Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2000-11-16
2003-05-06
Perkey, W. B. (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C349S009000
Reexamination Certificate
active
06557999
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of projection imaging systems, and more particularly to a system and method for reducing stray light in a projection system in order to improve projected image contrast.
BACKGROUND OF THE INVENTION
In Reflective LCD projection displays, also known as a liquid crystal on silicon (LCOS) display, a polarizing beamsplitter (PBS) is often used to separate the illumination, off light, and projected image beams. Great effort goes to maximizing system contrast ratio of white to dark. Most of the effort is directed towards the PBS, the RLCD panel, and other polarization optics near the panel. The effort is toward creating a good dark state when tie entire panel is driven to black.
In addition to full white to full black (sometimes called sequential) contrast, intrascene or checkerboard contrast significantly impacts picture quality. The projection lens is a key element for checkerboard contrast. As such the lens alone is often designed and tested for checkerboard contrast, or ghost images. In a reflective LCD system this is not enough. The reflections from the lens elements are reflected back towards the panel and can again be reflected forward. The result is a loss of checkerboard contrast and in some cases results in fairly strong ghost images.
In order to reduce the ambient reflection, one of a conventional techniques is by use of a circular polarizer, which reduces the reflection of the ambient light because the reverse of the handedness of polarization in reflection. The reduction of ambient reflection is significant. But for a high quality color display system, particularly for the high performance system utilizing the on-axis or near on-axis virtual image techniques, the reduction is still not adequate.
Ziegler discloses in U.S. Pat. No. 4,657,348 entitled “Arrangement to Remove Reflection from Liquid Crystal Displays (LCDs)” (issued on Apr. 14, 1987), expressly incorporated herein by reference, an optical arrangement to remove reflection from LCD display by employing a cover disk in front of the LCD, which is disposed obliquely to and apart from the LCD. A quarter wave retarding foil is disposed on the liquid crystal cell. The cover disk includes a polarizer, The quarter wave foil cooperates with the polanier to substantially remove the reflection from the light passing through the cover disk. Application of the polarizer together with the quarter wave (&lgr;/4) plate to remove the ambient reflection is thus well known in the art. However, such technique imposes several limitations for modem display devices, particularly for the virtual image color display systems. First of all, for a virtual image display, the blocking of the ambient reflection by this conventional method also reduces the brightness of the image display. It is caused by the reduction of the image reflection by the poplarizer and the &lgr;/4 retarding foil.
U.S. Pat. No. 5,786,934, (issued on Jul. 28, 1998, Chiu et al.), and U.S. Pat. No. 5,621,486 (issued Apr. 15, 1997, Doany et al.), each entitled “Efficient Optical System for a High Resolution Projection Display Employing Reflection Light Valves” (issued on Jul. 28, 1998, Chiu et al.), expressly incorporated herein by reference, provides an image projection system including a quarter wave plate positioned to suppress stay reflection from the projection lens.
U.S. Pat. No. 5,831,712, entitled “Optical Apparatus Having Ocular Optical System” (issued Nov. 3, 1998, Tabata et al.), expressly incorporated herein by reference, discloses a liquid crystal display device including a beam splitter prism, having quarter wave plates at its surfaces to suppress ghost images from ambient light.
U.S. Pat. No. 6,088,067, provides a “Liquid Crystal Display Projection System Using Multilayer Optical Film Polarizers”.
U.S. Pat. No. 5,278,532, entitled “Automotive Instrument virtual Image Display” (issued on Jan. 11, 1994, Hegg et al.) discloses a virtual image automotive instrument display system.
EP 0 991 281 A2 (20000504) relates to a projection-type display device having a beamsplitter prism, having a polarization filter as an analyzer at its output to block transmission of undesired light of a particular polarization axis.
JP-11015074 A (19990122) relates to a projected type color image display device for a liquid crystal projector having a selective reflector in the form of a sheet as an analyzer, which transmits only linearly polarized light components irrespective of incidence angle and reflects orthogonally crossing polarized light components.
JP-2000098322 A (20000314) relates to a liquid crystal projector having three liquid crystal panels to form an image of each color, with a dichroic prism including a quarter wave plate disposed after a lens on its output side.
JP-2000075246 A (20000314) relates to a projection-type display device, having a polarized plate arranged between a polarized beamsplitter and projection lens as an analyzer, so that reflected polarized light incident on the beamsplitter is absorbed.
JP-11352478 A (19991224) relates to a color light separator assembly in a liquid crystal projector for color image formation, having a polarizing plate to reflect blue light toward a prism having a quarter wavelength sheet on various surfaces, including the output surface facing toward the projection lens.
Image projection systems often provide a liquid crystal spatial light modulator (SLM), which requires illumination with a high intensity polarized light source. It is desirable to maintain a high contrast ratio, and internal reflections from the high power light source typically lead to stray light, which results in loss of contrast. When the internal reflections or stray light form an image in a focal plane of the system, ghost images often result. Although the lens elements of the projection lens are provided with anti-reflective coatings, and thus reflect but a very small percentage of light impinging thereon, even this small percentage of reflected light from the high brightness source is sufficient to be reflected by the OFF state of the SLM back onto the projection screen. This causes ghosts, which are, for example, unwanted weaker replications, which may be transformed, of the modulated image pattern, and are most noticeable in black areas of the screen when only a portion of the screen is white. The unwanted ghost is that portion of light retroreflecting from the air glass interface of a lens element, which then forms a focused image on the SLM surface. This image is reflected by the liquid crystal light valve through the lens and continues onto the screen. The problem is related to the lens element surface of all lenses that form images back onto the SLM, which then are imaged by the projection lens onto the screen. Even those surfaces that do not image may still result in stray light leading to loss of image contrast.
Accordingly, it is an object of the present invention to provide an image projection system reflective liquid crystal light valve projection system which avoids or minimizes above mentioned problems.
In is known to reduce internal reflections in image projection systems by changing the polarization state of such light so as to prevent it from being re-transmitted back through the polarizing beam splitter of the system. More specifically, a quarter wave plate may be positioned between the polarizing beam splitter and the projection lens, whereby light transmitted from the polarizing beam splitter to the lens obtains a circular polarization by the quarter wave plate, and light reflected from the lens back toward the polarizing beam splitter has a circular polarization of opposite handedness, resulting in light having an opposite linear polarization from the incident beam. The light then passes again through the quarter wave plate, resulting in light having a linear polarization with an axis opposite the original beam. This light is reflected away from the light valve by the polarizing beam splitter.
FIG. 1
illustrates a prior art liquid crystal light valve projection syst
Koninklijke Philips Electronics , N.V.
Perkey W. B.
Waxler Aaron
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