Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
1999-12-08
2002-02-12
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, C353S031000, C353S034000
Reexamination Certificate
active
06347014
ABSTRACT:
This application is based on application No. H10-351283 filed in Japan on Dec. 10, 1998, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination optical system for use in an optical apparatus such as a projection-type image display apparatus, and more particularly to an illumination optical system that makes uniform the polarization plane of the light emitted from a light source.
2. Description of the Prior Art
FIG. 12
shows an example of the construction of a conventional projection-type image display apparatus. This projection-type image display apparatus adopts an illumination method of a separate-pupils type, and is provided with an illumination optical system
90
, a cross dichroic prism
98
that serves both to separate and to integrate colors, three reflection-type liquid crystal panels
99
R,
99
G, and
99
B, and a projection optical system
100
. Moreover, a total-reflection mirror
101
for directing illumination light to the liquid crystal panels
99
R,
99
G, and
99
B is provided at the pupil position of the projection optical system
100
. The illumination optical system
90
is composed of a lamp
91
serving as a light source, a reflector
92
, a UV/IR cut filter
93
, a concave lens
94
, an integrator
95
, a polarization separation prism
96
, and a half-wave plate
97
.
The lamp
91
emits white light having random polarization planes. The reflector
92
reflects the light coming from the lamp
91
in such a way as to form it into a converging beam. The UV/IR cut filter
93
transmits only visible light. The concave lens
94
forms the light coming from the reflector
92
into a parallel beam and directs it to the integrator
95
.
The integrator
95
is composed of a first lens array
95
a
and a second lens array
95
b
, each having a plurality of lens cells, and the polarization separation prism
96
is disposed between these lens arrays
95
a
and
95
b
. The lens cells of the first lens array
95
a
individually focus the light coming from the concave lens
94
in the vicinity of the corresponding lens cells of the second lens array
95
b
, and the lens cells of the second lens array
95
b
individually direct the light passing therethrough to the whole surfaces of the liquid crystal panels
99
R,
99
G, and
99
B.
The polarization separation prism
96
is provided with a polarization separation surface
96
a
that reflects S-polarized light and transmits P-polarized light and a total-reflection surface
96
b
. The light having random polarization planes that has passed through the first lens array
95
a
is then separated into S-polarized light, which is reflected from the polarization separation surface
96
a
, and P-polarized light, which is transmitted through the polarization separation surface
96
a
. The P-polarized light is then reflected from the total-reflection surface
96
b
so as to travel in the same direction as the S-polarized light reflected from the polarization separation surface
96
a
, and then these two types of light enter adjoining lens cells of the second lens array
95
b
. The half-wave plate
97
is provided on the lens cells that receive the P-polarized light, and serves to convert the received P-polarized light into S-polarized light. Thus, the whole of the light originating from the illumination optical system
90
is now S-polarized.
The projection optical system
100
is composed of a front unit
100
a
, a rear unit
100
b
, and an aperture stop
100
c
. The pupil of the projection optical system
100
is located between the front unit
100
a
and the rear unit
100
b
, where the total reflection mirror
101
is so disposed as to close half of the pupil. The aperture stop
100
c
is disposed in the vicinity of the total-reflection mirror
101
. The cross dichroic prism
98
has a dichroic surface
98
R that selectively reflects red (R) light and a dichroic surface
98
B that selectively reflects blue (B) light, and the liquid crystal panels
99
R,
99
G, and
99
B are so arranged as to face the cross dichroic prism
98
each from a different direction.
The illumination light L
1
coming from the illumination optical system
90
is reflected from the total-reflection mirror
101
, then travels through the rear unit
100
b
to enter the cross dichroic prism
98
, and is then separated into R light, which is reflected from the dichroic surface
98
R, B light, which is reflected from the dichroic surface
98
B, and green (G) light, which is transmitted through the dichroic surfaces
98
R and
98
B. The thus separated R, G, and B light illuminates the liquid crystal panels
99
R,
99
G, and
99
B, respectively, and is reflected therefrom; meanwhile, the R, G, and B light is modulated by the corresponding liquid crystal panels
99
R,
99
G, and
99
B in accordance with the light components of the corresponding colors of the image to be displayed.
The R, G, and B light reflected from and thereby modulated by the liquid crystal panels
99
R,
99
G, and
99
B then enters the cross dichroic prism
98
again, and is integrated together by being reflected by or transmitted through the dichroic surfaces
98
R and
98
B so as to be formed into projection light L
2
. The projection light L
2
travels along a path symmetrical with the path of the illumination light Li with respect to the optical axis of the projection optical system
100
, and is then projected through the projection optical system
100
with enlargement. The projection optical system
100
focuses the projection light L
2
on a screen (not shown) and thereby displays a color image thereon.
In this projection-type image display apparatus, the whole of the illumination light L
1
is S-polarized when it enters the cross dichroic prism
98
. Now, suppose that the cutoff wavelength, at which the transmittance of the cross dichroic prism
98
for S-polarized light equals to 50% is, for example, 580 nm on the dichroic surface
98
R and 510 nm on the dichroic surface
98
B. Then, the G light that illuminates the liquid crystal panel
99
G covers a wavelength range from 510 to 580 nm, and its energy at wavelengths 510 and 580 nm is 50% of the energy it has before entering the cross dichroic prism
98
.
The G light, after being reflected from the liquid crystal panel
99
G so as to be formed into the projection light L
2
, passes through the dichroic surfaces
98
R and
98
B again. Here again, only 50% of the light having wavelengths of 510 and 580 nm is transmitted, and therefore the projection light, when it reaches the screen, has only 25% of its original energy at those wavelengths. Thus, the wavelength range of the G light included in the projection light is narrowed down to, for example, from 520 to 570 nm, within which the transmittance on the dichroic surfaces
98
R and
98
B is 70% or more (i.e. 50% or more on a two-way basis).
The same is true with the R and B light. Specifically, whereas the wavelength ranges of the illumination light L depend on the wavelengths at which the reflectance on the dichroic surfaces
98
R and
98
B equals to 50% (i.e. the R light covers a wavelength range from 580 nm and above, and the B light covers a wavelength range from 510 nm and below), the wavelength ranges of the projection light L
2
depend on the wavelengths at which the reflectance on the dichroic surfaces
98
R and
98
B equals to 70% (i.e. 50% on a two-way basis); for example, the R light covers a wavelength range from 590 nm and above, and the B light covers a wavelength range from 500 nm and below.
The specific values given above are simply estimates obtained for principal rays. In general, the characteristics of a dichroic surface depend on the angle of incidence of rays, and the transmittance and reflectance for rays incident on the dichroic surface at different angles from principal rays vary from the transmittance and reflectance for principal rays. If such variation is taken into consideration, the wavelength ranges of the R, G, and B light are narrowed down
Hayashi Kohtaro
Konno Kenji
Sawai Yasumasa
Epps Georgia
Minolta Co. , Ltd.
O'Neill Gary
Sidley Austin Brown & Wood
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