Optical system for use in an image projector

Details Optical system for use in an image projector Optical system for use in an image projector

1999-02-04

2001-02-13

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With field curvature shaping

C359S691000, C359S740000, C353S031000

Reexamination Certificate

active

06188524

ABSTRACT:

This application is based on application No. H10-046644 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system for use in an image projector (hereafter such an optical system will be referred to as a “projector optical system”), and more particularly to a projector optical system for use in an image projection apparatus (such as a liquid crystal projector) for projecting an image from a reflection-type display panel (such as a reflection-type liquid crystal panel) onto a screen.
2. Description of the Prior Art
As a method for achieving appropriate illumination in a projector optical system of the type that projects the image displayed on a reflection-type display panel onto a screen, U.S. Pat. No. 5,552,938 and Japanese Laid-Open Patent Application No. H5-203872 propose directing the light for illumination to the reflection-type display panel by the use of a polarized-light separating prism disposed in the position of the aperture stop of the projector optical system.
FIG. 5
shows the outline of the structure of such a projector optical system. This projector optical system is provided with a projection optical system and an illumination optical system. The projection optical system is composed of a front lens unit (GrF), a polarized-light separating prism (Pr
2
), an aperture stop (A), and a rear lens unit (GrR). The illumination optical system is composed of a condenser lens (CL).
The light beam from a light source (
1
) is formed into a substantially parallel beam by a reflector (
2
), and is then condensed by the condenser lens (CL) so as to form an image of the light source. The light source (
1
), the reflector (
2
), the condenser lens (CL), and the polarized-light separating prism (Pr
2
) are so arranged that the image of the light source is formed in the position of the aperture stop (A). Thus, this structure conforms to that of the so-called Koehler illumination. Of the light beam that is condensed to form the image of the light source, only the S-polarized light component is reflected by the polarized-light separating prism (Pr
2
). The light beam reflected from the polarized-light separating prism (Pr
2
) then passes through the rear lens unit (GrR), and then enters a color separating/integrating prism (Pri), where the light beam is separated into three light beams of different colors so as to illuminate the display surfaces of three reflection-type display panels (PR, PG, and PB) individually, with each light beam illuminating the entire display surface of the corresponding display panel.
Since these display panels (PR, PG, and PB) employ reflection-type liquid crystal panels, the light beam that illuminates each of the display panels (PR, PG, and PB) is, when reflected therefrom, partially P-polarized and partially S-polarized according to the pattern formed by the pixels of the display panel. The light beams reflected from the individual display panels are then, by the color separating/integrating prism (Pr
1
), integrated into a single light beam to be projected (hereafter referred to as the “projection light beam”), which then passes through the rear lens unit (GrR). Thereafter, of this projection light beam, only the P-polarized light component is allowed to pass through the polarized-light separating prism (Pr
2
). Here, note that the front lens unit (GrF) is designed to be substantially afocal so that the rays from around the center of each of the display panels (PR, PG, and PB) pass through the polarized-light separating prism (Pr
2
) as a nearly parallel beam. After passing through the polarized-light separating prism (Pr
2
), the projection light beam passes through the front lens unit (GrF), and then forms a display image on a screen (S).
The method, as adopted in the above-described conventional example, that uses a polarized-light separating prism (Pr
2
) to select and project, out of the light beam reflected from each display panel (PR, PG, or PB), only the light component obtained from a particular polarizing surface suffers from low contrast in the projected image (hereafter referred to as the “projection contrast”) because of the disturbance of the polarizing surface caused by slight double refraction and the like occurring in the lens elements constituting the rear lens unit (GrR). The projection contrast tends to be lower the greater the number of constituent lens elements of the rear lens unit (GrR). Accordingly, by reducing the number of constituent lens elements of the rear lens unit (GrR), it is possible to prevent degradation of the projection contrast, but this simultaneously makes it difficult to obtain satisfactory optical performance.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a projector optical system that offers satisfactory optical performance despite having a projection optical system with a rear lens unit composed of as few lens elements as possible.
To achieve the above object, according to one aspect of the present invention, a projector optical system is provided with a reflection-type display panel, an illumination optical system for illuminating the reflection-type display panel, and a projection optical system for projecting the images displayed on the reflection-type display panel onto a screen. The projection optical system is composed of, from the screen side, a front lens unit and a rear lens unit. A light-deriving means for directing illumination light toward the reflection-type display panel is disposed between the front and rear lens units. Additionally, the following condition is fulfilled:
0.2<&phgr;F/&phgr;<0.9
where
&phgr;F represents the optical power of the front lens unit of the projection optical system; and
&phgr;represents the optical power of the entire projection optical system.


REFERENCES:
patent: 5552938 (1996-09-01), Sugawara
patent: 5626409 (1997-05-01), Nakayama et al.
patent: 5969875 (1999-10-01), Sugawara
patent: 5-203872 (1993-08-01), None
patent: 6-265842 (1994-09-01), None

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