Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2002-10-07
2004-03-16
Adams, Russell (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C359S490020, C359S490020, C362S019000, C349S009000
Reexamination Certificate
active
06705731
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application is based on Japanese Patent Application No. 2001-314371 filed in Japan on Oct. 11, 2001, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection type display apparatus. For example, the present invention relates to the projection type display apparatus such as a liquid crystal projector for projecting and displaying a two-dimensional image of an illuminated liquid crystal panel onto a screen.
2. Description of the Related Art
A projection type display apparatus, which uses a plurality of polarizing beam splitters (PBS) for projection of an illumination to a light valve (for example, a liquid crystal panel) or a light that an image is modulated by the light valve, is conventionally known (for example, Japanese Unexamined Laid-Open Patent Publication Nos. 2000-330196, 2000-321662, etc.). Its schematic optical structure is shown in FIG.
6
. In
FIG. 6
, the reference numeral (
1
) is a light source, (
2
a
) is a first lens array, (
2
b
) is a second lens array, (
3
) is a superimposing lens, (
4
a
) is a first dichroic mirror, (
4
b
) is a second dichroic mirror, (
6
R), (
6
G) and (
6
B) are field lenses, (
7
a
) and (
7
b
) are pre-polarizing beam splitters (pre-PBS), (
8
R), (
8
G) and (
8
B) are main polarizing beam splitters (main PBS), (
9
R), (
9
G) and (
9
B) are reflection type light valves (for example, reflection type liquid crystal panels), (
11
) is a cross dichroic prism, (
12
) is a projection lens, (
13
R), (
13
G) and (
13
B) are spacer glasses and AX is an optical axis.
A spatial energy distribution of a light emitted from the light source (
1
) is averaged by the first lens array (
2
a
) and the second lens array (
2
b
). Lights emitted from respective cells of the second lens array (
2
b
) are superimposed on the light valves (
9
R), (
9
G) and (
9
B) by the superimposing lens (
3
). Meanwhile, a light emitted from the superimposing lens (
3
) is color-separated into lights of primary colors: red (R); green (G); and blue (B) corresponding to three primary colors by first and second dichroic mirrors (
4
a
) and (
4
b
).
The light of the primary color B is reflected by the first dichroic mirror (
4
a
) and by the pre-PBS (
7
a
), and passes through the field lens (
6
B). Meanwhile, the lights of the primary colors G and R are reflected by the first dichroic mirror (
4
a
) and by the pre-PBS (
7
b
), and are color-separated into G and R by the second dichroic mirror (
4
b
). The light of the primary color G is reflected by the second dichroic mirror (
4
b
), and passes through the field lens (
6
G). The light of the primary color R transmits through the second dichroic mirror (
4
b
), and passes through the field lens (
6
R). Since polarizing directions of the lights entering the main PBSs (
8
R), (
8
G) and (
8
B) are previously adjusted by the reflection at the pre-PBSs (
7
a
) and (
7
b
), higher contrast can be obtained.
The field lenses (
6
R), (
6
G) and (
6
B) change the illumination light into a telecentric light flux and allow a projection light to enter a pupil of the projection lens (
12
) by the power of the light flux. The primary color lights of RGB which pass through the field lenses (
6
R), (
6
G) and (
6
B), respectively, enter the main PBSs (
8
R), (
8
G) and (
8
B). The main PBSs (
8
R), (
8
G) and (
8
B) allow polarized components (P polarized lights) which are not necessary for illuminations to the light valves (
9
R), (
9
G) and (
9
B) to transmit and remove polarized components, and allows only polarized components (S polarized lights) which are necessary for the illuminations to the light valves (
9
R), (
9
G) and (
9
B) to reflect and enter the light valves (
9
R), (
9
G) and (
9
B), respectively.
The respective light valves (
9
R), (
9
G) and (
9
B) modulate the primary color lights (S polarized lights), polarizing directions of which are aligned, by means of selective polarizing control according to display of pixels of a two-dimensional image (namely, ON/OFF for each pixel), and emit a reflected light composed of two kinds of polarized lights (P polarized light and S polarized light). The primary color lights emitted from the light valves (
9
R), (
9
G) and (
9
B) again enter the main PBSs (
8
R), (SG) and (
8
B), respectively. The main PBSs (
8
R), (
8
G) and (
8
B) reflect and remove a polarized component (S polarized light) which is unnecessary for projection and allow only a polarized component (P polarized light) which is necessary for projection to transmit and enter the spacer glasses (
13
R) (
13
G) and (
13
B).
The spacer glasses (
13
R) (
13
G) and (
13
B) are jointed to the main PBSs (
8
R), (
8
G) and (
8
B), respectively, and to the cross dichroic prism (
11
), and their gaps are kept stable and suitable so that shift of the pixels for the respective colors is prevented. The primary color lights which transmit through the spacer glasses (
13
R), (
13
G) and (
13
B), respectively, enter the cross dichroic prism (
11
) to be color-synthesized. The color-synthesized projection light in the cross dichroic prism (
11
) is projected onto a screen (not shown) by the projection lens (
12
).
The contrast which is generally obtained in the projection type display apparatus shown in
FIG. 6
is determined by a relationship between an extinction ratio due the PBS system from the light source to the light valves (illumination side extinction ratio) and an extinction ratio due to the PBS system from the light valves to the screen (projection side extinction ratio). Concretely, an inferior value of both values determines the contrast of the projection type display apparatus. Therefore, in the case where the projection type display apparatus having high contrast is obtained, it is necessary that both the illumination side extinction ratio and the projection side extinction ratio show high values. Further, when the illumination side extinction ratio and the projection side extinction ratio are almost equivalent to each other, the projection type display apparatus having high contrast can be obtained most efficiently.
In addition, there are two kinds of methods, a method of taking out a necessary polarized light by means of transmission (the method using a P polarized light) and a method of taking out a necessary polarized light by means of reflection (the method using a S polarized light), in a PBS for determining the contrast of the projection type display apparatus. However, in the case of the method using the S polarized light, a high extinction ratio can not be secured with a single PBS. The reason for this will be explained below. When a transmittance of the P polarized light in PBS is Tp, a transmittance of the S polarized light is Ts, a reflectance of the S polarized light is Rs and a reflectance of the P polarized light is Rp, an extinction ratio in the case of using the P polarized light is obtained by Tp/Ts, and an extinction ratio in the case of using the S polarized light is obtained by Rs/Rp. According to PBS characteristics, since the transmittance Ts of the S polarized light can be very low, the extinction ratio Tp/Ts in the case of using the P polarized light can be high. However, since the reflectance Rp of the P polarized light cannot be as low as the transmittance Ts of the S polarized light, the extinction ratio Rs/Rp in the case of using the S polarized light is lowered.
In the projection type display apparatus shown in
FIG. 6
, since the illumination sides of the pre-PBSs (
7
a
) and (
7
b
) and the main PBSs (
8
R), (
8
G) and (
8
G) are used in the method of taking out a necessary polarized light by means of reflection, the high illumination side extinction ratio cannot be secured due to the above reason. Moreover, since the lights which have passed through the pre-PBSs (
7
a
) and (
7
b
) pass through the second dichroic mirror (
4
b
) and the field lenses (
6
R), (
6
G) and (
6
B), a phase shift occurs at the time of transmission and reflection,
Adams Russell
Dalakis Michael
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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