Optics: image projectors – Reflector
Reexamination Certificate
2002-01-28
2003-05-06
Cariaso, Alan (Department: 2875)
Optics: image projectors
Reflector
C353S102000, C362S551000, C362S268000
Reexamination Certificate
active
06558007
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an illumination optical system that directs light with a substantially even intensity distribution to a target to be illuminated, and also relates to an image projection apparatus that projects light representing an image by reflecting the light from an illumination optical system by using an image display device that reflects light in two directions in such a way that the light reflected in one of those directions represents an image.
2. Description of the Prior Art
In recent years, image projection apparatus provided with a reflection-type image display device called a DMD (digital micromirror device) have been developed. A DMD has a reflective surface composed of a large number of minute mirror elements arranged in a two-dimensional array, and each mirror element is rotatable about an axis perpendicular to the normal thereto so that its direction (the direction of its normal) can be controlled individually. The rotation axes of all the mirror elements are parallel, and the direction of each mirror element is chosen between two predetermined directions according to the image signal. Thus, the illumination light fed to the DMD is reflected in two directions in such a way that light representing the desired image is reflected in one direction and unnecessary light, i.e. light that does not represent the desired image, is reflected in the other direction.
In the following descriptions, light representing the desired image is referred to as the ON light, unnecessary light is referred to as the OFF light, the direction of a mirror element in which it generates the ON light is referred to as the ON direction, and the direction of a mirror element in which it generates the OFF light is referred to as the OFF direction. The range in which a mirror element can change its direction (i.e. the range of its rotation angles) is narrow; specifically, the angle difference between the ON and OFF directions is typically about 10°, and thus the angle difference between the ON and OFF light is about 20°.
The direction of each mirror element is switched at high speed. The amount of ON light is determined by the time for which a mirror element is directed in the ON direction. Thus, by controlling the time for which each mirror element is directed in the ON direction according to the image signal, it is possible to make the individual mirror elements reflect different amounts of ON light so that the ON light as a whole represents an image.
An illumination optical system for illuminating a DMD is arranged in such a way that illumination light is directed to the DMD from a direction perpendicular to the rotation axes of the mirror elements thereof, and the DMD is controlled in such a way that the angle of the OFF light relative to the illumination light is larger than the angle of the ON light relative to the illumination light. That is, the principal rays of the illumination light, ON light, and OFF light lie on the same plane, and the ON light travels between the illumination light and the OFF light.
The illumination light may be directed to the DMD from a direction nearly perpendicular to the DMD or from a direction considerably inclined relative to the DMD. When the illumination light is directed to the DMD from a direction nearly perpendicular thereto, the angle difference between the illumination light and the ON light is small, and thus these two beams of light partially overlap with each other. This makes it necessary to separate the illumination light and the ON light, and this is achieved by exploiting the total reflection of light in a prism. Specifically, a prism is arranged immediately in front of the DMD, and the illumination light is directed into the prism from the side so that the illumination light is totally reflected by the oblique surface of the prism and is thereby directed to the DMD; on the other hand, the ON light is transmitted through the prism so as to be directed to a projection optical system.
When the illumination light is directed to the DMD from a direction considerably inclined relative thereto, the angle difference between the illumination light and the ON light is large, preventing these two beams of light from overlapping with each other. Thus, the illumination light can be shone directly on the DMD. This eliminates the need to use a prism, and thus helps simplify the optical arrangement.
To obtain evenly bright images, it is preferable that the illumination light have as even an intensity distribution as possible on the DMD. To achieve this, the illumination optical system is provided with, in addition to a light source that emits the illumination light, an integrator for making the intensity distribution of the illumination light from the light source even and a relay optical system for making the exit surface of the integrator substantially conjugate with the DMD. To permit the light from the light source to be used efficiently for the illumination of the DMD, the exit surface of the integrator is given a shape that is geometrically similar to the DMD. The relay optical system is generally composed of a front lens unit disposed on the entrance side, a rear lens unit disposed on the exit side, and an aperture stop for obtaining an f-number identical with that of the projection optical system. The aperture stop is disposed in the optical path leading from the front lens unit to the rear lens unit, in the vicinity of the front lens unit.
In the above described illumination optical system provided with a prism, in which the illumination light is introduced from a direction nearly perpendicular to the DMD, the image formed on the exit surface of the integrator by the relay optical system is largely parallel to the DMD, and thus the illumination light has a substantially even intensity distribution on the DMD. Moreover, almost all the illumination light can be shone on the DMD, and thus the DMD can be illuminated brightly.
FIG. 12
schematically shows the optical arrangement of an image projection apparatus provided with an illumination optical system in which illumination light is introduced from a direction considerably inclined relative to the DMD. This image projection apparatus
5
is provided with an illumination optical system
6
, a DMD
40
, and a projection optical system
50
. The illumination optical system
6
is composed of a light source
10
, an integrator
20
, and a relay optical system
60
. The light source
10
is composed of a lamp
11
and a reflector
12
having the shape of an ellipsoid of revolution, with the lamp
11
disposed at the first focal point of the reflector
12
.
The integrator
20
is a rod having a rectangular cross section, and is arranged in such a way that the entrance-side end surface
21
thereof is disposed at the second focal point of the reflector
12
. The light emitted from the lamp
11
and reflected from the reflector
12
converges on the end surface
21
, and enters the integrator
20
. The light then, by being totally reflected from the side surfaces of the integrator
20
, reaches the exit-side end surface
22
thereof. The number of times that different portions of the light are totally reflected from the side surfaces of the integrator
20
depends on the angles of incidence at which they strike the end surface
21
. Thus, central and peripheral portions of the light mix on the exit-side end surface
22
, smoothing the intensity distribution of the light (making it even).
The relay optical system
60
is composed of a front lens unit
61
, a rear lens unit
62
, and an aperture stop
63
. The front and rear lens units
61
and
62
each include a rotationally symmetrical lens element only. The relay optical system
60
forms the image of the exit-side end surface
22
of the integrator
20
on a plane P perpendicular to the optical axis Ax of the relay optical system
60
.
The DMD
40
is arranged with its center located at the intersection between the optical axis Ax of the relay optical system
60
and the plane P on whi
Nakagawa Tomoko
Sawamura Shigeru
Takamoto Katsuhiro
Alavi Ali
Cariaso Alan
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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