are combined optically with component-color beam recombination optics and projected through a single projection lens.
With increasing demands for higher resolution, higher pixel count, and smaller devices, the difficulty of designing projection lenses for color digital-graphic projection displays has increased. One characteristic of a projection lens that becomes emphasized is a characteristic referred to as “lateral color.” Lateral color refers to a chromatic aberration involving a dependence in lateral magnification of a graphic on the wavelength or color or color of the graphic. Such a change in lateral magnification with color typically results in a change in the convergence of the superimposed component-color imaged graphics from red, green, and blue digital-graphic encoding beam-modulator panels from the center of a projected graphic display outwards. So, if digital-graphic-encoding panels are positioned so that there is perfect convergence of the component color pixels of a graphic in the center of the display, the color convergence can be off by a pixel or more at the edge of the display. Such variation in color convergence degrades graphic quality, particularly for projection of computer graphics displays.
Limiting lateral-color misconvergence is a significant issue in the development of three-panel projection monitors. For example, a projection monitor employing panel arrays having 1200×1600 pixels, with each pixel being about 10 microns square, and subject to a design objective of a half pixel or less of lateral-color misconvergence presents a difficult objective for a projection lens maker to achieve. A conventional projection monitor with such panel arrays might have a lateral-color misconvergence at the corners of the graphic of about 12 microns measured at the panel arrays, which is greater than a pixel. Lateral-color misconvergence of such magnitude is a significant problem in computer display monitor applications, where icons and menus are typically placed at the edges of the graphic display. In general, to compensate for lateral color effects, a projection lens would have to increase in size and contain additional elements, which results in an increase in design complexity and cost. As a practical matter, lens design alone is not enough to meet the demands of increased display resolution.
An emerging direction in electronic projection displays is reflective-polarization-modulator based systems employing reflective liquid-crystal polarization modulators. Many of the architectures based on reflective polarization modulators employ arrangements where the distance from the polarization-modulator panel to the projection lens is large compared to the focal length of the projection lens. This is especially true for rear projection displays, such as projection monitors and projection television. In general, the greater the back-focal distance, the more difficult it is to produce a projection lens. Another optical design constraint that makes a projection lens a challenge to produce for such systems is a requirement for “telecentricity.” A telecentric lens system has an entrance pupil at infinity. In terms of chief rays—also termed “principal rays”—having an entrance pupil at infinity means that the chief rays are parallel to the optical axis, thus resulting in every point on an image possessing the same set of angles or pupil properties.
A conventional digital-graphic projector employing reflective liquid-crystal polarization modulators and a telecentric projection lens 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 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 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 component-color 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 a color-component output face of the color splitting/combining prism assembly. According to the '789 patent, the images of the reflective liquid-crystal polarization modulators are brought into coincidence upon the projection screen by mechanical adjustment of the polarization modulators relative to the color-component output faces of the 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 graphic. In particular, for each pixel of the final imaged graphic 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 final imaged graphic. Such illuminated pixels are referred to as “light” pixels. Conversely, for each pixel of the final imaged graphic 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 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 final image graphic. Such non-illuminated pixels are referred to as “dark” pixels.
The color-component light beam thus spatially selectively polarization modulated by a liquid-crystal polarization modulator of the digital-graphics projector of the '789 patent is reflected f
Epps Georgia
Koninklijke Philips Electronics , N.V.
Seyrafi Saeed
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