Optical image projector

Details Optical image projector Optical image projector

1998-08-24

2002-07-02

Dowling, William (Department: 2851)

Optics: image projectors

Composite projected image

Multicolor picture

C353S038000, C353S102000

Reexamination Certificate

active

06412951

ABSTRACT:

This application is based on patent application No. 9-228386 filed in Japan, the contents of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention relates to an optical image projector of the ray splitting type or light mixing type for projecting an optical image, which is used, for example, in a projector for displaying a color video image and adopts a dichroic mirror.
As one kind of optical image projector, there have been marketed liquid crystal projectors. A liquid crystal projector acts as a means for displaying a large image and enlargedly projects an optical image from liquid crystal panels which is obtained by modulating the luminance of illumination light in accordance with a video signal on a screen via a projection lens.
The construction of such a liquid crystal projector is described with reference to FIG.
8
. In
FIG. 8
, light irradiated from an illumination optical system
51
is split into rays in three wavelength ranges of R (red), G (green) and B (blue) by dichroic mirrors
52
,
53
.
Specifically, a ray in the wavelength range of R reflected by the dichroic mirror
52
illuminates a liquid crystal panel
56
for R after being reflected by a full-reflection mirror
54
and transmitting through a field lens
55
. Rays in the wavelength ranges of G and B transmit through the dichroic mirror
52
. Thereafter, the ray in the wavelength range of G illuminates a liquid crystal panel
58
for G after being reflected by the dichroic mirror
53
and transmitting through a field lens
57
. Further, the ray in the wavelength range of B illuminates a liquid crystal panel
64
after transmitting through the dichroic mirror
53
, being introduced to a relay optical system including two lenses
59
,
60
and two full-reflection mirrors
61
,
62
and then transmitting through a field lens
63
.
Further, optical images of the respective colors formed by the three liquid crystal panels
56
,
58
,
64
are combined into one image by a dichroic prism
65
.
Specifically, the optical image of R formed by the liquid crystal panel
56
is incident on the dichroic prism
65
, propagates straight therein and is reflected at right angles by a first dichroic coating surface
65
a
after being incident thereon at 45° to emerge toward the projection lens
66
. Further, the optical image of G formed by the liquid crystal panel
58
is incident on the dichroic prism
65
, propagates straight therein without being reflected by the first and second dichroic coating surfaces
65
a
,
65
b
to emerge toward the projection lens
66
. Furthermore, the optical image of B formed by the liquid crystal panel
64
is incident on the dichroic prism
65
, propagates straight therein and is reflected at right angles by the second dichroic coating surface
65
b
after being incident thereon at 45° to emerge toward the projection lens
66
.
As described above, the optical images of the respective colors formed by the three liquid crystal panels
56
,
58
,
64
for R, G, B are caused to emerge toward the projection lens
66
after being combined by the dichroic prism
65
with an optical axis and the directions of the optical images aligned. The combined optical image is enlargedly projected on the screen via the projection lens
66
.
In the case that an optical integrator including first and second lens arrays is used as the illumination optical system
51
, the first lens array splits light from a light source and incident thereon into a plurality of rays by a plurality of lenses thereof, and a plurality of rays from the first lens array are projected on the display surfaces of the respective liquid crystal panels
56
,
58
,
64
in a superposed manner by the second lens array.
However, in the above conventional liquid crystal projector, the rays incident on the first and second dichroic coating surfaces
65
a
,
65
b
are not necessarily completely parallel. This results in color nonuniformity.
In
FIG. 9
, the construction of
FIG. 8
is simplified in order to simplify the following description. In other words, a typical arrangement of the first and second dichroic mirrors
52
,
53
is provided between the field lens
57
and a second lens array
72
for introducing the rays to the liquid crystal panel
58
for G, so that the rays from the second lens array
72
can transmit through both the first and second dichroic mirrors
52
,
53
.
As shown in
FIG. 9
, the liquid crystal panel
58
for G is telecentrically illuminated via the field lens
57
by setting an angle distribution of the rays from a plurality of secondary light source images
73
formed in the vicinity of the first lens array
72
by a plurality of lenses of the first lens array
71
within a specified range. Assuming that L denotes a distance between the second lens array
72
and the field lens
57
, a focal length f
1
of the field lens
57
is set at L in order to ensure the telecentric illumination. A most intensive ray al which is from point a in the position of an aperture of the first lens array
71
and is at the center of an intensity distribution of light energy which contributes most to the projected image is incident on the dichroic prism
65
as converged light although it should be perpendicularly incident on the display surface of the liquid crystal panel
58
, i.e., should be incident on the dichroic coating surfaces
65
a
,
65
b
of the dichroic prism
65
at 45° In other words, at points A, B located at opposing ends of the liquid crystal panel
58
, the most intensive ray al contributing most to the projected image and located at the center of the intensity distribution of light energy is incident on the dichroic coating surfaces
65
a
,
65
b
at 45° ±&agr;. Thus, wavelengths to be cut off by the dichroic coating surfaces
65
a
,
65
b
are shifted due to an incident angle dependency of the cutoff values of the dichroic coating surfaces
65
a
,
65
b
. As a result, the color is differed in positions of the projected image corresponding to points A and B, causing a color nonuniformity in the projected image.
The cutoff values of the dichroic coating surfaces
65
a
,
65
b
have incident angle dependencies. Thus, if the cutoff value for an incident angle of 45° is set at 580 nm for the entire dichroic coating surface
65
a
, the cutoff value is shifted as much as the incident angle is shifted. Then, as shown in
FIG. 10
, points of inflection of the spectral distribution of the projected light shift between the opposite ends (left and right ends) of the screen, which causes a color nonuniformity. As a result, the color becomes nonuniform in the projection screen.
Japanese Unexamined Patent Publication No. 4-142530 discloses a projection type liquid crystal display device using a coating of a varying thickness. In this device, a wavelength selecting characteristic in each position is made equal by changing the wavelength selecting characteristic of a dielectric multi-layered coating of a dichroic prism for the image combination according to an inclination of a main ray to a projection lens, thereby avoiding the creation of color nonuniformity in a projected image. Such a method using the coating of a varying thickness also has the problem of a difficult maintenance of the coating characteristics.
Also, Japanese Unexamined Patent Publication No. 4-223456 discloses a projection type liquid crystal display device additionally including a trimming filter. In this device, there are provided a dichroic mirror for selectively transmitting or reflecting a light at a specified wavelength, a first optical device constructed by a dichroic prism for the image combination, and a trimming filter as a second optical device for cutting off light components in a wavelength range corresponding to a change in the incident angle of a light on the first optical device such that a wavelength range of the transmitted light or reflected light falls within a specified region even if the incident angle of the light on the first optical device varies. Accordingly, a projected image free from color no

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