Illumination system and projector

Details Illumination system and projector Illumination system and projector

2000-11-02

2002-11-12

Perkey, W. B. (Department: 2851)

Optics: image projectors

Prism in light path

C353S084000, C353S099000

Reexamination Certificate

active

06478431

ABSTRACT:

This application is based on application No. H11-321408 filed in Japan on Nov. 11, 1999, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination system and a projector employing it, and more particularly to an illumination system provided with a color wheel that permits projection of color images and to a projector employing such an illumination system so as to be able to project color images.
2. Description of the Prior Art
FIGS. 24 and 25
show the optical construction of a conventional projector.
FIG. 24
is a front view of the projector as seen from the back side of the DMD™ (Digital Micromirror Device™ manufactured by Texas Instruments Incorporated) (
3
) provided therein as a display device, and
FIG. 25
is a top view of the projector as seen from above. This projector has an illumination system, a DMD™ (
3
) for modulating the illumination light emitted from the illumination system, and a projection optical system (
4
) for projecting the light modulated by the DMD™ (
3
). The illumination system is composed of a lamp (LA), a transmission-type color wheel (CWT), an integrator rod (IR), a first to a third relay lens (RL
1
~RL
3
), a turning prism (
1
), and a TIR (total internal reflection) prism (
2
).
The illumination light emitted from the lamp (LA) first passes through the color wheel (CWT). The color wheel (CWT) is composed of a plurality of color filters transmitting light of different colors (such as R, G, and B), and is so constructed as to rotate, by the action of a motor or the like, about a rotation axis (ax) in such a way that the DMD™ (
3
) is illuminated with light of colors sequentially switched with time. The illumination light having exited from the color wheel (CWT) is, by passing through the integrator rod (IR), made uniform in terms of its brightness distribution on the display surface of the DMD™ (
3
). The illumination light having exited from the integrator rod (IR) passes through the first and second relay lenses (RL
1
, RL
2
), and then enters the turning prism (
1
) having the shape of a quadrangular prism. The illumination light having entered the turning prism (
1
) is totally reflected from a reflecting surface (RT), is then mirror-reflected from a reflecting surface (RM), and is then transmitted through the reflecting surface (RT) so as to exit from the turning prism (
1
).
The illumination light having been deflected by the turning prism (
1
) passes through the third relay lens (RL
3
), and then enters the TIR prism (
2
), where the illumination light has its direction turned. The TIR prism (
2
) is composed of a first prism (
2
a
) and a second prism (
2
b
)(FIG.
25
), and serves to separate light traveling toward and away from the DMD™ (
3
). To the first prism (
2
a
), the third relay lens (RL
3
) is cemented so that the illumination light having passed through the third relay lens (RL
3
) directly enters the first prism (
2
a
). The illumination light having entered the first prism (
2
a
) is totally reflected from a reflecting surface facing the second prism (
2
b
) those surfaces of the first prism (
2
a
) and the second prism (
2
b
) that face each other are arranged substantially parallel to each other with a predetermined air gap secured in between, and then illuminates the DMD™ (
3
) from an oblique direction at an angle of 45° relative thereto. Then, the illumination light is optically modulated by being reflected from the DMD™ (
3
).
The DMD™ (
3
) is so constructed that each of the micromirrors provided therein is in one of two differently inclined states, namely either in an ON state or in an OFF state. The micromirrors in their ON state reflect the illumination light toward the inside of the projection optical system (
4
), and the micromirrors in their OFF state reflect the illumination light toward the outside of the projection optical system (
4
). Thus, the portion of the illumination light reflected by the micromirrors in their ON state passes through the TIR prism (
2
), i.e. first the first prism (
2
a
) and then the second prism (
2
b
), then enters the projection optical system (
4
), and eventually forms a display image on a projection surface (not shown).
Nowadays, there is a keen demand for slim and compact mobile projectors on the market. However, in the case of the conventional projector described above, because the color wheel (CWT) is arranged vertically so as to jut upward or downward (FIG.
24
), even if the other components are miniaturized in the vertical direction, it is impossible to make the entire projector slimmer. This is true also in cases where a reflection-type color wheel is used. With a color wheel (CWT) having a smaller diameter, the illumination system can be made accordingly slimmer and thus the entire projector can be made accordingly slimmer. However, the smaller the diameter of the color wheel (CWT) is made, the more serious the problem described below becomes.
Assume that, as shown in
FIG. 26A
, the color wheel (CWT) is composed of three color filters (F
1
~F
3
) transmitting light of different colors, namely R, G, and B. In this case, when the illumination light (LB) enters the boundary between two color filters, the illumination light (LB) exiting therefrom has mixed colors. This degrades the color purity of the projected image. If the DMD™ (
3
) is kept in an OFF state while the illumination light (LB) is passing through a portion around the boundary between two color filters (F
1
~F
3
) (e.g. the portion indicated as “off area” in
FIG. 26A
, such mixing of colors can be prevented. However, while the DMD™ (
3
) is in its OFF state, no image is projected. In addition, as shown in
FIG. 26B
, the smaller the diameter of the color wheel (CWT) is made, the greater the ratio of the angle of the area in which the DMD™ (
3
) needs to be kept in its OFF state (off area) to the angle of the entire color wheel (CWT). This makes the projected image dimmer. In this way, making the diameter of the color wheel (CWT) smaller causes degradation in brightness or color purity of the projected image, and therefore expectations are high for a method that permits a projector to be made slimmer without making the diameter of the color wheel (CWT) used therein smaller.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an illumination system for use in a projector which permits the projector to be made slimmer without making the diameter of the color wheel used therein smaller. A second object of the present invention is to provide a slim and compact projector employing such an illumination system so as to offer projected images without degradation in brightness or color purity.
To achieve the above object, according to one aspect of the present invention, an illumination system is provided with: a light source for emitting illumination light, a color wheel that transmits or reflects the illumination light while rotating about a rotation axis in such a way that illumination light exiting from the color wheel has colors that are switched sequentially with time, a first deflecting member for deflecting the illumination light in such a way that the illumination light enters the color wheel, and a second deflecting member for deflecting the illumination light having exited from the color wheel, wherein the illumination light entering the first deflecting member and the illumination light exiting from the second deflecting member are both substantially perpendicular to the rotation axis of the color wheel.
According to another aspect of the present invention, a projector is provided with: a light source for emitting illumination light; a color wheel that transmits or reflects the illumination light while rotating about a rotation axis in such a way that illumination light exiting from the color wheel has colors that are switched sequentially with time; a first deflecting member for deflecting the illumination light in such a way that the illumination light enters the

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