Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
1999-06-24
2001-02-20
Dowling, William (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S020000, C353S038000, C353S069000
Reexamination Certificate
active
06190013
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a polarized beam splitter capable of efficiently splitting a polarized beam into S-components and P-components over an entire visible spectrum, and an illumination optical system and a projector provided with such a polarized beam splitter.
A polarized beam splitter splits a plane wave into two components: (S-component and P-component) by reflecting the S-component of the plane wave incident on a polarizing multilayered film at a specified incident angle and causing the P-component thereof to transmit. Known polarized beam splitters include a prism type, in which the polarizing multilayered film is held between a pair of prisms, and a plate type, in which an incident angle of the plane wave on the polarizing multilayered film provided on a flat base plate becomes a Brewster angle.
There have been proposed a variety of apparatuses in which a polarized beam splitter is incorporated as an element in order to utilize the aforementioned polarized light splitting characteristic, i.e. the characteristic of splitting the plane wave into the S-component and P-component, thereby displaying an excellent performance in cooperation of the polarized beam splitter. One of such proposed apparatuses is, for example, a liquid crystal projector.
The liquid crystal projector is an equipment for projecting a large image on a screen. An image, having a shape similar to that of an image desired to be projected, is formed on a liquid crystal panel; and an illumination light, aligned to a specified polarized light component by an illumination optical system, is produced and projected onto the liquid crystal panel. A light emitted from such a liquid crystal panel is introduced to the screen via a projection lens, thereby projecting an enlarged version of the image on the liquid crystal panel onto the screen. In other words, in this liquid crystal projector, the illumination optical system is constructed by a light source, the polarized beam splitter, and a half-wave plate, and the light from the light source is split into an S-component and a P-component by being incident on the polarized beam splitter at a specified incident angle. Thereafter, only one component, e.g. only the P-component is caused to transmit the half-wave plate, thereby converting it to S-component to produce a light aligned to the S-component. This light is projected to the liquid crystal panel as an illumination light.
In order to project a color image onto the screen, it is necessary to prepare liquid crystal panels for each of three primary color components (R(red)-component, G(green)-component and B(blue)-component) and to illuminate the respective liquid crystal panels with corresponding illumination lights. To this end, a white light source needs to be used as a light source and a light from the white light source needs to be split into S-component and P-component by the polarized beam splitter over the entire visible spectrum (400 nm to 700 nm). Further, this splitting efficiency, i.e. a splitting ratio, is closely related to the intensity of the illumination light to the liquid crystal panel. When the splitting ratio decreases, the intensity of the illumination light actually projected on the liquid crystal panel decreases. This darkens the image projected onto the screen, with the result that a satisfactory image cannot be obtained. Accordingly, the polarized beam splitter used in the color liquid crystal projector needs to have a polarized light splitting characteristic of efficiently splitting the incident light into S-component and P-component over the entire visible spectrum.
An incident angle &thgr; of the light from the white light source on the polarizing multilayered film is normally set at 45°, and the illumination optical system is designed such that the splitting ratio is at maximum at this incident angle &thgr;. However, in reality, not only the rays at incident angle &thgr;, but also rays displaced from those rays by about ±3° are incident on the polarizing multilayered film. In the conventional polarized beam splitters, no sufficient investigation and consideration are made for such incident rays oblique to those incident at 45°. If the oblique incident rays are incident on the polarizing multilayered film, the splitting ratio is considerably reduced, causing problems similar to the above.
Further, even if the liquid crystal panel surface is uniformly illuminated, a satisfactory image may not be obtainable due to a nonuniform luminance of an optical image projected onto the screen.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a beam splitter, an illumination optical system, and a projector which have overcome the problems residing in the prior art.
It is another object of the present invention to provide a beam splitter which can efficiently split a beam into an S-component and a P-component over the entire visible spectrum.
It is still another object of the present invention to provide an illumination optical system which can illuminate a specimen brightly with a visible light having its polarization direction aligned.
It is yet another object of the present invention to provide a projector which can project a satisfactory image onto a screen.
According to an aspect of the present invention, a beam splitter comprises: a first transparent base; a second transparent base; and a polarizing multilayered film placed between the first and second transparent bases.
The polarizing multilayered film includes high refraction layers, each having a refractive index n
H
, and low refraction layers, each having a refractive index n
L
lower than the refractive index n
H
, the high refraction layers and the low refraction layers being alternately laminated to one another. The refractive index n
H
and the refractive index n
L
of the laminated layers satisfy the following equation:
2
π
sin
-
1
&LeftBracketingBar;
n
H
2
-
n
L
2
n
H
2
+
n
L
2
&RightBracketingBar;
≥
0.33
.
Alternatively, the polarizing multilayered film may include: a first alternately laminated film for splitting a light beam of a wavelength range having a center wavelength &lgr;
0
, and having high refraction layers, each having a refractive index n
H
, and low refraction layers, each having a refractive index n
L
lower than the refractive index n
H
, the high refraction layers and the low refraction layers being alternately laminated to one another; and a second alternately laminated film for splitting a light beam of a wavelength range having a center wavelength &lgr;
1
greater than the center wavelength &lgr;
0
, and having high refraction layers, each having a refractive index n
H
, and low refraction layers, each having a refractive index n
L
lower than the refractive index n
H
, the high refraction layers and the low refraction layers being alternately laminated to one another. The refractive index n
H
and the refractive index n
L
of the laminated layers in an aspect of the invention satisfy the following equation:
2
π
sin
-
1
&LeftBracketingBar;
n
H
2
-
n
L
2
n
H
2
+
n
L
2
&RightBracketingBar;
≥
0.178
.
Alternatively, further, the polarizing multilayered film may include: a first alternately laminated film for splitting a light beam of a wavelength range having a center wavelength &lgr;
0
, and having high refraction layers, each having a refractive index n
H
, and low refraction layers, each having a refractive index n
L
lower than the refractive index n
H
, the high refraction layers and the low refraction layers being alternately laminated to one another; a second alternately laminated film for splitting a light beam of a wavelength range having a center wavelength &lgr;
0
, greater than the center wavelength &lgr;
0
, and having high refraction layers, each having a refractive index n
H
, and low refraction layers, each having a refractive index n
L
lower than the refractive index n
H
, the high refraction layers and the low refraction layers being alternately laminated
Sawai Yasumasa
Tani Hakuzo
Dowling William
Minolta Co. , Ltd.
Sidley & Austin
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