Television – Video display – Cathode-ray tube
Reexamination Certificate
1999-01-14
2001-01-16
Hsia, Sherrie (Department: 2714)
Television
Video display
Cathode-ray tube
C348S782000, C348S816000, C348S809000
Reexamination Certificate
active
06175392
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to projection display technology and to a system for improving the quality of a color image projected by a projection display.
BACKGROUND OF THE INVENTION
In a projection display, a full color image on the screen is achieved by overlaying separate images containing red, green and blue components. These component images are projected by a common projection lens onto a distant screen for viewing. As described in the reference “Applied Photographic Optics,” pages 103-111, Sidney F. Ray, Focal Press, 1994, it is a feature of glass that its refractive index changes with the wavelength of light, and a lens may, therefore, have a different magnification for red, green and blue images. Therefore, it is commonly seen that the red, green and blue component image do not exactly overlap especially at the edges of the screen. This is a serious problem for some applications and it is necessary to construct a projection lens of considerable complexity to attempt to remedy this (color convergence) problem.
To minimize this effect, a lens may consist of a number of elements employing different glasses so as to color correct the lens at two or more specific wavelengths. As displays of higher resolution are developed, the demands on the projection lens become more severe.
In applications such as rear projection monitor or TV, the viewer may approach closely to the screen and misconvergence detracts noticeably from the quality of the image. In electronic theater applications, misconvergence at the edges of the screen is objectionable since some viewers may sit significantly to the side of the screen.
FIG. 1
illustrates the imperfect overlapping of, for example, red and blue images at a screen. The dots may be made to overlap perfectly at the center of the screen by adjustment of the position of the objects being imaged, however, there is a displacement (misconvergence) between the different colored dots toward the edge of the screen.
FIG. 2
shows a highly simplified projection system
50
where a target or panel A,
100
, is imaged in red light by a projection lens
200
onto a distant screen
300
. The outline of the imaging panel
100
is shown as a square
400
on the screen. A second panel B,
140
, is imaged in blue light via a dichroic mirror
180
that reflects blue light, but is transparent to red light. In this way, an image
440
of the panel is produced on the screen in proximity to the image
400
.
In a projector, the panels
100
,
140
are variously described as light valves or spatial light modulators. The panel may be transmissive in type, that is, light is transmitted through the panel or reflective in type where light is reflected from the panel. The light valves are designed to produce a pattern or image within the light valves from an electronic or other source, that is small in size, e.g., one inch in width. This image is projected onto a distant screen
300
with a magnification of order 20 to 200 times or more.
It can be seen that in the example shown in
FIG. 2
, the size of the images
400
and
440
are slightly different due to the variation in lens strength with color of light. It is the case that the magnifications cannot be equalized by movement of one of the panels
100
or
140
since there is only one position of correct focus for the panels. To vary the magnification, it is necessary to vary the distance from lens
200
to the screen
300
separately for the two panels, however this is not possible. In a projector, red, green and blue images are required and the optical system may include more than two panels that are imaged onto the screen.
It would be highly desirable to provide an apparatus for improving the color convergence of projection display systems that is simple and does not require modification of the projection optical system.
SUMMARY OF THE INVENTION
The present invention is a system and method for improving color convergence in projection display systems. The system employs one or more mirror units disposed for reflecting light from a projection lens to a display screen, with each mirror unit including one or more reflective layers adjustably positioned for changing the path length of selected one or more color components of a color image to be displayed. Changing the path length of the selected one or more color components of the color image effectively changes the magnification of those selected color components according to:
δ
v
v
=
δ
m
m
where v is path length traversal of the light from the projection lens to the display screen, &dgr;v is change in path length effected by the positioning of the reflective layers, m is the magnification factor, and &dgr;m is the change in magnification factor of a color component required to correct color convergence of a displayed image. Once a desired change in path length is calculated to adjust magnification of the color component to improve color convergence, the relative spacing of the one or more reflective layers in each mirror unit is determined.
It is clear that the deflection of rays by means of one or more plane (i.e., flat) mirrors does not introduce distortion of projected images, but serves only to redirect the rays from the projection lens.
Advantageously, the incorporation of the special mirror device does not require modification of the projection optical/display system. That is, for small corrections to the magnification of the different colored images, the deforming effect due to change in distance to the screen is not objectionable. However, for highly critical applications as, for example, in an electronic cinema installation, the imaging panel in a projector may be moved slightly to achieve correct magnification and the sharpest image.
REFERENCES:
patent: 2493200 (1950-01-01), Land
patent: 4621293 (1986-11-01), Matis
patent: 5161042 (1992-11-01), Hamada
patent: 5467154 (1995-11-01), Gale
patent: 5499061 (1996-03-01), Oku
patent: 5548349 (1996-08-01), Mizuguchi
David M. Shofi, Esq.
Hsia Sherrie
International Business Machines - Corporation
Scully Scott Murphy & Presser
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