Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
2000-08-11
2002-10-29
Adams, Russell (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S031000, C353S081000, C353S102000, C349S005000, C349S061000, C359S242000
Reexamination Certificate
active
06471356
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
None
TECHNICAL FIELD
This invention relates to image display systems and, in particular, to an image projection system implemented with a reflective light modulator and having an optical system that reduces weight and packaging profile.
BACKGROUND OF THE INVENTION
The following description is presented with reference to an image projector implemented with a reflective light modulator of a digital micromirror device (“DMD”) type but is applicable also to image projectors implemented with other types of reflective light modulators. Image projectors currently implemented with DMDs require that the projector housing or DMD-illuminating light beam-directing optics contained within the projector housing be tilted at a 45 degree angle relative to a support table on which the image projector rests. This is done to cause the illuminating light to impinge on the DMD from either above or below its light reflecting surface and thereby provide a correct orientation of the DMD relative to a projection screen on which an image can be viewed. Inclining the projector or its components causes the projector to occupy an undesirably tall space when it is in use. Currently available single DMD projectors are taller than 10 cm in their operating positions. Using a tilting mechanism to thin the profile to less than 10 cm requires a tilting mechanism that raises the operating height by a corresponding amount.
FIGS. 1A
,
1
B,
1
C, and
1
D are respective isometric, frontal, side elevation, and top plan views of such a prior art image projector
10
, which includes a high power lamp
12
positioned at the focus of an elliptical reflector
14
to produce a high intensity illumination beam characterized by a principal ray
16
that propagates through a rotating color wheel disk
18
of a color wheel assembly
20
. Disk
18
includes at least three sectors, each tinted in a different one of three primary colors to provide a field sequential color image capability for image projector
10
. The illumination beam propagates through an integrator tunnel
22
to create at its output end a uniform illumination pattern that lens elements
24
,
26
, and
28
image onto a DMD
30
.
The illumination beam propagating from integrator tunnel
22
is directed by a mirror
32
that is inclined so that the illumination beam propagates upwardly at a 45 degree angle relative to the plane of the supporting table for image projector
10
and exits lens element
26
toward a prism assembly
40
. Prism assembly
40
is composed of prism components
42
and
44
that are spaced apart by an air space interface
46
. After reflection by mirror
32
, principal ray
16
of the illumination beam strikes a surface of lens element
28
.
An incident light beam derived from principal ray
16
propagates through prism component
42
and, by total internal reflection, reflects off of a surface
50
at air space interface
46
to form a reflected incident light beam. The reflected incident beam propagates through prism component
42
to strike DMD
30
. DMD
30
in its “on” light reflecting state (on-state) reflects an imaging light beam propagating normal to the plane of DMD
30
through prism component
42
and, without total internal reflection, through air space interface
46
into prism
44
to exit through an exit face
60
of prism component
44
. The imaging light beam that passes through exit face
60
is characterized by a principal ray
62
and propagates through a projection lens
64
to a projector screen (not shown) to display an image to a viewer. DMD
30
in its “off” light reflecting state (off-state) reflects light by total internal reflection off of a face
68
of prism component
44
.
The angles of the faces and the shapes of prism components
42
and
44
are selected so that the incident light beam, reflected incident light beam, and imaging light beam propagating within prism assembly
40
are coplanar. The arrangement of the components of image projector
10
results in the upward inclination of prism assembly
40
and thereby dictates for a housing (not shown) of projector
10
a minimum height that is greater than a minimum height that would be possible with an uninclined prism assembly and principal rays
16
and
62
propagating along essentially the same vector.
Reducing its height is one step in achieving true portability for image projector
10
. Reducing its weight (mass) would be another beneficial step toward true portability. The image projector market is demanding projectors that can be carried along with a companion laptop computer in a briefcase.
SUMMARY OF THE INVENTION
An object of this invention is, therefore, to provide a truly portable image projector apparatus.
Another object of this invention is to provide an image projector weighing less than about 2.27 kilograms (five pounds) and having a height less than about 6.35 centimeters (2.5 inches).
A further object of this invention is to provide an image projection optical system suitable for use with a light-weight and compact projection lens.
This invention is suitable for use in an image projector employing a reflective light modulator, such as a DMD, and a prism assembly that illuminates the DMD and receives a reflected imaging light bundle for directing toward a projection lens.
The prism assembly includes compensating and output prism components having opposed surfaces separated by an air gap. The prism assembly sets up a correct illumination angle on the DMD and then separates incident illumination light from reflected imaging light by total internal reflection discrimination. The illumination light enters the prism assembly and reflects by total internal reflection off a top surface of the compensating prism component. The top surface has a compound angle that directs the light toward the DMD at the correct angle for illumination. For each micromechanical mirror of the DMD in its on-state, the illumination light reflects nearly normal to the horizontal datum plane to form imaging light that reenters the prism assembly through the output prism component. Because the angle of incidence at the air gap is greater than the critical angle, the imaging light reflects off the air gap and propagates through the output prism component.
A focusing lens of this invention is disposed between the prism assembly and the DMD to refract all the reflected imaging light into a converging imaging light bundle as it propagates into a small diameter light entry pupil of a light-weight, compact projection lens.
In an alternative embodiment, the focusing lens is implemented as a curved surface on the side of the prism assembly facing the reflective light modulator. This embodiment reduces weight, complexity, and space, but adds to the cost and complexity of the prism assembly.
An advantage of the focusing lens of this invention is that the projection lens requires a light entry pupil having a diameter that is only 30 to 50 percent the diameter of prior art entry pupils.
Another advantage of the focusing lens of this invention is that the projection lens has a mass and a length that is about 50 to 75 percent of prior art projection lenses.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.
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patent: 6155687 (2000-12-01), Peterson
patent: 6179424 (2001-01-01), Sawamura
patent: 6264331 (2001-07-01), Sawai et al.
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“The Computer Videomaker Handbook” 2ndEdition , Chapter 1, “All About Lenses” by Jim Stinson, pp. 3-11, copyright 2001 by Butterworth-Hei
Gohman Jeffrey A.
Peterson Mark
Adams Russell
InFocus Corporation
Koval Melissa J
Stoel Rives LLP
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