Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-06-02
2001-12-04
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
C353S020000, C349S009000
Reexamination Certificate
active
06327093
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image display apparatus, such as a liquid crystal projector, for projecting a light beam supplied from a light source via an image display element on a screen as a magnified image.
In a liquid crystal display (LCD) device having pixel electrodes arranged orderly in a matrix, driving voltages corresponding to image signals are applied to the respective pixel electrodes, to change the optical characteristics of a liquid crystal material sealed in the LCD device. In this way, the LCD device displays images, characters, and the like. The application of individual driving voltages to the pixel electrodes is realized by a simple matrix method or an active matrix method. In the latter method, the LCD device is provided with nonlinear two-terminal elements or three-terminal elements.
In the active matrix method, required are switching elements such as MIM (metal-insulator-metal) elements and thin film transistors (TFTs), as well as wiring electrodes for supplying driving voltages to the pixel electrodes.
Such an active matrix LCD device has drawbacks as follows. When intensive light is incident on the switching element, the resistance of the switching element in the OFF state lowers, resulting in discharging a charge accumulated during voltage application. Moreover, light leaks in the black state from the portions of the liquid crystal material corresponding to the regions where the switching elements and the wiring electrodes are formed, because such liquid crystal portions are not applied with a normal driving voltage and thus fail to participate in the display operation done by the other portions. This results in lowering the contrast ratio.
To overcome the above drawbacks, a transmission LCD device is required to provide a light-shading section called a black matrix
602
as shown in
FIG. 31
for shading the above regions from incident light. Referring to
FIG. 31
, the black matrix
602
is formed on a counter substrate that faces via a liquid crystal layer a TFT substrate on which the switching elements such as TFTs
601
and pixel electrodes
605
are formed. The TFTs
601
as well as gate bus lines
603
and source bus lines
604
intrinsically have the light-shading property. Therefore, in the case of the transmission LCD device, the formation of the light-shading black matrix
602
further reduces the occupation of an effective pixel aperture in the entire area of a pixel, that is, the numerical aperture.
It is difficult to reduce the sizes of the switching elements and wiring electrodes below a certain level due to the restrictions in the electrical performance and production technology. Therefore, as the pitch of the pixel electrodes is made smaller in the course of implementation of more precise, smaller LCD devices, the numerical aperture is further reduced.
A reflection LCD device has been developed to solve the above problem.
In a reflection LCD device, as shown in
FIG. 32
, reflection type pixel electrodes
655
can be formed over TFTs
651
as the switching elements, thereby allowing for a larger numerical aperture than that of the transmission LCD device described above. The reflection LCD device is therefore very effective in improving the brightness of a projection LCD apparatus.
Applications of such a reflection LCD device to a projection LCD apparatus are proposed in Electronic display Forum 97 (pp. 3-27 to 3-32) and Japanese Laid-Open Patent Publication No. 4-338721.
The proposal in the Electronic Display Forum 97 is as shown in
FIG. 33. A
light beam emitted from a light source
701
is separated into light beams of the three primary colors of red (R), green (G), and blue (B) by dichroic mirrors. The three light beams are guided to be incident on corresponding polarization beam splitters (PBS)
702
. The PBS
702
splits the incident light beam into two linearly polarized light components in directions orthogonal to each other. One of the light components is incident on a corresponding reflection LCD element
704
. After being reflected from the reflection LCD elements
704
, the R, G, and B light beams with a modulated polarization direction are incident again on the respective PBSs
702
. The light beams are then combined by a cross dichroic mirror
703
, and the combined light beam is projected on a screen via a projection lens
705
.
The proposal of Japanese Laid-Open Patent Publication No. 4-338721 is as shown in
FIGS. 34A and 34B
. A light beam emitted from a light source
101
is split into two linearly polarized light beams by a PBS
105
. One of the two light beams is separated into R, G, and B light beams by a color separation/combination element (a cross dichroic prism in
FIG. 34A
, and a Phillips type prism in FIG.
34
B). After being reflected from respective reflection LCD elements
107
-R,
107
-G, and
107
-B, the color light beams are combined by the color separation/combination element. The combined light beam is incident again on the PBS
105
, where only the light components of which polarization direction has been modulated are incident on a projection lens
108
and projected on a screen.
The above method is implemented as a 3-panel type liquid crystal projector, which allows efficient use of the R, G, and B light beams from the light source and thus display of markedly bright images.
The 3-panel type liquid crystal projector uses all the R, G, and B light beams for display at all times (spatial color mixing). Another method called a time sequential method or a field sequential method is known, where a light beam for display is time-divided into the R, G, and B light beams to effect color display.
For example, in a liquid crystal projector disclosed in Japanese Laid-Open Patent Publication No. 5-158012, R, G, and B light components of a light beam emitted from a light source are sequentially selected in a time-division manner to be sequentially incident on a PBS. The PBS splits the incident light beam into a p-polarized light component and an s-polarized light component, and outputs one of the two light components (in the disclosed example, the s-polarized light component) to be incident on an image display element. The image display element receives a signal synchronized with the presently incident color light beam. Display is made for each cycle of the R, G, and B light beams as a unit (called a frame; the unit of display for each of the R, G, B light beams is called a field).
For the time-division display of the R, G, and B light beams, the disclosed liquid crystal projector uses a combination of a dichroic mirror and a shutter. The dichroic mirror separates white light emitted from the light source into the R, G, and B light beams. The shutter allows/blocks transmission of the separated color light beams. As an alteration, a rotary color filter
393
having R, G, and B transmission regions as shown in
FIG. 39
may be used. The projector of this type (a single-panel type projector, named against the above 3-panel type projector) may be constructed of only one reflection image display element and only one PBS. Moreover, it requires no individual optical systems for color separation and color combination, allowing for realization of an inexpensive, compact system.
However, the above conventional apparatuses have the following problems.
In the proposal in the Electronic Display Forum 97, three PBSs for the R, G, and B colors are required, together with the optical systems for color separation and the cross dichroic prism for color combination. This greatly increases not only the cost but also the size of the system.
In the proposal in Japanese Laid-Open Patent Publication No. 4-338721, the size of the system can be reduced because color separation and combination are implemented by one element and only one PBS is required. However, the construction using the cross dichroic prism has the following problem. Normally, light is incident on a color separation plane of the cross dichroic prism at an angle of 45°. This increases the polarization dependence of the spectral cha
Katoh Hiromi
Nakanishi Hiroshi
Epps Georgia
Nixon & Vanderhye P.C.
Seyrafi Saied
Sharp Kabushiki Kaisha
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