Optical: systems and elements – Glare or unwanted light reduction – With absorption means
Reexamination Certificate
2000-05-19
2001-05-15
Sikder, Mohammad Y. (Department: 2872)
Optical: systems and elements
Glare or unwanted light reduction
With absorption means
C359S603000, C359S884000
Reexamination Certificate
active
06231195
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optical systems, particularly imaging and projection systems utilizing digital micromirror arrays, and particularly to improving the light throughput and/or efficiency of such DMA-based optical systems.
BACKGROUND OF THE INVENTION
Digital Micromirror Array (DMA) technology comprises an array of individual moveable micromirrors over memory cells of a CMOS static RAM, for example the digital micromirror device (DMD) produced by Texas instruments. Electrostatic forces based on the data in each memory cell tilt each individual micromirror on torsional hinges, typically by and angle of ±10 degrees from the plane of the overall DMA. The individual micromirrors are switched between the “on” and “off” state, while DMA components other than the mirrors can be found in a “flat” orientation with no tilt.
In DMA-based optical systems and devices (e.g., light projectors) the ±10 degrees (total 20°) tilt of the micromirrors is the factor limiting the maximum aperture of the objective lens projecting the light reflected from the DMA surface and imaging it on a screen. According to the prior art shown in
FIG. 1
, incident light
100
is passed through an illuminating lens
103
and focused on a digital micromirror array (DMA)
102
. It is then reflected depending on the positions of the micromirrors.
Normally, desired light reflected from micromirrors in the “on” position, i.e., the “on” reflected light cone
104
, is collected by an objective lens
106
. The maximum vertex (and consequent width) of the “on” reflected light cone
104
, corresponds to the objective lens
106
aperture, which is typically F/2.8. It is noted that the F/# is an optical term known in the art and defined as the focal length of the lens divided by the diameter of the lens. The “speed” of a given lens refers to the F/#. In general, the “faster” the lens, the lower the F/#.
Similarly, undesired light reflected away from the objective lens
106
from micromirrors in the “off” position, forms the “off” reflected light cone
110
. Undesired light reflected by “flat” components of the DMA (i.e., those components of the DMA other than the “on” and “off” micromirrors) form the “flat” reflected light cone
108
. These undesired “flat” and “off” light cones
108
and
110
are diverted away from objective lens
106
to a light absorber
112
.
FIG. 1
shows the illuminating light
100
and all possible reflected light cones (
104
,
110
,
108
) based on the mirrors in the on and off positions, as well as contributed by reflections from flat components of the DMA.
Prior art imaging systems such as illustrated in
FIG. 1
typically are optimized for a standard 10° tilt of the micromirror by using an objective lens
106
with an aperture of F/2.8 since at this aperture the objective lens
106
collects only the “on” reflected light cone
104
and there is no overlap with undesired light in either the “flat” or “off” light cones. It is to be noted that
FIG. 1
is a schematic representation insofar as the light cone vertex angles are illustrated to be larger than 20° simply to facilitate drawing clarity and emphasis. In reality, these vertex angles are in fact approximately 20°. Each light cone then emerges tilted from the adjacent light cone by approximately 20° so that the light cones emanate from DMA
102
as closely as possible to one another without overlap, as shown. Thus, there is a 20° rotation of the “flat” light cone relative to its adjacent “on” light cone, and further of the “off” light cone relative to its adjacent “flat” light cone.
To understand this fully, it is important to note that for a given degree of tilt of one mirror relative to another, the reflected light beams from a common incident light source will differ in angle from one another by twice this degree of tilt (i.e., the reflection angle is doubled), as can be seen by considering basic optical principles of light reflection. The 20° vertex of each light cone results from choosing illuminating lens
103
so as to focus the incident light
100
on DMA
102
with a similar 20° vertex; the 20° tilting of each light cone relative to its adjacent cone results from the 10° difference between the “on” micromirrors and the “flat” DMA components, and between the “flat” DMA components and the “off” micromirrors, which difference becomes doubled upon the reflection of the incident light
100
.
Therefore, typical prior art DMA-based systems, with a representative 10° micromirror tilt producing a 20° angular reflection between adjacent light cones, are limited to using an objective lens aperture no better than F/2.8. Using a faster lens with a larger aperture in combination with using a wider vertex angle for all of the reflected light cones would produce overlap between the light cones, and thereby degrade the image produced since the lens would capture light from the undesired light cones and the resultant image would include artifacts from the undesired overlapping light cones. The prior art is thus lacking a means of benefiting from the use of an objective lens faster and wider than that which corresponds to no overlap between reflected light cones, without introducing image degrading artifacts that essentially negate the improvements normally gained by increasing the lens aperture in a projection system.
More generally, a micromirror
102
with any given degree of tilt imposes a corresponding limiting objective lens
106
aperture based on the requirement that the undesired “flat”
108
and “off”
110
reflected light cones do not overlap with the desired “on” reflected light cone
104
, since the latter fills the area collected by objective lens
106
.
OBJECTS OF THE INVENTION
It is therefore desirable to provide an improved DMA-based imaging or projection system than can use a larger aperture objective lens without the negative limitations found in the prior art.
It is also desirable to provide an improved DMA-based imaging or projection system for enhanced light efficiency.
SUMMARY OF THE INVENTION
Several embodiments of the invention disclosed and claimed herein comprise a light-absorbing mask preventing light from at least one overlap region where a desired light cone intersects with at least one undesired light cone from passing through an objective lens of an optical system. These embodiments also further allow substantially all remaining light from the desired light cone which does not intersect with any of the undesired light cones to pass through the objective lens. By eliminating light collected from “off” state micromirrors and from flat components of the DMA, degradation of image quality is avoided. Further, the masked objective lens, with a larger aperture, enhances the light collection from the desired light cone.
These and equivalent embodiments of the present invention can be utilized in all DMA-based optical systems, including imaging and projection devices, in order to improve light throughput and system efficiency by a significant factor. This feature is especially important in night vision systems and other low light applications. It is also of value in light projectors and other DMA-based imaging devices.
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InterScience, Inc.
Sikd-er Mohammad Y.
Yablon Jay R.
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