Arrangement of &lgr;/2 retardation plate in projector

Details Arrangement of &lgr;/2 retardation plate in projector Arrangement of &lgr;/2 retardation plate in projector

2000-07-19

2003-11-11

Ngo, Julie (Department: 2871)

Liquid crystal cells, elements and systems

Liquid crystal system

Projector including liquid crystal cell

C349S005000, C349S009000, C349S117000

Reexamination Certificate

active

06646690

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a projector that projects and displays images.
2. Description of the Related Art
A projector usually comprises an illuminating optical system and liquid crystal panels for modulating light from the illuminating optical system responsive to image information (an image signal). The modulated light is then projected onto a screen by a projection lens.
FIG. 10
is an explanatory diagram showing the main part of a conventional projector. This projector comprises three liquid crystal light valves
900
R,
900
G and
900
B, a cross-dichroic prism
920
and a projection lens
940
. Colored light of the three colors red (R), green (G) and blue (B) emitted from an illuminating optical system (not shown) is caused to pass through the liquid crystal light valves
900
R,
900
G and
900
B and thus modulated responsive to image information. The modulated light (modulated light beams) is combined by the cross-dichroic prism
920
and the composite light is projected by the projection lens
940
. A color image is thus displayed upon the screen SC. Note that in
FIG. 10
, it is assumed that s-polarized colored light R, G and B is incident upon the liquid crystal light valves
900
R,
900
G and
900
B.
The second liquid crystal light valve
900
G comprises a liquid crystal panel
901
G and polarizing plates
902
Gi and
902
Go at the incident and exit sides of the liquid crystal panel
901
G, respectively. The polarization axes of the polarizing plates
902
Gi and
902
Go are set at right angles with each other. The incident side plate
902
Gi transmits s-polarized light whereas the exit side plate
902
Go transmits p-polarized light. With such a configuration, the s-polarized light incident on the second liquid crystal light valve
900
G is converted to p-polarized light and then emitted.
The first liquid crystal light valve
900
R comprises a liquid crystal panel
901
R, a p-polarized light transmitting polarizing plate
902
Ri, an s-polarized light transmitting polarizing plate
902
Ro, and a &lgr;/2 retardation plate
903
R. The &lgr;/2 retardation plate
903
R is affixed to a glass plate
908
, and the first polarizing plate
902
Ri is affixed to the &lgr;/2 retardation plate
903
R. The s-polarized light incident on the glass plate
908
is converted to p-polarized light by the &lgr;/2 retardation plate
903
R, is transmitted through the p-polarized light transmitting polarizing plate
902
Ri and is then converted to s-polarized light by the liquid crystal panel
901
R and the s-polarized light transmitting polarizing plate
902
Ro and emitted. The third liquid crystal light valve
900
B has the same structure as the first light valve
900
R.
The dichroic films of the cross-dichroic prism reflect s-polarized light more effectively than p-polarized light, and transmit p-polarized light more effectively than s-polarized light. Thus, the s-polarized light emitted from the first and third liquid crystal light valves
900
R and
900
B and the p-polarized light emitted from the second liquid crystal light valve
900
G can be efficiently combined by the cross-dichroic prism
920
.
In this Specification, the term “liquid crystal light valve” denotes a liquid crystal light valve in the broad sense, and it refers to a unit that comprises at least a liquid crystal panel and polarizing plates, and that may further comprises a &lgr;/2 retardation plate. Note that a liquid crystal light valve in the narrow sense refers to a unit that includes a liquid crystal panel and polarizing plates and that does not include a &lgr;/2 retardation plate.
The light illuminating the polarizing plate of the liquid crystal light valve generates heat to cause distortion of the polarizing plate occurs. Note that the amount of this distortion depends mainly on the intensity and distribution of the light which illuminates the polarizing plate.
In the aforementioned conventional projectors, the &lgr;/2 retardation plates
903
R and
903
B contained in the first and third liquid crystal light valves
900
R and
900
B are sandwiched between the glass plate
908
and the polarizing plates
902
Ri and
902
Bi. For this reason, the distortion of the polarizing plate
902
Ri causes distortion of the &lgr;/2 retardation plate
903
R.
FIG. 11
is a plan view of the polarizing plate
902
Ri provided on the incident side of the first liquid crystal light valve
901
R shown in
FIG. 10
when viewed from the −x direction. Note that in the direction going back into the plane of the paper (the +x direction) from the polarizing plate
902
Ri, the &lgr;/2 retardation plate
903
R and the glass plate
908
are disposed in this order. As shown by the broken lines in the figure, the heating of the polarizing plate
902
Ri is normally relatively large near the center and relatively small in the periphery. In this case, the polarizing plate
902
Ri becomes distorted in the directions indicated by arrows. At this time, when the polarizing plate
902
Ri becomes distorted, the &lgr;/2 retardation plate
903
R sandwiched between the polarizing plate
902
Ri and glass plate
908
also become distorted in the directions indicated by arrows. This phenomenon can occur in the same manner even in the case wherein the intensity distribution of the light beams illuminating the liquid crystal light valves is relatively uniform.
In the case wherein the &lgr;/2 retardation plate
903
R is distorted in this manner, the distorted portions of the &lgr;/2 retardation plate
903
R (
FIG. 10
) cannot efficiently convert the incident s-polarized light into p-polarized light. Thus, the intensity of the p-polarized light emitted from the p-polarized light transmitting polarizing plate
902
Ri on the incident light becomes lower, and so does the intensity of s-polarized light emitted from the spolarized light transmitting polarizing plate
902
Ro on the exit side. In this manner, nonuniformities occur in the brightness of the modulated light beam emitted from the liquid crystal light valve
900
R. When such a modulated light beam is used to display a color image on the screen SC, chrominance nonuniformities occur within the displayed image.
SUMMARY OF THE INVENTION
An object of the present invention is thus to provide a technique for reducing chrominance nonuniformities in the images displayed by a projector.
At least part of the above and other objects of the present invention is attained by a projector for projecting and displaying a color image. The first projector comprises: an illuminating optical system that emits illuminating light; a color-separating optical system that separates the illuminating light emitted from the illuminating optical system into a first through third beam of colored light, each of which has one of three color components; first through third electro-optical devices that modulate the first through third beams of colored light separated by the color-separating optical system according to image information to form a first through third beam of modulated light; a color combiner that combines the first through third beams of modulated light by reflecting the first and third beams of modulated light while transmitting the second beam of modulated light; a projecting optical system that projects the composite light emitted from the color combiner; and a &lgr;/2 retardation plate provided on at least one of an incident side and an exit side of both of the first and third electro-optical devices, the &lgr;/2 retardation plate having a first surface open to the atmosphere and a second surface in contact with a transmissive member.
Since the &lgr;/2 retardation plate is provided with one surface open to the atmosphere and its other surface in contact with a transmissive member, the distortion of the &lgr;/2 retardation plate is reduced, and the chrominance nonuniformities in the images displayed are reduced accordingly.
In a preferred embodiment of the invention, a direction of polarization of the first and third beams of modulated light incident on the color c

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