U.S. patent application number 09/993034 was filed with the patent office on 2003-05-08 for image-forming system with enhanced gray levels.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Brazas, John C. JR., Kowarz, Marek W..
Application Number | 20030086177 09/993034 |
Document ID | / |
Family ID | 25539025 |
Filed Date | 2003-05-08 |
United States Patent
Application |
20030086177 |
Kind Code |
A1 |
Kowarz, Marek W. ; et
al. |
May 8, 2003 |
IMAGE-FORMING SYSTEM WITH ENHANCED GRAY LEVELS
Abstract
An image-forming system with enhanced gray levels, including: a
primary beam of light having a primary intensity value; a secondary
beam of light having a secondary intensity value significantly less
than the primary intensity value; a first modulator array of
discrete devices receiving the primary beam of light and producing
an output with coarse gray levels; a second modulator array of
discrete devices receiving the secondary beam of light and
producing an output with fine gray levels; a controller for
synchronously controlling the first and the second modulator array;
and optics that combine the output with fine gray levels and the
output with coarse gray levels to form an image with the enhanced
gray levels.
Inventors: |
Kowarz, Marek W.;
(Henrietta, NY) ; Brazas, John C. JR.; (Hilton,
NY) |
Correspondence
Address: |
Thomas H. Close
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
25539025 |
Appl. No.: |
09/993034 |
Filed: |
November 6, 2001 |
Current U.S.
Class: |
359/618 ;
348/E9.027 |
Current CPC
Class: |
H04N 9/3105 20130101;
H04N 9/3123 20130101; H04N 9/3188 20130101 |
Class at
Publication: |
359/618 |
International
Class: |
G02B 027/10 |
Claims
What is claimed is:
1. An image-forming system with enhanced gray levels, comprising: a
first light source that can emit a primary beam of light having a
primary intensity value; a second light source that can emit a
secondary beam of light having a secondary intensity value
significantly less than the primary intensity value; a first
modulator array of discrete devices receiving the primary beam of
light and producing an output with coarse gray levels; a second
modulator array of discrete devices receiving the secondary beam of
light and producing an output with fine gray levels; a controller
for synchronously controlling the first and the second modulator
array; and optics that combine the output with fine gray levels and
the output with coarse gray levels to form an image with the
enhanced gray levels.
2. The image-forming system claimed in claim 1, wherein the first
and the second light source are combined as a single light source
that can emit a source beam; and wherein a beam divider receives
the source beam and divides the source beam unequally into the
primary and the secondary beam of light.
3. The image-forming system claimed in claim 1, wherein the
controller receives image data with enhanced gray levels and
separates the image data appropriately to produce the output with
coarse gray levels and the output with fine gray levels.
4. The image-forming system claimed in claim 1, wherein the first
and the second modulator array modulate incident light on a linear
scale such that a separation between adjacent gray levels is a
constant value.
5. The image-forming system claimed in claim 1, wherein a ratio of
the secondary intensity value to the primary intensity value equals
1/2.sup.N, wherein N is a number of linear bits of a single
modulator.
6. The image-forming system claimed in claim 1, wherein a ratio of
the secondary intensity value to the primary intensity value is
greater than 1/2.sup.N and is less than 1/2, wherein N is a number
of linear bits of a single modulator; such that there is overlap of
the coarse and the fine gray levels.
7. The image-forming system claimed in claim 1, wherein the coarse
and the fine gray levels are produced by pulse width
modulation.
8. The image-forming system claimed in claim 1, wherein the first
and the second modulator array are arrays of electromechanical
grating devices.
10. The image-forming system claimed in claim 1, wherein the first
and the second modulator array are arrays of micro-mirror
devices.
11. The image-forming system claimed in claim 1, wherein the
image-forming system displays an image on a screen.
12. The image-forming system claimed in claim 1, wherein the
image-forming system prints an image.
13. An image-forming system with enhanced gray levels, comprising:
a polarization beam splitter that receives light of a chosen
polarization and divides the polarized light unequally, based on
the chosen polarization, into a primary beam of light having a
primary intensity value, and a secondary beam of light having a
secondary intensity value significantly less than the primary
intensity value; a first modulator array of discrete devices
receiving the primary beam of light and producing an output with
coarse gray levels; a second modulator array of discrete devices
receiving the secondary beam of light and producing an output with
fine gray levels; a controller for synchronously controlling the
first and the second modulator array; and optics that combine the
output with fine gray levels and the output with coarse gray levels
to form an image with enhanced gray levels.
