Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-12-31
2004-04-20
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S295000, C359S231000
Reexamination Certificate
active
06724515
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electromechanical grating devices for light modulation. More particularly, the invention relates to the formation of gray levels in a projection display system containing a linear array of electromechanical grating devices.
BACKGROUND OF THE INVENTION
Many projection display systems employ a spatial light modulator to convert electronic image information into an output image. At present in such systems, the light source is typically a white light lamp and the spatial light modulator is typically an area array containing liquid crystal devices or micromirror devices. Alternative display system architectures with one or more laser sources and spatial light modulators that are linear arrays of electromechanical grating devices show much promise for the future. To display high quality motion images, the individual devices of these different spatial light modulators must be capable of rapidly producing a large number of gray levels in the image. The limit on the number of possible gray levels is usually dictated either by the device dynamics or by the speed of electronic components within the display system.
Prior inventions have disclosed schemes for increasing the number of gray levels in the image without increasing the speed of the modulating elements or of the associated electronics. These schemes vary the illumination that is incident on the spatial light modulator during a frame. Specifically, according to U.S. Pat. No. 5,812,303, issued to Hewlett et al. on Sep. 22, 1998, entitled, “LIGHT AMPLITUDE MODULATION WITH NEUTRAL DENSITY FILTERS,” additional gray levels can be obtained with a micromirror device by using a variable neutral density filter to generate bright and dark gray levels. The dark gray scale is obtained by attenuating the illumination for some time during the display of a frame. The bright gray scale has no attenuation. In practice, the invention is implemented by rotating a filter wheel with a multi-segment neutral density filter in synchronization with the data stream.
An alternative approach employs a pulsating light source such as a pulsed laser to reduce speed requirements on the electronic components, as described in U.S. Pat. No. 5,668,611, issued to Ermstoff et al. on Sep. 16, 1997, entitled “FULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING PULSE RATE MODULATED ILLUMINATION.” The illumination on the spatial light modulator is adjusted by varying the pulse rate or pulse count. Moreover, the average brightness of the light source is determined by the number of pulses. A complementary method uses direct intensity modulation of the light source to obtain multiple levels of brightness, as disclosed in U.S. Pat. No. 5,903,323, issued to Ernstoff et al. on May 11, 1999, entitled “FULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING TIME MODULATED ILLUMINATION.” Both U.S. Pat. No. 5,668,611 and U.S. Pat. No. 5,903,323 address the specific problem of having a large enough time window for the electronic components to load new image data bits into the spatial modulator.
Each of the above described inventions trade light source efficiency for improved gray levels or reduced electronic component speed requirements. However, efficient use of the light source is needed for high-quality projection displays in order to maximize brightness and color saturation of the projected image.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE” issued Oct. 23, 2001. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device, with GEMS standing for grating electromechanical system. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated thrughout most of an optical system and enables a simpler optical system design with smaller optical elements. Display systems based on a linear array of conformal GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAM”, issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,476,848, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SEGMENTED WAVEPLATE”, issued Nov. 5, 2002.
The conformal Grating Electromechanical System (GEMS) devices disclosed in U.S. Pat. No. 6,307,663 are illustrated in
FIGS. 1-3
.
FIG. 1
shows two side-by-side conformal GEMS devices
5
a
and
5
b
in an unactuated state. The conformal GEMS devices
5
a
and
5
b
are formed on top of a substrate
10
covered by a bottom conductive layer
12
, which acts as an electrode to actuate the devices
5
a
,
5
b
. The bottom conductive layer
12
is covered by a dielectric protective layer
14
followed by a standoff layer
16
and a spacer layer
18
. On top of the spacer layer
18
, a ribbon layer
20
is formed which is covered by a reflective layer and conductive layer
22
. The reflective and conductive layer
22
provides electrodes for the actuation of the conformal GEMS devices
5
a
and
5
b
. Accordingly, the reflective and conductive layer
22
is patterned to provide electrodes for the two conformal GEMS devices
5
a
and
5
b
. The ribbon layer
20
, preferably, comprises a material with a sufficient tensile stress to provide a large restoring force. Each of the two conformal GEMS devices
5
a
and
5
b
has an associated elongated ribbon element
23
a
and
23
b
, respectively, patterned from the reflective and conductive layer
22
and the ribbon layer
20
. The elongated ribbon elements
23
a
and
23
b
are supported by end supports
24
a
and
24
b
, formed from the spacer layer
18
, and by one or more intermediate supports
27
that are uniformly separated in order to form equal-width channels
25
. The elongated ribbon elements
23
a
and
23
b
are secured to the end supports
24
a
and
24
b
and to the intermediate supports
27
. A plurality of standoffs
29
is patterned at the bottom of the channels
25
from the standoff layer
16
. These standoffs
29
reduce the possibility of the elongated ribbon elements
23
a
and
23
b
sticking when actuated.
A top view of a four-device linear array of conformal GEMS devices
5
a
,
5
b
,
5
c
and
5
d
is shown in FIG.
2
. The elongated ribbon elements
23
a
,
23
b
,
23
c
, and
23
d
are depicted partially removed over the portion of the diagram below the line A—A in order to show the underlying structure in an active region
8
. For best optical performance and maximum contrast, the intermediate supports
27
should preferably be completely hidden below the elongated ribbon elements
23
a
,
23
b
,
23
c
, and
23
d
. Therefore, when viewed from the top, the intermediate supports
27
should not be visible in the gaps
28
between the conformal GEMS devices
5
a
-
5
d
. Here, each of the conformal GEMS devices
5
a
-
5
d
has three intermediate supports
27
with four equal-width channels
25
. The center-to-center separation &Lgr; of the intermediate supports
27
defines the period of the conformal GEMS devices in the actuated state. The elongated ribbon elements
23
a
-
23
d
are mechanically and electrically isolated from one another, allowing independent operation of the four conformal GEMS devices
5
a
-
5
d
. The bottom conductive layer
12
of
FIG. 1
can be common
Eastman Kodak Company
Hanig Richard
Shaw Stephen H.
Sugarman Scott J.
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