TIR prism for DMD projector

Details TIR prism for DMD projector TIR prism for DMD projector

2002-11-22

2004-04-27

Adams, Russell (Department: 2851)

Optics: image projectors

Composite projected image

Multicolor picture

C353S034000, C353S037000, C353S038000, C353S047000, C353S050000, C353S081000, C359S460000, C359S449000, C348S771000, C362S268000, C362S290000, C362S337000, C362S339000

Reexamination Certificate

active

06726332

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to micro-display projection systems and, in particular, to micro-display projection systems that use a digital micromirror device (DMD) and a TIR prism.
BACKGROUND OF THE INVENTION
A. Prior Art Projection Displays Employing DMDs
As known in the art, a digital micromirror device (DMD) comprises a panel which selectively reflects illumination light to produce image light, said panel comprising a plurality of selectively adjustable reflecting elements arranged in a common plane, said elements being adjustable between at least a first position and a second position.
A typical projection display using a DMD (e.g., a DMD from TEXAS INSTRUMENTS) has illumination from a light source (e.g., a high pressure mercury arc lamp), a color wheel for field sequential color, and an illumination path that has an integrator and relay optics. The illumination light from the light source strikes the imager display and is modulated by the micro-mirrors at each pixel. Each flipping mirror can direct the illumination reflected from its surface so that it goes towards the projection lens and screen or off into a reject state where it is blocked from getting to the screen.
There are a number of ways to keep the incident illumination bundle of light separated from the outgoing imaging light, i.e., the light that gets to the screen. The first uses physical separation of the illumination bundle and the imaging bundle. The pupil in the projection lens is then located so as to accept light from pixels that are turned “on” and reject light from any other direction.
The second method uses a TIR prism to separate the illumination light from the imaging light reflected from the DMD imager. A TIR prism has a face that is close to the critical angle of reflection, i.e., it has a face at which light at some angles undergoes total internal reflection and light at other angles passes through the face. The original disclosure of the use of a TIR prism with a DMD imager was in commonly-assigned U.S. Pat. No. 5,552,922 to Simon Magarill. Other patents in which TIR prisms are used with DMD imagers include: Magarill, U.S. Pat. No. 5,604,624; Peterson et al., U.S. Pat. No. 6,185,047; Poradish et al., U.S. Pat. No. 6,249,387; Fielding et al., U.S. Pat. No. 6,250,763; Okamori et al., U.S. Pat. No. 6,349,006; and Magarill, U.S. Pat. No. 6,461,000.
In what has now become the standard configuration for TIR prisms used with DMD imagers, the illumination light comes in at an angle that “totally reflects”, while in the imaging path, the light that goes to the screen passes through the TIR surface without any reflections. Again, the pupil in the projection lens is located so as to accept light from pixels that are turned “on” and reject light, if any, from other directions.
FIGS. 1A and 1B
are side and front views, respectively, showing a DMD projection system
9
inside a rear-projection cabinet
11
.
FIG. 2
is an isometric close-up of the same elements, without the cabinet. These two figures represent the elements that one would find in the cabinet of a rear projection television which uses a DMD and a TIR prism assembly typical of the prior art.
As shown in these figures, prism assembly
13
sits in front of DMD imager
15
. Projection lens
17
selects the imaging light and sends it to fold mirrors
19
,
21
and then to screen
23
. All the parts of a prior art Illumination system can be seen in the figures, namely, a light source
25
(e.g., a high pressure mercury arc lamp), a color wheel
27
with motor
33
, an integrating tunnel
29
, relay lenses
31
, a fold mirror
35
, and a further relay lens
37
.
B. Operation and Orientation of Illumination Light, Imaging Light, and TIR Surface in the Prior Art
On all DMD imagers produced to date, the square pixel micro-mirrors tilt about their corners. This means that the illumination light needs to come at the device at 45 degrees to the device's horizontal (see discussion below).
FIG. 3
is a front view of a DMD imager which illustrates how the 45 degree requirement has been achieved in the prior art. In this figure,
39
is a single micro-mirror pixel,
41
shows the mirror flipping axis (axis of motion),
43
illustrates the TIR surface reflection,
45
shows the illumination direction,
47
shows an “undesirable” light source orientation (see discussion below),
49
shows the illumination-path fold mirror used in the prior art, and
51
shows the “desirable” light source orientation achieved in the prior art through the use of the illumination-path fold mirror (see discussion below).
To interact properly with the micro-mirrors, the illumination light has to come in perpendicular to flipping axis
41
. As a result, illumination direction
45
ends up being at 45° to the horizontal axis
53
of the imager
15
, i.e., the horizontal axis defined by the imager's horizontal edges. In a system that uses a TIR prism, the air gap surface is inclined across this same imager axis.
Without fold mirror
49
, the illumination axis
45
will go all the way back to the start of the illumination, i.e., to light source
47
in its “undesirable” orientation in FIG.
3
. For many light sources and, in particular, for light sources which employ an arc burner (e.g., high pressure mercury arc lamps), operation of the source is impaired if the arc burner is not substantially horizontal. It is for this reasons that the orientation of light source
47
in
FIG. 3
is considered “undesirable.”
As shown in
FIG. 3
, in the prior art, the need for a substantially horizontal orientation for the light source's axis was achieved by the use of an illumination-path fold mirror
49
. This mirror allowed the light source
51
to have an orientation such that the angle of the source's axis relative to the horizontal axis
53
of imager
15
was acceptable.
FIG. 4
is a further illustration of the use of TIR prisms in the prior art, in this case, an isometric view of just one of the prisms
13
a
making up prism assembly
13
, with DMD imager
15
being shown below the prism in this figure. In actual use, the DMD imager
15
and the prism assembly will typically be located vertically.
FIG. 4
shows the illumination axis
55
coming into the prism
13
a
and reflecting down to the device at the TIR surface
57
. For illustration, a projection
59
of the illumination axis on the plane of the device is shown as a dashed line in FIG.
4
. Note that that the projection is at 45 degrees to the horizontal axis
53
of the imager.
FIG. 4
also shows flipping axis
41
which again is at 45 degrees to the horizontal axis
53
of the imager. If the positive direction of the horizontal axis is to the right in
FIG. 4
, then the flipping axis is at +45°, while the projection
59
of the illumination axis is at −45°.
FIGS. 5A
,
5
B, and
5
C illustrate light paths through various TIR prism assemblies used in the prior art, where in each figure a single micro-mirror
67
in its “on” state (+10 degrees) is shown.
FIG. 5A
shows the case where illumination light
61
reflects from TIR surface
57
and imaging light
63
passes through the TIR surface. For a prism composed of acrylic (n=1.493) and for a DMD device with a 10 degree mirror tilt and a F/3.0 light cone, the prism angle is about 35 degrees.
FIG. 5B
shows the case where illumination light
61
passes through TIR surface
57
and imaging light
63
reflects from the TIR surface. The mirrors of the imaging device operate in the same manner for this approach as for the approach of FIG.
5
A. Thus, the illumination still has to come in at an angle for the tilted “on-state” mirrors and the imaging (on-state) light still comes straight up off the device (normal to the device plane). To satisfy these conditions, the prisms making up the prism assembly have different configurations for the
FIG. 5B
approach than for the
FIG. 5A
approach.
In particular, for the
FIG. 5B
approach, the prism angle is different from that of the
FIG. 5A
approach because the imaging light coming st

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