Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
1998-06-17
2001-02-06
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S031000, C349S009000
Reexamination Certificate
active
06183090
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection type image display apparatus, such as a liquid crystal projector, which modulates light from a light source by an image display element, and enlarges and projects the modulated light by a projection lens.
2. Description of the Related Art
Examples of projection type image display apparatuses having an optical system which performs polarization separation and synthesis of light from a light source are disclosed, for example, in Japanese Laid-Open Publication No. 63-39294 and Japanese Laid-Open Publication No. 8-160374.
First, a projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 63-39294 is described below.
A schematic view of the structure of the apparatus is shown in FIG.
1
.
A light beam emitted by a light source
12
passes through a collimating lens
13
, and is separated into two orthogonal linearly polarized light beams by a polarization beam splitter
14
(henceforth referred to simply as “PBS”) having a square pole shape. One of the separated polarized light beams is reflected by the PBS
14
, and is incident upon a color separating prism
15
. The color separating prism
15
includes a first prism
15
A, a second prism
15
B and a third prism
15
C. On a second plane
15
e
of the first prism
15
A, a thin dichroic interference film which reflects a blue light beam and transmits a light beam having a longer wavelength than that of the blue light beam, is vapor-deposited. There is a gap between the first prism
15
A and the second prism
15
B. On a plane
15
f
between the second prism
15
B and the third prism
15
C, a thin dichroic interference film which reflects a red light beam and transmits a green light beam is deposited. Therefore, when white light is incident upon an incident plane
15
a
, blue light included therein is reflected by the plane
15
e
, is totally and internally reflected by the plane
15
a
and heads towards an output plane
15
b
. Green light which passed through the plane
15
f
heads towards an output plane
15
d
. Reference numerals
16
,
17
and
18
represent liquid crystal display elements for displaying a blue image, a red image and a green image respectively. Dielectric reflecting mirrors
19
,
20
and
21
are provided on the back of the liquid crystal display elements
16
,
17
and
18
respectively. The light beam separated by each of the prisms
15
A,
15
B and
15
C passes through the liquid crystal display elements
16
,
17
and
18
respectively, is reflected by the reflecting mirrors
19
,
20
and
21
respectively, and passes through the liquid crystal display elements
16
,
17
and
18
respectively again. The light beams reflected by the liquid crystal display elements
16
,
17
and
18
are synthesized by the color separating prism
15
, and the synthesized light beam is incident upon the PBS
14
again. The component of the incident light beam which had the polarization direction thereof modulated by the liquid crystal display elements
16
,
17
and
18
in response to an image signal, passes through the PBS
14
and is projected on a screen
7
by a projection lens
6
.
Next, a projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 8-160374 is described below.
A schematic view of the structure of the apparatus is shown in FIG.
2
.
Reference numeral
22
represents a projection light source which includes a light source and a reflecting mirror for converging the light beam emitted by the light source. The emitted light beam passes through an IR-UV cutting filter
22
′ which blocks unnecessary infrared radiation and ultraviolet radiation, and enters PBS
23
, where the light beam is separated by a polarization separation plane
23
-
1
into a P polarized light beam and an S polarized light beam which are orthogonal to each other.
The S polarized light beam which was reflected by the polarization separation plane
23
-
1
is separated into red, green and blue light beams by dichroic mirrors
24
,
25
and
26
respectively. These separated light beams are respectively incident upon a liquid crystal display panel for red
27
, a liquid crystal display panel for green
28
and a liquid crystal display panel for blue
29
, where the polarization direction of the light beams are rotated in response to an image signal. The resultant light beams are incident upon dichroic mirrors
30
,
31
and
32
respectively, and then upon another PBS
33
. The P polarized light beam passes through the PBS
33
, and is enlarged and projected on a screen
7
by a projection lens
34
. The P polarized light beam which passed through the PBS
23
is incident upon a liquid crystal display panel
35
. The component of the incident light beam which had the polarization direction thereof modulated by the liquid crystal display panel
35
, is reflected by the PBS
33
, passes through the projection lens
34
, and is enlarged and projected on the screen
7
as a luminance signal light beam of an image.
The projection type image display apparatuses such as the above have the following drawbacks.
As described above, these image display apparatuses use a PBS having a square pole shape as a polarization selective reflection element having a function of reflecting or transmitting light in accordance with the polarization direction thereof.
Such a PBS is made by depositing a dielectric multi-layer film on a surface of one of two glass prisms made of optical glass (e.g., BK7), and attaching the two prisms together so that the deposited surface constitutes an interface therebetween. For example, the glass prism is made by the following procedure: a large glass block is made by cooling glass which has been melted at a high temperature; a small glass block is scraped from the large glass block; and a surface of the small glass block is polished. When cooling and solidifying the glass, an annealing process is adopted in order to remove any deformations. However, it is difficult to remove the deformations completely. Furthermore, such deformations generally occur unevenly in the prism. These deformations cause birefringence in the light in the prism. Since the deformations are not uniform, the degree of birefringence and the direction of the principal axis of birefringence exhibit an uneven distribution.
In the case where a light beam is incident upon a uniaxial birefringent material, there arises a difference in the travelling speed between ordinary ray and extraordinary ray. This causes, for example, a phase contrast between the ordinary ray and the extraordinary ray in the case where a linearly polarized light beam which is diagonal to the principal axis of birefringence. Such a phase contrast adversely affects the polarization state of the incident light beam.
Problems found in the case where birefringence remains in the PBS will now be described, using the projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 63-39294 as an example (see FIG.
1
).
Ideally, the light beam emitted by the light source
12
is incident upon the PBS
14
and is separated by the plane of the dielectric multi-layer film into two linearly polarized light beams whose polarization directions are orthogonal to each other. Only the component of the light beam which had the polarization direction thereof rotated by the liquid crystal display panels
16
,
17
and
18
in response to image information and is now orthogonal to the polarization direction of the incident light, passes through the PBS
14
, and is enlarged and projected on the screen
7
by the projection lens
6
.
However, in the case where birefringence remains in the prism constituting the PBS
14
, while a linearly polarized light beam travels through the prism, a light beam orthogonal to the polarization direction of the incident light arises due to the birefringence. Accordingly, the component of the light beam which did not have the polarization direction thereof modulated by the liquid crystal display pa
Hamada Hiroshi
Kato Hiromi
Nakanishi Hiroshi
Takahara Ikuo
Dowling William
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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