Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2000-06-14
2002-08-06
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C353S034000
Reexamination Certificate
active
06429906
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to a projection display that projects an image produced by liquid-crystal light valves.
BACKGROUND OF THE INVENTION
Conventional full-color projection displays using reflective light valves, such as that of Unexamined Japanese Patent Document 63-39294 are known.
FIG. 13
shows an arrangement of such a conventional projection display. A white illumination-light flux is emitted from a light source
223
that comprises, for example, a halogen lamp. The illumination-light flux typically passes through a collimating lens
222
operable to make parallel the rays comprising the illumination-light flux. The illumination-light flux then enters a polarizing beamsplitter (PBS)
221
disposed along the optical axis O of a color-separation optical system
211
.
S-polarized light of the illumination-light flux is reflected by the PBS
221
and is incident on the color-separation optical system
211
. The s-polarized illumination-light flux incident on the color-separation optical system
211
is separated into the three primary colors, red (R), blue (B), and green (G), as follows.
The color separation optical system
211
includes a first prism
211
A, a second prism
211
B, and a third prism
211
C, each disposed as shown in
FIG. 13. A
surface
211
e
of the first prism
211
A is coated with a dichroic film that reflects blue light but transmits light with longer wavelengths (i.e., red and green light). There is a gap between the first prism
211
A and the second prism
211
B. A dichroic film that reflects red light but transmits green light is coated on a surface
211
f
of the second prism
211
B, between the second prism
211
B and the third prism
211
C.
As the illumination-light flux reflected from the PBS
221
enters through an incidence surface
211
a
of the first prism
211
A, blue light is reflected by the surface
211
e
and is then reflected inwardly by the surface
211
a
toward an emergence surface
211
b
of the first prism
211
A. Red light that passes through the surface
211
e
of the first prism
211
A is reflected by the surface
211
f
and is then reflected inwardly by the surface of the second prism
211
B between the first and the second prisms. The inwardly reflected red light then exits through an emergence surface
211
c
of the second prism
211
B. Green light that passes through the surface
211
e
of the first prism
211
A and through the surface
211
f
of the second prism
211
B travels toward an emergence surface
211
d
of the third prism
211
C.
Reference numerals
212
,
213
, and
214
denote two-dimensional reflection-type liquid crystal light valves (LCLVs) for displaying a blue light image, a red light image, and a green light image, respectively. Each of the reflective-type LCLVs has a dielectric reflecting layer
215
,
216
, and
217
, respectively, formed on the back of a respective transmission-type LCLV so that the LCLVs
215
,
216
,
217
operate as reflection-type LCLVs. As each color of light enters a respective LCLV, the light is modulated by the respective LCLV. Hence, each color's video signal is converted into an image that has a transmission-rate distribution at the respective LCLV.
The modulated color light is then reflected and changed in polarization state by 90°. That is, the s-polarized light is converted by the LCLV to p-polarized light. The modulated and reflected color lights travel along reverse paths through the first, second, and third prisms
211
A,
211
B,
211
C, respectively, to be combined into a single light flux. The resultant combined, single light flux emerges from the incidence surface
211
a
of the first prism
211
A. The light flux whose polarization state has been converted is transmitted through the PBS
221
and projected on a screen
225
by a projection lens
224
.
A problem with the conventional example shown in
FIG. 13
is its inability to provide sufficiently high-contrast projected images. The conventional projection display described herein does not project an ideal “black” image on the screen for the following reasons.
As linearly polarized light fluxes are incident on the dichroic films, after being passed through the PBS
221
, the light flux is in part transmitted and in part reflected by the dichroic films. A light flux incident on a dichroic film and having a plane of polarization that is not entirely s-polarized or p-polarized with respect to the dichroic film is separated into s-polarized light and p-polarized light by the dichroic film. In addition, reflection and transmission by the dichroic film impose a phase difference between the s-polarized light and p-polarized light. As a result, the light flux exiting the dichroic film is typically elliptically polarized. Hence, the light flux transmitted by the PBS
221
includes light of undesirable polarization. The PBS
221
then directs the undesirable polarized light toward the screen
225
. Accordingly, an ideal black image is not projected on the screen
225
and image contrast is degraded.
The light flux from the light source
223
is split into polarized components by the polarizing beamsplitter
221
, and one of the polarized components is subsequently color-separated and color-combined by the prisms
211
A,
211
B,
211
C. The polarizing beamsplitter
221
analyzes the color-combined light flux that is directed to the screen
225
. A rotation of the plane of polarization of the light flux at the prisms
211
A,
211
B,
211
C results in a degradation of image contrast as well. In order to prevent such a rotation of the plane of polarization, it is necessary to make the prisms
211
A,
211
B,
211
C from a material having an extremely low birefringence, increasing material and fabrication costs. Even when low-birefringence materials are used, birefringence is not completely eliminated, and image contrast is degraded.
The invention provides projection displays that reduce image contrast deterioration caused by polarization changes in color-separation and color-combining optical systems. Furthermore, the invention provides projection displays that do not exhibit image-contrast degradation caused by birefringence in the color-separation and color-combining optical systems.
SUMMARY OF THE INVENTION
Projection displays according to the invention preferably comprise a color-separation optical system having a plurality of substantially parallel dichroic mirrors. The color-separation system separates a light flux from a light source into multiple (e.g., first, second, and third) color components. Alternatively, the dichroic mirrors of the color-separation system are arranged to form a crossed dichroic-mirror or prism.
A separate light valve is provided for individually modulating each corresponding color component. Multiple (e.g., first, second, and third) polarizing beamsplitters are provided to polarize the color components before the color components are incident to corresponding light valves; the polarizing beamsplitters further serve to analyze the color components after modulation and reflection by the light valves. Because each polarizing beamsplitter is used with a single color component, the polarizing beamsplitters can have performance superior to that of a polarizing beamsplitter to be used with multiple color components.
A color-combining optical system is provided to re-combine the color components after the color components are modulated and analyzed. The color-combining optical system preferably comprises an L-shaped dichroic prism having a plurality of substantially parallel dichroic reflecting surfaces. Alternatively, a plurality of substantially parallel dichroic mirrors can be provided or a plurality of substantially parallel dichroic films. Similarly, crossed-dichroic mirrors can be used instead of a crossed-dichroic prism. Because the color-combining system receives the color components after analysis by the polarizing beamsplitters, birefringence and other polarization effects in the color-combining system have little effect on image contrast. Expensive, low-birefringence
Hattori Tetsuo
Sekine Atsushi
Klarquist & Sparkman, LLP
Nikon Corporation
Parker Kenneth
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