Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2001-07-16
2004-02-03
Adams, Russell (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S081000
Reexamination Certificate
active
06685322
ABSTRACT:
RELATED APPLICATION
This application is based on Patent Application No. 2000-236430 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 comprising a plurality of prisms, and to a projection-type image display device provided with this optical system.
2. Description of the Related Art
There are projection-type image display devices which direct illumination light to a reflection-type display element, modulate the illumination light by a projection image displayed on the display element, and display the projection image on a screen by projecting the reflected modulation light through a projection optical system. Reflection-type liquid crystal displays (LCD), or digital micro mirror devices™ (DMD™, both DMD ™ and digital micro mirror devices™ are trademarks of Texas Instruments) are used as the display element.
A reflection-type LCD modulates illumination light entering from an approximately perpendicular direction via a liquid crystal layer displaying the projection image, and reflects the image in an approximately perpendicular direction. A DMD™ has micro mirror elements of variable directionality arranged in a plurality of rows on a plane. The direction of each mirror element is alternatively selectable from among two specified directions, which are selected in accordance with the displayed image. Illumination light comprises light of a displayed projection image reflected in one direction and modulated light of an undisplayed projection image reflected in another direction. The range of variance of direction of the mirror elements is very slight at the micro level, and the DMD™ receives illumination light from a near-perpendicular direction, and reflects light in a second near-perpendicular direction.
In this way illumination light of modulated light from one direction must be directed to the display element in a projection-type image display device which modulates illumination light by a reflection-type display element. Accordingly, an optical system for directing illumination light to the display element without blocking the modulated light must be arranged medially to the display element and the projection optical system, and an optical system comprising a plurality of prisms is often used for this purpose.
An example of a conventional optical system is shown in FIG.
7
. Parts (a) and (b) of
FIG. 7
represent mutually intersecting cross section views of an optical system
50
. The optical system
50
comprises two prisms
51
and
52
. Prism
51
has three surfaces
51
a
,
51
b
, and
51
c
, and the surfaces
51
a
and
51
b
form an acute angle therebetween. Prism
52
also has three surfaces
52
a
,
52
b
, and
52
c
, and the surfaces
52
a
and
52
b
form an acute angle therebetween.
Prisms
51
and
52
are arranged such that surface
51
b
confronts surface
52
b
with an interval of a small distance therebetween. That is, a small air gap G
P
is formed between surface
51
b
and surface
52
b
. Surface
51
b
and surface
52
b
are mutually parallel, and the size (thickness) of the air gap G
P
is constant regardless of position.
The angle formed by the surfaces
51
a
and
51
b
of the prism
51
and the angle formed by the surfaces
52
a
and
52
b
of the prism
52
are equal to each other, and, accordingly, the surfaces
51
a
and
52
a
are parallel. An optical axis perpendicular to the surface
51
a
of prism
51
is referred to as optical axis Ax of the optical system
50
. A display element is arranged perpendicularly to the optical axis Ax on the surface
51
a side of the optical system
50
, and a projection optical system is arranged such that the optical axis of the projection optical system is parallel to the optical axis Ax on the surface
52
a
side of the optical system
50
. Accordingly, the air gap G
P
, and the surfaces
51
b
and
52
b
, forming this air gap G
P
, are oblique to the optical axis of the projection optical system.
The direction of the optical axis Ax is referred to as the X direction, the direction perpendicular to the optical axis Ax within a plane perpendicular to the air gap G
P
is referred to as the Y direction, and the direction perpendicular to the optical axis Ax within a plane parallel to the air gap G
P
is referred to as the Z direction. Part (a) of
FIG. 7
represents the cross section in the X-Y plane, and part (b) of
FIG. 7
represents the cross section in the X-Z plane.
The light for illuminating the display element passes through the surface
51
c
of the prism
51
in the optical system
50
. The light passing through the surface
51
c
[and entering] enters the prism
51
and reaches the surface
51
b
. The incidence angle of light on the surface
51
b
is set so as to exceed the critical angle, and the light is completely reflected by the surface
51
b
. The light completely reflected by the surface
51
b
reaches the surface
51
a
, is transmitted through the surface
51
a
, and impinges the display element approximately perpendicularly thereto.
Light impinging the display element is modulated and reflected by the projection image displayed on the display element. The modulated reflected light impinges the surface
51
a
, and passes through the prism
51
, reaching the surface
51
b
. The entrance angle of this light on the surface
51
b
is less than the critical angle, and the light is transmitted through the surface
51
b
, crosses the air gap G
P
, and impinges the surface
52
b
of the prism
52
. The light impinging the prism
52
reaches the surface
52
a
, is transmitted therethrough, impinges the projection optical system, is projected therefrom, and forms a projection image on the screen.
The modulated light is refracted when transmitted through the surfaces
51
b
and
52
b
. However, since the surfaces
51
b
and
52
b
are parallel, the optical path is also parallel both before passing through surfaces
51
b
and
52
b
and after passing through surfaces
51
b
and
52
b
. Since the air gap G
P
is oblique to the optical axis Ax, the size of the shift in the optical path before transmission through the air gap G
P
and after transmission through the air gap G
P
is different in the mutually perpendicular Y direction and Z direction. Therefore, although the light has the same point of origin, the origin point in the Y direction is positioned nearer to the projection optical system than the origin point in the Z direction. The shift of the Y direction and Z direction origin points in the optical axis Ax direction is referred to as the interval difference. The origin point in the Z direction is one point, however, the origin point in the Y direction is broadened.
Since the shift of these origin points causes distortion in the image formed by the projected light and reduces the quality of the displayed projection image, this shift must be suppressed as much as possible. For this reason, the size of the air gap is very small, approximately 10 &mgr;m in a conventional optical system.
FIG. 8
shows the relationship between the amount of defocus and the optical transfer function (OTF) when the size of the air gap G
P
is set to this degree. In
FIG. 8
, the curves marked by the symbols XY and XZ represent the OTF within the XY plane and the XZ plane, respectively. Both the amount of defocus of the horizontal axis or the OTF of the vertical axis is standardized when there is no air gap within the optical system. In
FIG. 8
, there is no great difference in the OTF of the XY plane and the OTF of the XZ plane, and excellent image forming performance is obtained.
When a color projection image is provided, light from a light source emitting white light is split into red (R), green (G), and blue (B) light, and each color light after splitting is modulated by separate reflection-type display elements. In this case an optical system having two air gaps of constant size are used, and a dichroic film is provided on one surface of each air gap, and, for example, red light is
Fujiki Katsunari
Sawamura Shigeru
Takamoto Katsuhiro
Adams Russell
Minolta Co. , Ltd.
Sever Andrew
Sidley Austin Brown & Wood LLP
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