Optics: image projectors – Distortion compensation
Reexamination Certificate
2001-01-12
2004-02-10
Dowling, William (Department: 2851)
Optics: image projectors
Distortion compensation
C353S081000
Reexamination Certificate
active
06688748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to projection systems, and more particularly to a novel off axis projection system including a de-centered field lens group.
2. Description of the Background Art
Reflective liquid crystal displays (LCDs) provide many advantages over transmissive LCDs, and are, therefore, becoming increasingly more popular for use in projection systems. For example, transmissive displays typically have a limited aperture ratio (i.e., the total area available for light to shine through a pixel) and require pixel fill to separate the pixels, resulting in a pixelated image. The limitations of transmissive displays pose formidable problems in building bright, high-resolution displays at a reasonable cost. Reflective LCDs, on the other hand, include an array of highly reflective mirrors manufactured on a standard processed CMOS silicon chip back plane driver, using sub-micron metalization processes recently developed by VLSI process engineers, and do not, therefore, suffer from the limitations of the transmissive displays.
Although superior to transmissive displays in brightness and resolution, reflective displays do pose additional system design problems. For example,
FIG. 1
shows a prior art, on-axis projector system
100
to include an illumination source
102
, a polarizing beam splitter
104
, a color separator
106
, a plurality of liquid crystal displays (LCDs)
108
(
r, g,
and
b
), and projection optics
110
. Illumination source
102
generates a source beam of white light and directs the source beam toward polarizing beam splitter
104
, which passes one portion of the source beam having a first polarity, and redirects another portion (an illumination beam) of the source beam having a second polarity along a system axis
112
, toward color separator
106
. Color separator
106
separates the illumination beam into its red, green, and blue components, and directs each of these colored illumination beams to a respective one of LCDs
108
(
r, g,
and
b
). Each of LCDs
108
(
r, g,
and
b
) is controlled by a system, e.g., a computer or other video signal source (not shown), and modulates the polarity of selective portions (i.e., pixels) of the colored illumination beams to form colored imaging beams, which are reflected back toward color separator
106
. Color separator
106
recombines the colored imaging beams to form a composite imaging beam and directs the composite imaging beam back along the optical axis
112
of projection optics
110
, toward polarizing beam splitter
104
, which passes only the modulated portions of the composite imaging beam to projection optics
110
. Projection optics
110
then focuses the modulated portions of the composite imaging beam onto a display surface (not shown).
Because the illumination beams and the imaging beams in system
100
both travel along the same path (i.e., axis
112
), projection system
100
is considered an “on-axis” system. On-axis projection systems generally require a polarizing beam splitter such as polarizing beam splitter
104
, and, therefore, suffer from the following limitations. First, polarizing beam splitters are highly angular sensitive. Second, polarizing beam splitter
104
must perform both the polarizing function and the analyzing function, and must, therefore, work well for both orthogonal states (S & P) of polarization, thus requiring undesirable manufacturing compromises. Furthermore, polarizing beam splitter
104
introduces a significant path length through glass, which can induce undesirable aberrations in the incident and imaging beams, due to stress induced birefringence. Finally, polarizing beam splitters are very expensive, compared to, for example, polymer based polarizing films.
FIG. 2
shows an off-axis projection system
200
that does not require a polarizing beam splitter. Projection system
200
includes an illumination source
202
, a condenser lens
204
, a polarizer
206
, a field lens
207
, a reflective LCD
208
, an analyzer
210
, and a projection lens group
212
. Illumination source
202
generates an illumination beam
214
that is focused by condenser lens
204
to pass through polarizer
206
, and impinge on LCD
208
at a non-perpendicular angle (non-zero angle of incidence). LCD
208
modulates illumination beam
214
to form an imaging beam
216
, and reflects imaging beam
216
toward projection lens group
212
. Field lens
207
is disposed adjacent reflective LCD
208
, and focuses the aperture stop (not shown) of illumination source
202
at a field stop (not shown) near the rear of projection lens group
212
. The angular separation between illumination beam
214
and imaging beam
216
allows for the separation of polarizer
206
and analyzer
210
.
Projection lens group
212
focuses imaging beam
216
to project a magnified image of LCD
208
on a display surface
220
. In a configuration such as system
200
, with a net average angle between LCD
208
and imaging beam
216
, projection lens group
212
would typically be used as shown (i.e., not symmetrical about its optical axis
218
) to avoid keystone distortion. Imaging beam
216
thus forms a non-zero angle with optical axis
218
of projection lens group
212
.
The complexity of projection lens group
212
depends on the amount of angular separation between its optical axis
218
and the axis of imaging beam
216
. In particular, for an angular separation between imaging beam
216
and optical axis
218
of projection lens group
212
adequate to permit a separate polarizer and analyzer (e.g., 12°), the total design field-of-view for the projection lens would be on the order of 30% larger than a similar on-axis system projecting a similar image on display surface
220
. The resulting projection lens group
212
would tend to have excessive distortion, would be more complex, and would be more expensive than that required for the similar on-axis system. For typical distortion limits of <0.25% in display applications, size benefits on the order of 30% reduction in track length can be achieved if the field-of-view is reduced.
What is needed, therefore, is a projection system, which allows the angular separation of the illumination beam and the imaging beam, without displacing and/or distorting the projected image, and without increasing the required field-of-view of the projection lens.
SUMMARY
The present invention overcomes the problems associated with the prior art by providing a novel system and method for using off axis illumination in a reflective projection system. The invention facilitates the angular separation of an illumination beam and an imaging beam, without displacing and/or distorting the projected image, and easing the design requirements for a projection lens group.
The projection system includes an illumination source for emitting an illumination beam, a reflective display device for modulating the illumination beam to form a reflected imaging beam, a projection lens group, and a field lens group. The field lens group is de-centered with respect to the optical axis of the projection lens group and is disposed to bend the illumination beam and the imaging beam. The field lens group redirects the illumination beam to illuminate the display device at a non-zero angle of incidence, and redirects the reflected imaging beam along an optical path parallel to the optical path of the projection lens group. In a particular embodiment the display device is disposed on the optical axis of the projection lens group. In a more particular embodiment, the redirected portion of the optical path of the reflected imaging beam is coincident with the optical axis of the projection lens group. In another particular embodiment, the display device is tilted with respect to the optical axis of the projection lens group to accommodate the tilt in the focal plane of the projection lens group caused by the redirection of the imaging beam.
In one embodiment, the field lens group includes a centered field lens and an optical wedge. In an alterna
Bone Matthew F.
Francis Melvin
Lewis Isabella T.
Aurora Systems, Inc.
Dowling William
Henneman & Saunders
Henneman, Jr. Larry E.
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