Optical: systems and elements – Projection screen – Unitary sheet comprising plural refracting areas
Reexamination Certificate
1998-04-17
2001-07-03
Mahoney, Christopher E. (Department: 2851)
Optical: systems and elements
Projection screen
Unitary sheet comprising plural refracting areas
Reexamination Certificate
active
06256145
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rear projection display apparatus and a screen unit that are able to project a magnified image from an image source onto a transmissive screen via a projection lens.
2. Description of the Related Art
A well-known type of rear projection display apparatus projects image light emitted from an image source formed of a CRT (Cathode Ray Tube) and an optical system using optical elements, such as a liquid crystal panel, onto a transmissive screen via a projection lens to form a magnified image thereon.
Since a screen unit in such a rear projection display apparatus is required to have various functions for projecting a good image, and includes in combination a plurality of screens having specific functions.
In general, the screen unit includes a Fresnel lens sheet located on the side of the image source, and a lenticular lens sheet located at the rear of the Fresnel lens sheet, as disclosed in, for example, Japanese Unexamined Patent Publication No. 6-6739.
FIG. 7
is a perspective view showing the structure of a screen unit
30
including a Fresnel lens sheet
31
, a lenticular lens sheet
32
, and the like.
Image light projected by a projection lens (not shown), which is placed after the aforesaid CRT or optical system, is gathered and directed toward a viewer by Fresnel lenses in the Fresnel lens sheet
31
, which corrects variations in luminance from point to point on the viewed screen.
The lenticular lens sheet
32
placed behind the Fresnel lens sheet
31
contains a light diffusing agent therein. Image light is transmitted through the lenticular lens sheet
32
while being diffused by the light diffusing agent, thereby projecting an image. The lenticular lens sheet
32
also controls the light diffusion properties in the horizontal and vertical directions, and causes the light to converge by the lensing action of a plurality of lenticular lens elements located on the incident side.
Furthermore, external-light absorbing layers
32
a
are formed in vertical stripes at portions on the emergent side of the lenticular lens sheet
32
other than portions where the light converges so that they cover predetermined areas on the emergent side, thereby reducing the influence of external light from outside the display apparatus without blocking image light to be projected onto the screen, and improving the image contrast.
Although it is only necessary to increase the covering ratio of the external-light absorbing layers
32
a
in order to reduce the influence of external light and to improve the image contrast, if the covering ratio exceeds a predetermined value, image light is blocked by the external-light absorbing layers
32
a
, and is decreased in amount. Therefore, in a rear projection display apparatus using a CRT as an image source, the covering ratio of the external-light absorbing layers
32
a
is limited to, for example, about 40% according to the structure of the optical system and the like.
In a rear projection display apparatus using three CRTs corresponding to three colors R, G, and B, as shown in
FIG. 8
, image lights of three colors emitted from a red CRT
40
R, a green CRT
40
G, and a blue CRT
40
B are magnified and projected onto a screen unit
30
, which is composed of a Fresnel lens, a lenticular lens and the like, via projection lenses
41
R,
41
G, and
41
B, respectively.
Since the CRTs
40
R,
40
G, and
40
B are required to be placed so that their centers are opposed to about the center of the screen unit
30
, as is shown, the CRT
40
G is placed in front of the screen unit
30
, and the CRTs
40
R and
40
B on both sides of the CRT
40
G are placed at an angle of, for example, about 10° with respect to the screen unit
30
. Thereby the colored lights are incident on the screen unit
30
at different angles and are not parallel to each other in front of the screen unit
30
, and therefore, the covering ratio is set at about 40% in consideration of these circumstances, as mentioned above.
In recent years, a rear projection display apparatus has also been known that uses as an image source an optical system composed of a liquid crystal panel and the like and that magnifies and projects an image formed by the optical system by a single projection lens.
FIG. 9
is a schematic view of the rear projection display apparatus in which an optical system
51
composed of three liquid crystal panels corresponding to colors R, G, and B and a screen unit
30
are placed in a housing
50
.
An image formed by the optical system
51
located in the lower part of the housing
50
is magnified by a projection lens
52
, reflected by a mirror
53
as shown by the arrows, and reaches the screen unit
30
.
The configuration of the optical system
51
will now be described with reference to FIG.
10
.
In a light source
55
, a lamp
56
formed of, for example, a metal halide lamp, is placed at the focal point of a parabolic mirror, and light almost parallel to the optical axis of the parabolic mirror is emitted from an opening of the lamp
56
. Unnecessary beams, in the infrared and ultraviolet regions, of the light emitted from the light source
55
, are blocked by an UV-IR cutting filter
57
, and only the effective beams are directed to a lens array section
58
.
The lens array section
58
includes optical elements, such as a PBS (Polarizing Beam Splitter), and it polarizes P-polarized light and S-polarized light emitted from the light source
55
and outputs, for example, P-polarized light.
That is, the lens array section
58
allows the light from the light source
55
passed through the UV-IR cutting filter
57
to be polarized into P-polarized light and to be effectively and uniformly radiated onto effective apertures of liquid crystal panels
62
,
63
, and
68
.
Between the lens array section
58
and the effective apertures of the liquid crystal panels
62
,
63
, and
68
, dichroic mirrors
59
and
60
are placed to separate the light emitted from the light source
55
into lights of red, green, and blue light.
In this example shown in
FIG. 10
, the dichroic mirror
59
first reflects red light R, and transmits green light G and blue light B. The red light R reflected by the dichroic mirror
59
is deflected by 90° by a mirror
61
, and enters the red liquid crystal panel
62
.
On the other hand, the green light G and the blue light B transmitted through the dichroic mirror
59
are separated by the dichroic mirror
60
. That is, the green light G is reflected, deflected by 90°, and directed to the green liquid crystal panel
63
. The blue light B passes through the dichroic mirror
60
, travels straight, and is directed to the blue liquid crystal panel
68
via a relay lens
64
, a mirror
65
, a relay lens
66
, and a mirror
67
.
The colored light modulated by the liquid crystal panels
62
,
63
, and
68
is synthesized by a crossed dichroic prism
69
serving as a light synthesizing element. The crossed dichroic prism
69
is composed of a reflecting plane
69
a
and a reflecting plane
69
b
. The red light R and the blue light B are reflected toward the projection lens
52
by the reflecting plane
69
a
and the reflecting plane
69
b
, respectively, and the green light G is transmitted through the reflecting planes
69
a
and
69
b
. Therefore, the R, G, and B light is combined, and magnified and projected by the projection lens
52
onto the screen unit
30
shown in FIG.
9
.
In such a rear projection display apparatus, since image light is magnified and projected by the single projection lens
52
, it enters the screen unit
30
from one direction. Therefore, it is possible to set the covering ratio of the external-light absorbing layers
32
a
to, for example, about 80%, which is considerably higher than that of the aforesaid display apparatus using three CRTs shown in FIG.
8
.
In order to maximize the covering ratio of the external-light absorbing layers
32
a
without blocking image light, as disclosed in, for example, Japanese Examined Patent Publication No. 7-1902
Kono Toshiya
Nakanishi Yasuaki
Mahoney Christopher E.
Sonnenschein Nath & Rosenthal
Sony Corporation
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