Optical: systems and elements – Lens – Reverse telephoto
Reexamination Certificate
2001-08-22
2003-06-10
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Reverse telephoto
C359S663000, C359S651000
Reexamination Certificate
active
06577455
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a constitution of a wide angle projection lens having a large image circle, which is mounted in a projection display device using a light valve, and a projection display device mounted with this wide angle projection lens.
2. Description of the Background Art
FIG. 17
is an illustration showing an eccentric projection optical system in a projection display device using a background-art reflection light valve. This figure shows an illumination luminous flux
91
illuminating a reflection light valve
6
, a reflection luminous flux
92
reflected by the reflection light valve
6
and entering a projection lens
8
and an optical axis
93
of the projection lens
8
.
The illumination light
91
is irradiated diagonally with respect to the reflection light valve
6
, and the luminous flux
92
reflected by the reflection light valve
6
, being two-dimensionally modulated in a surface of the reflection light valve
6
, is enlargedly projected on a not-shown screen by the projection lens
8
.
A display area of the reflection light valve
6
is perpendicular to the optical axis
93
of the projection lens
8
and disposed eccentrically with its center point displaced downward with respect to the lens optical axis
93
. This is intended to avoid physical interference between an illumination optical system generating the illumination light
91
and the projection lens
8
, prevent generation of eclipse of the illumination luminous flux or projection luminous flux and obtain a uniform projection image.
In order to dispose the reflection light valve
6
eccentrically with respect to the projection lens
8
, as discussed above, it is necessary to enlarge the specifications on the image circle of the projection lens
8
(an area where the aberration of the projection lens
8
is well corrected). This situation will be discussed, referring to
FIGS. 18A
,
18
B,
19
A and
19
B.
FIGS. 18A and 18B
show image circles IMC of the projection lens
8
projecting the light valve
6
with aspect ratio of 4:3. Specifically,
FIG. 18A
shows a case where the center point of the light valve
6
coincides with the center point of the image circle IMC (on-axis projection) and
FIG. 18B
shows a case where the center point of the upper long side of the light valve
6
coincides with the center point of the image circle IMC (off-axis projection). It can be seen from these figures that the diameter of the image circle in the on-axis projection is 5 while that of the image circle in the off-axis projection is 7.2, being larger by 44%.
FIGS. 19A and 19B
show image circles IMC of the projection lens
8
projecting the light valve
6
with aspect ratio of 16:9. Specifically,
FIG. 19A
shows a case where the center point of the light valve
6
coincides with the center point of the image circle IMC (on-axis projection) and
FIG. 19B
shows a case where the center point of the upper long side of the light valve
6
coincides with the center point of the image circle IMC (off-axis projection). It can be seen from these figures that the diameter of the image circle in the on-axis projection is 18.36 while that of the image circle in the off-axis projection is 24.08, being larger by about 31%.
Further, it is understood from the above examples that the diameter of the image circle becomes larger when the degree of eccentricity of the light valve
6
is larger.
Since the off-axis projection system has a simple constitution and a projection light corresponding to the center point of the screen can go out, being tilted upwards with respect to the optical axis
93
of the projection lens, there is a case where the off-axis projection system is adopted in a small-size front projection type projector. In this system, the image circle of the projection lens
8
becomes larger than the diagonal size of the light valve
6
. This makes it difficult to realize both reduction in focal length required for size-reduction of a body incorporating the whole optical system and excellent imaging, especially in a case of applying the off-axis projection system to a rear projection type projector.
As a background-art example of projection lens for DMD (Digital Micromirror Device) projection, for instance, one disclosed in Japanese Patent Application Laid Open Gazette No. 2000-275729 is well known. This background-art example proposes a constitution of a projection lens with half angle of projection view &ohgr;=27.3 degrees at F/3 with respect to the tilt angle of ±10 degrees of micromirrors constituting the DMD. This also discloses a lens constitution intended for reduction in interference between the illumination luminous flux and the projection luminous flux by disposing the stop of the whole lens system on the DMD side. Since an element having micromirrors with the tilt angle of ±12 degrees is being developed through the research and development of DMD, however, the specification of about F/2.4 is required for a projection lens in order to achieve a high-intensity projector while ensuring sufficient projection luminous flux. Further, the half angle of projection view of 27.3 degrees in the above example, which makes the projection distance with respect to a predetermined screen size too large in a rear projection type projector, has a problem in being used for a small-size rear projection type projector. It is desirable to ensure a half angle of view of 40 degrees or more for a projection lens.
Items of basic specification required for a projection lens of off-axis projection system used in a rear projection type display device are as follows.
(1) Wide angle of view: a projection lens with short focal length is needed in order to mount an optical system in a small-size body with the projection distance shortened. As discussed above, it is desirable that the half angle of view &ohgr; of lens>40 degrees at F/2.4.
(2) Small diameter of a lens on the incident side: it is desired that the diameter of a projection lens on the incident side of the luminous flux (light valve side) should be as small as possible in order to avoid luminous flux eclipse caused by interference between the illumination optical system and the projection optical system.
(3) Long back focal length: combined with the requirement (2), it is required that the back focal length on the light valve side should be set longer in order to avoid luminous flux eclipse caused by the interference between the illumination optical system and the projection optical system.
(4) Large image circle: required is a lens system with aberration well corrected in a large image circle in order to be used for the off-axis projection.
In the rear projection type display device required is a projection lens further having the following fundamental properties besides satisfying the above basic specifications relating to (1) wide angle of view, (2) small diameter of a lens on the incident side, (3) long back focal length and (4) large image circle.
(a) Low chromatic aberration: it is needed to control the chromatic difference of magnification, typically to about pixel-pitch level, or desirably to half the pixel-pitch level or lower in order to sufficiently reduce the difference in projection magnification among primary color pixels of a projection image, and it is further needed to control the longitudinal chromatic aberration to be smaller so that the focuses of primary colors may coincide with one another.
(b) Low distortion aberration: it is required that a projection lens should be designed to control the displacement of pixel off from an ideal point due to distortion aberration typically to about 1% or lower since distortions at peripheral portions in a screen are visible as a rectangular projection image is projected inside a frame of a projection screen in a rear projection type projector.
(c) High resolution: in order to enlargedly project an original image of a high-density and multipixel light valve element having million-level pixels which has been increasingly developed
Birch Stewart Kolasch & Birch, LLP.
Epps Georgia
Mitsubishi Denki & Kabushiki Kaisha
Spector David N.
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