Projection optical apparatus

Details Projection optical apparatus Projection optical apparatus

2000-07-21

2002-05-07

Dowling, William (Department: 2851)

Optics: image projectors

Miscellaneous

C353S081000, C353S099000, C348S771000

Reexamination Certificate

active

06382799

ABSTRACT:

This application is based on application No. H11-211527 filed in Japan on Jul. 27, 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 a projection optical apparatus, and particularly to a projection optical apparatus provided with a reflective type spatial light modulator, such as a Digital Micromirror Device™ (DMD™, manufactured by Texas Instruments Incorporated, hereafter referred to simply as a digital micromirror device or DMD), having a large number of micromirrors that vary the angle of refection of illumination light (i.e. the light with which they are illuminated) in accordance with a video signal so as to reflect only signal light (i.e. those portions of the illumination light that convey the image carried by the video signal) toward a projection optical system.
2. Description of the Prior Art
Recently, much attention has been paid to DMDs as light modulation devices for use in projection optical apparatuses. A DMD has a display surface having a large number of micromirrors arranged in a matrix, with each micromirror constituting a pixel of a displayed image. To achieve light modulation, the inclination of the individual micromirrors is driven and controlled independently so that each micromirror is in one of two differently inclined states at a time, namely either in an ON state or in an OFF state. Micromirrors in their ON state reflect illumination light toward the inside of a projection optical system, and micromirrors in their OFF state reflect illumination light toward the outside of the projection optical system. Thus, only those portions of illumination light that have been reflected by micromirrors in their ON state travel through the projection optical system to reach a projection surface (for example, a screen surface), thereby forming a displayed image, which is a pattern consisting of differently bright spots, on the projection surface.
A first conventional example of a projection optical apparatus provided with a DMD as described above is shown in FIG.
16
. In
FIG. 16
, at (A) is shown the positional relationship between the image circle (
3
) formed by a projection optical system (PL) and a DMD (
2
), and at (B) is shown a principal portion of the projection optical apparatus as seen from a direction perpendicular to the optical axis (AX
1
) of the projection optical system (PL). The projection optical system (PL) has a non-telecentric construction. There is also provided an illumination optical system (IL) that is composed of lens elements including a D-shaped lens element (the other lens elements are not shown in the figure) and that has an optical axis AX
2
. The light having passed through the illumination optical system (IL) illuminates the DMD (
2
) from an oblique direction at an angle of 45° relative thereto. The DMD (
2
) has micromirrors that are each so configured as to be in one of two differently inclined states relative to an axis (ax) at a time, namely either in an ON state or in an OFF state. Thus, micromirrors in their ON state reflect the light toward the inside of the projection optical system (PL), and micromirrors in their OFF state reflect the light toward the outside of the projection optical system (PL). As a result, the light reflected by micromirrors in their ON state forms a displayed image on a projection surface (
1
).
In this first conventional example, to achieve separation of the optical path of the projection optical system (PL) from the optical path of the illumination optical system (IL), it is necessary to secure an unduly large image circle diameter. Specifically, as shown at (A) in
FIG. 16
, only a small portion of the image circle (
3
) can be used for the arrangement of the DMD (
2
). For this reason, in a projection optical apparatus of a rear projection type that requires a wide angle of view, it is difficult to make the projection optical system (PL) satisfactorily inexpensive. Moreover, to avoid interference between the projection optical system (PL) and the illumination optical system (IL), it is necessary to design the projection optical system (PL) to have an aperture stop in a rear portion thereof, or to use a D-shaped lens element. However, as shown at (B) in
FIG. 16
, using a D-shaped lens element makes it difficult to obtain a uniform distribution of brightness.
FIGS. 17 and 18
show a principal portion of a second conventional example.
FIG. 17
shows the optical path of projection light (i.e. the light to be projected) when the micromirrors of a DMD (
2
) are in an ON state, and
FIG. 18
shows the optical path of projection light when the micromirrors of a DMD (
2
) are in an OFF state. This projection optical apparatus is provided with a TIR (total internal reflection) prism (PR) composed of a first prism (PR
1
) and a second prism (PR
2
), and with a projection optical system (PL) having an optical axis AX. Here, if the mirror rotation angles of the DMD (
2
) (i.e. the rotation angles of the micromirrors constituting the DMD (
2
)) are ± 10°, using a TIR prism (PR) makes it possible to obtain the maximum f
umber of f/3.0 in a completely telecentric construction. This makes it possible to make efficient use of light and thereby obtain satisfactory brightness in projected images.
However, in a projection optical apparatus of a one-chip type like this second conventional example, where the distance from the DMD (
2
) to the projection optical system (PL) is short, as shown in
FIG. 18
, of the light reflected by OFF-state micromirrors, portions traveling close to the projection optical system (PL) and portions internally reflected from a side surface of the second prism (PR
2
) enter the projection optical system (PL). These portions of reflected light act as ghost light and thereby degrade contrast. Moreover, a construction telecentric toward the DMD (
2
) requires that, in a telecentric-side portion thereof, rays pass through a positive lens element at comparatively great heights. This makes it difficult to reduce lateral chromatic aberration.
FIGS. 19 and 20
show a principal portion of a third conventional example.
FIG. 19
shows the optical path of projection light when the micromirrors of a DMD (
2
) are in an ON state, and
FIG. 20
shows the optical path of projection light when the micromirrors of a DMD (
2
) are in an OFF state. In this third conventional example, as compared to the second conventional example, a longer back-focal length is secured in the projection optical system (PL). This makes it possible to prevent those portions of the light reflected by OFF-state micromirrors which travel close to the projection optical system (PL) from entering the projection optical system (PL). However, this requires that the projection optical system (PL) be made larger and thus makes it difficult to make the projection optical system (PL) satisfactorily inexpensive, and in addition degrades lateral chromatic aberration.
As described above, in the second conventional example, part of the light reflected by OFF-state micromirrors enters the projection optical system (PL), and this degrades contrast. On the other hand, in the third conventional example, it is inevitable to make the projection optical system (PL) larger, and also it is difficult to correct aberrations. To solve these problems, U.S. Pat. No. 5,633,755 proposes a video projector that is so constructed that the light reflected by micromirrors is directed back to the source of the light. However, even in this construction, part of the constituent optical components are shared between an illumination optical system and a projection optical system, and thus degradation of contrast is inevitable owing to interfacial ghosts appearing in those shared components.
In a projection optical apparatus, not only is it necessary to solve problems as mentioned above, but it is also necessary, when the projection optical apparatus is applied to a digital television monitor or printer, to meet high standards in terms of optical perfo

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