Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2003-06-18
2004-08-10
Dowling, William C. (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S033000, C349S009000
Reexamination Certificate
active
06773111
ABSTRACT:
RELATED APPLICATIONS
This application claims the priorities of Japanese Patent Application No. 2002-180177 filed on Jun. 20, 2002 and Japanese Patent Application No. 2003-152704 filed on May 29, 2003, which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection type display apparatus for projecting an image displayed on a reflection type light valve using a liquid crystal or the like onto a screen under magnification; and, more specifically, to a projection type display apparatus having a configuration suitable for widening a projection lens.
2. Description of the Prior Art
Recently, the market for projectors has been greatly expanding as personal computers have come into wider use. Known as a light valve for converting images into signals and carrying out optical modulation are transmission and reflection type liquid crystal display devices and DMD devices in which minute mirrors are regularly arranged. Among them, the reflection type liquid crystal display devices are suitable for making highly efficient, minute pixels, and have become a focus of attention as a light valve providing high-quality images. A characteristic feature of the reflection type liquid crystal display device lies in that they distinguish ON and OFF states of a pixel from each other according to polarization directions by utilizing an optical rotation of the liquid crystal.
Known as an example of a projection type display apparatus using a reflection type liquid crystal light valve is one using four polarization beam splitters. Its schematic configuration will be explained with reference to FIG.
13
. This drawing shows a state where, in a projection type display apparatus, white light from a light source is decomposed into three color luminous fluxes of R, G, and B, which are then caused to carry image information items by respective reflection type liquid crystal panels corresponding to the individual color light components, and thereafter combined together, so as to enter a projection lens. The paths of individual color light components are schematically illustrated, whereas solid and dotted lines indicate two kinds of polarization states with polarization directions different from each other. In the following explanation, the solid and dotted lines will refer to S-polarized light and P-polarized light, respectively.
Of the white light emitted from a light source which is not depicted, only S-polarized light is transmitted through a polarizing plate
241
, or its polarization direction is adjusted by the polarizing plate
241
after being converted into S-polarized light in an upstream stage which is not depicted. The resulting light component is made incident on a polarizing beam splitter prism (hereinafter referred to as PBS)
243
by way of a specific wavelength polarization converter
242
. The specific wavelength polarization converter
242
is a device for converting polarization of light in a specific wavelength band, e.g., converting the polarization of G light into P polarization.
Of the luminous flux incident on the PBS
243
, the G light, which is in P polarization, is transmitted through the PBS
243
, and further a PBS
219
, so as to irradiate a reflection type liquid crystal panel (hereinafter referred to as LCD)
221
G for G light. On the other hand, B light and R light, which are in S polarization, are reflected within the PBS
243
, so as to be made incident on a PBS
226
. A specific wavelength polarization converter
225
is disposed upstream the PBS
226
, so as to convert the polarization of B light into P polarization, for example. As a consequence, the B light is transmitted through the PBS
226
, so as to irradiate an LCD
221
B for B light, whereas the R light is reflected by the PBS
226
, so as to irradiate an LCD
221
R for R light.
The luminous fluxes carrying respective image information items corresponding to the individual color light components are turned into polarized light components different from those emitted from the PBS
219
,
226
. Therefore, the G light is reflected within the PBS
219
, the B light is reflected within the PBS
226
, and the R light is transmitted through the PBS
226
, so as to be made incident on a PBS
247
.
For improving the contrast of projected images, quarter wave plates
220
a
,
220
b
,
220
c
are disposed upstream the LCDs
221
G,
221
B,
221
R. Polarizing plates
218
,
222
are disposed on the light entrance side (PBS
243
side) and light exit side (PBS
247
side) of the PBS
219
, so as to adjust deviations in polarization directions. A specific wavelength polarization converter
227
is disposed between the PBSs
226
and
247
, so as to convert the polarization of B light into P polarization.
The G light, which is in S polarization, is reflected within the PBS
247
, whereas the B light and R light, which are in P polarization, are transmitted therethrough, whereby three color light components are emitted as a composite luminous flux. This luminous flux is transmitted through a specific wavelength polarization converter
245
, where the polarization of G light is converted into P polarization, and then a polarizing plate
246
, where deviations in polarization directions caused by the PBSs are adjusted, so as to be projected by a projection lens
230
.
In such a configuration, four PBSs are employed so as to utilize differences in polarization directions of individual color light components in decomposing and combining colors. Therefore, as compared with the case using dichroic devices, polarization characteristics can be maintained more favorably, and the light utilization efficiency can theoretically be made higher. In practice, however, polarization characteristics are hard to maintain when light is transmitted through large polarizing beam splitters, so that the polarization characteristics are likely to be disturbed, whereby the light utilization efficiency cannot always be made higher. Also, the projection type display apparatus may lower its contrast because of disorders in polarization characteristics.
For eliminating such disorders in polarization characteristics, it has been proposed to use a PBS made of a material having a smaller photoelastic constant. However, materials having a very small photoelastic constant may be not only heavy and expensive, but also environmentally problematic since they contain a large amount of lead and the like. Japanese Unexamined Patent Publication No. 2001-154152 discloses a configuration in which the number of PBSs for use is reduced to 2 or 3, in order to overcome these problems.
Meanwhile, projection lenses for recent projection type display apparatus have been prone to widen lenses by shortening their focal length, so that a large image can be constructed even when the distance from the projection lenses to a screen is short. In particular, the demand for widening is strong in projection lenses employed in front type projectors for use in limited spaces such as those for home use, and rear projection TVs which are required to be made smaller and thinner.
However, it is quite difficult to design wide lenses in projection type display apparatus having the above-mentioned configuration. These apparatus are configured so as to dispose at least a PBS on the LCD side and a PBS, a dichroic mirror, or the like on the projection lens side between the LCD and the lens located closest to the light source in the projection lens. When widening the projection lens while maintaining an absolute distance from such a liquid crystal panel to the projection lens, not only the projection lens but also the whole apparatus is likely to become large. This may not be overcome by the lens design alone under circumstances where the whole apparatus is required to be made smaller.
The technique disclosed in the above-mentioned publication disposes a field lens in the vicinity of a light valve, and describes operations and effects obtained when the field lens is considered to be integrated with the projection lens. Tho
Dowling William C.
Fuji Photo Optical Co., Ltd.
Snider Ronald R.
Snider & Associates
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