Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2000-10-19
2002-02-19
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S633000
Reexamination Certificate
active
06349006
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a prism, projection optical system using the prism, and projection display device using the projection optical system.
2. Description of the Background Art
Projectors (projection display devices) are now arousing interest as a large screen display. For instance, CRT projectors using a compact CRT of high definition and high intensity, liquid crystal projectors using a liquid crystal panel, and DMD (Digital Micromirror Device) projectors using a DMD, have been commercialized.
Further, the category called “data projector” which not only responds to AV sources such as movies and TV programs, but also projects computer images, is extending rapidly in the market. Its noticeable performance improvements have been made for increasing brightness and contrast of projected image planes, as well as resolution, and uniformity in brightness.
Of these, uniformity in brightness is being one of the most basic requirements as the market of data projectors extends. For instance, in liquid crystal projectors, uniform illuminating technologies such as fly eye integrators have been introduced to attain the compatibility with improvement in brightness.
Meanwhile, light valves such as LCDs and DMDs can be broadly divided into two classes: transmission types and reflection types. The former has a feature that an illuminating optical axis for illuminating a light valve and the optical axis of a projection lens can be coaxially disposed with ease.
It is therefore relatively easy to design an illuminating optical system. This is advantageous in ensuring a basic performance of illuminating the light valve uniformly and brightly.
On the other hand, the latter is inherently disadvantageous because it is often difficult to coaxially dispose an illuminating optical axis and the optical axis of a projection lens, which involves configuration of a complex optical system.
One such illuminating optical system of the reflection type light valve is described in detail in A. G. Dewey, “Projection Systems for Light Valves,” Proc. SID, vol. 18/2, pp.134-146, 1977.
FIGS. 28
to
31
are schematic diagrams of illuminating optical systems which are proposed as an illuminating system for reflection type light valve, in the above literature.
Conventional Technique I
FIG. 28
illustrates a typical representative of off-axis illuminating optical systems, which is characterized by illuminating a light valve from a direction that departs from its normal.
In
FIG. 28
, reference numeral
300
denotes a light valve, numeral
800
denotes a light source, numeral
801
denotes a condenser lens, numeral
802
denotes the final lens of a projection lens on the side on which the light valve
300
is disposed, and numeral
810
denotes an image of the light source
800
. Numeral
600
denotes the normal of the light valve
300
, and numeral
601
denotes an optical axis of the projection lens.
A bundle of illuminating rays indicated schematically by solid lines in
FIG. 28
exits from the light source
800
, and is then condensed by the condenser lens
801
and enters the light valve
300
. This bundle of rays has the smallest diameter immediately before it enters the final lens
802
, thereby forming the image
810
of the light source
800
.
Accordingly, the projection lens takes an arrangement of a post-stop lens in which the stop is disposed in the vicinity of the final lens
802
. The optical axis
601
of the projection lens is parallel to the normal
600
of the light valve
300
, however, since these
600
and
601
are not coaxial, the travel direction of the projected light is inclined with respect to the normal
600
, as indicated by the dotted arrow.
In addition, it is suited to avoid the physical impingement between the projecting lens and the illuminating optical system or the bundle of illuminating rays, because the diameter of the final lens
802
can be reduced depending on the post-stop type.
Conventional Technique II
FIG. 29
is a schematic diagram of an off-axis illuminating optical system different from that in FIG.
28
. The references used for
FIG. 28
have been retained for similar parts in
FIG. 29
, and description thereof is thus omitted.
In the point that the stop of a projecting lens is disposed within a projection lens system, this optical system has a high possibility that it will be configured by one lens type, the design of which is easier than that of the system in FIG.
28
.
Thus, the off-axis optical system as disclosed in Conventional Techniques I or II is excellent in minimizing a focal shift in image planes, because even when projection is made in such an attitude of looking up at the screen, the optical system is less susceptible to keystone distortion (trapezoidal distortion) by which an image plane can be distorted in a trapezoid. That is, it can be said that each optical system is suitable for the front projection type projector which has a high necessity for providing a predetermined elevation angle in the projection direction with respect to the axis of the projector (i.e., which is usually the axis along the vertical direction). In contrast, with either of the optical systems, it is inherently difficult to increase uniformity in illumination because the light valve is illuminated obliquely.
Conventional Technique III
The aforesaid literature further proposes the type in which a prism as shown in
FIG. 30
is inserted, and the type in which a reflection mirror is disposed within a bundle of rays as shown in FIG.
31
. In
FIG. 30
, reference numeral
803
denotes a prism, numeral
602
denotes an illuminating light ray, numeral
603
denotes a light ray in the direction of the normal of a light valve
300
, which ray is diffracted due to insertion of the prism
803
, and numeral
604
illustrates a projected light ray schematically.
It is described that this system can control to some extent the angle formed by the illuminating light ray and projected light ray, thereby increasing the degree of freedom of the optical system's configuration, whereas this system exerts a great influence on the astigmatism and chromatic aberration of the projection lens, resulting in poor practicability.
Conventional Technique IV
FIG. 31
illustrates a self convergent relay optical system in which with a mirror disposed at the position of a stop in a projection lens, an image of a light source to be formed on the stop is reflected by a light valve
300
, and the image is formed again in the vicinity of the mirror.
Compared to the three illuminating optical systems in the foregoing Conventional Techniques I to III, this system is superior in the prevention of distortion of projected images and in illumination performance. However, the following drawback is pointed out. Specifically, when an illuminating light ray reflected by the mirror enters the lens
802
, a reflected light occurs on the surface of the lens
802
, and this reflected light becomes a ghost light and reaches the screen.
Like this example, if the lens
802
is disposed in the vicinity of the reflection type light valve
300
, a sufficient consideration should be given to the influence which can be caused by light passing through the lens
802
two times during its going and returning.
Any of the four illuminating optical systems thus discussed briefly in the foregoing Conventional Techniques I to IV, has difficulty in attaining both the uniform illumination to the light valve and the prevention of impingement between the illuminating optical system and projection lens system.
Note that one optical system which overcomes the above-mentioned drawbacks and is suitable for illuminating variable mirror elements, e.g., DMDs, is disclosed in U.S. Pat. No. 5,604,624.
Conventional Technique V
FIG. 32
is a longitudinal section of a conventional reflection-type light valve illuminating optical system disclosed in U.S. Pat. No. 5,604,624.
In
FIG. 32
, reference numeral
140
denotes a prism, numerals
141
and
142
denote the side surfaces of the prism
140
, n
Okamori Shinji
Shikama Shinsuke
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