Optical: systems and elements – Single channel simultaneously to or from plural channels – By refraction at beam splitting or combining surface
Reexamination Certificate
1999-09-22
2001-02-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By refraction at beam splitting or combining surface
C359S649000, C353S081000
Reexamination Certificate
active
06185047
ABSTRACT:
TECHNICAL FIELD
This invention relates to image display systems and, in particular, to an image projection system implemented with a reflective light element and packaged to operate lying flat with very low profile on a support table.
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. With reference to
FIGS. 1A
,
1
B,
1
C, and
1
D, a prior art image projector
10
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.
SUMMARY OF THE INVENTION
The invention is an image projection system implemented with a projector engine using a reflective light modulator, preferably a Digital Micromirror Device (DMD), and operating lying flat with very low profile on a support table. The invention overcomes the above-described disadvantage of previous DMD projectors that require either tilting all or part of the projection system 45 degrees relative to a support table top or packaging the projection system in a thick box that allows light to impinge on the DMD from above or below its light reflecting surface. This is accomplished with a prism assembly that sets up the correct illumination angles for the DMD and directs imaging (output) light along approximately the same vector as that of illumination (input) light incident to the prism assembly.
The prism assembly includes compensating and output prism components having opposed surfaces separated by a light beam separation boundary, which is preferably an air space. The prism assembly sets up a correct illumination angle on the DMD and then separates illumination light from imaging light by total internal reflection discrimination. In a preferred embodiment, illumination light travels upwardly at 8 degrees relative to the surface of a support table (hereafter referred to as the horizontal datum plane) and in a direction such that its projection onto the horizontal datum plane is parallel to the projection of the optical axis of a projection lens that receives light exiting the prism assembly. 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 relative to the three-dimensional DMD coordinate system a compound angle that directs the light toward the DMD at the correct angle for illumination. In a preferred embodiment, the angle of this first reflected light beam is tilted 24 degrees (16 degrees in the prism glass) from the normal of the horizontal datum plane and is less than the critical angle of the glass from which the first prism component is formed at the air gap interface surface. The projection of this first reflected light beam onto the horizontal datum plane is rotated 40 degrees from the projection of the optic axis of the projection lens onto the same horizontal datum plane. The light passes, therefore, through the air space between the first and second prism components. For each micromechanical mirror of the DMD in its on-state, the illumination light reflects at 4 degrees from the normal of the horizontal datum plane to form imaging light, the projection of which onto the horizontal datum plane is parallel to the projection of the optical axis of the projection lens. The imaging light reenters the prism assembly through the output prism component. Because the angle of incidence at the air gap interface surface is greater than the critical angle, the imaging light reflects off the air gap and propagates through the output prism component. The imaging light exits the prism assembly, traveling upwardly at +4 degrees from the horizontal datum plane toward a projection lens. The illumination light and imaging light do not propagate along a common plane within the prism assembly, but the vectors of the illumination light entering and the imaging light exiting the prism assembly are approximately the same.
The DMD in a preferred implementation is positioned face up and, therefore, can be mounted on one printed circuit board that covers the interior bottom of the projection system. This arrangement is less expensive than the alternative of using a high-density connector at right angles to the DMD control electronics for the printed circuit
Engle Scott
Peterson Mark
Potekev Franc
Epps Georgia
InFocus Corporation
Schwartz Jordan M.
Stoel Rives LLP
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