14. The image-forming system claimed in claim 13, further
comprising a laser system emitting polarized light.
15. The image-forming system claimed in claim 13, wherein the
chosen polarization is chosen by selecting an orientation of a
waveplate.
16. An image-forming system claimed in claim 13, wherein the
polarization beam splitter combines the output of fine gray levels
and the output of course gray levels to produce enhanced gray
levels.
17. The image-forming system claimed in claim 13, wherein the
controller receives image data with enhanced gray levels and
separates the image data appropriately to produce the output with
coarse gray levels and the output with fine gray levels.
18. The image-forming system claimed in claim 13, wherein the first
and the second modulator array modulate incident light on a linear
scale such that a separation between adjacent gray levels is a
constant value.
19. The image-forming system claimed in claim 13, wherein a ratio
of the secondary intensity value to the primary intensity value
equals 1/2.sup.N, wherein N is a number of linear bits of a single
modulator.
20. The image-forming system claimed in claim 13, wherein a ratio
of the secondary intensity value to the primary intensity value is
greater than 1/2.sup.N and less than 1/2, wherein N is a number of
linear bits of a single modulator; such that there is overlap of
the coarse and the fine gray levels.
21. The image-forming system claimed in claim 13, wherein the
coarse and the fine gray levels are produced by pulse width
modulation.
22. The image-forming system claimed in claim 13, wherein the first
and the second modulator array are arrays of electromechanical
grating devices.
23. The image-forming system claimed in claim 13, wherein the first
and the second modulator array are arrays of micro-mirror
devices.
24. The image-forming system claimed in claim 13, wherein the
image-forming system displays an image on a screen.
25. The image-forming system claimed in claim 13, wherein the
image-forming system prints an image.
26. A display system with enhanced gray levels, comprising: a
polarization beam splitter that receives light of a chosen
polarization and divides the polarized light unequally, based on
the chosen polarization, into a primary beam of light having a
primary intensity value, and a secondary beam of light having a
secondary intensity value significantly less than the primary
intensity value; a first modulator array of electromechanical
grating devices receiving the primary beam of light and producing
an output with coarse gray levels; a second modulator array of
electromechanical grating devices receiving the secondary beam of
light and producing an output with fine gray levels; a controller
for synchronously controlling the first and the second modulator
array; and optics that combine the output with fine gray levels and
the output with coarse gray levels to form an image with enhanced
gray levels.
27. The display system claimed in claim 26, further comprising a
laser system emitting polarized light.
28. The display system claimed in claim 26, wherein the chosen
polarization is chosen by selecting an orientation of a
waveplate.
29. The display system claimed in claim 26, wherein the
polarization beam splitter combines the output of fine gray levels
and the output of coarse gray levels to produce enhanced gray
levels.
30. The display system claimed in claim 26, wherein the first and
the second modulator array modulate incident light on a linear
scale such that a separation between adjacent gray levels is a
constant value.
31. The display system claimed in claim 26, wherein a ratio of the
secondary intensity value to the primary intensity value is greater
than 1/2.sup.N and less than 1/2, wherein N is a number of linear
bits of a single modulator; such that there is overlap of the
coarse and the fine gray levels.
32. The display system claimed in claim 26, wherein the coarse and
the fine gray levels are produced by pulse width modulation.
33. A color display system for providing enhanced gray levels,
wherein the color display system includes a controller and a
plurality of single color subsystems, wherein each one of a
plurality of single color subsystems comprises: a beam divider that
receives light of a single color and divides the light unequally
into a primary beam of light having a primary intensity value, and
a secondary beam of light having a secondary intensity value
significantly less than the primary intensity value; a first
modulator array of discrete devices receiving the primary beam of
light and producing an output with coarse gray levels; a second
modulator array of discrete devices receiving the secondary beam of
light and producing an output with fine gray levels; and optics
that combine the output with fine gray levels and the output with
coarse gray levels to form an image with enhanced gray levels.
34. The color display system claimed in claim 33, wherein each one
of a plurality of single color subsystems further comprises a laser
system emitting light of a single color.
35. The color display system claimed in claim 33, wherein the beam
divider is a polarization beam splitter that receives light of a
chosen polarization and divides the polarized light unequally,
based on the chosen polarization.
36. The color display system claimed in claim 35, wherein the
chosen polarization in each one of a plurality of single color
subsytems is chosen by selecting an orientation of a waveplate.
37. The color display system claimed in claim 35, wherein the
polarization beam splitter in each one of a plurality of color
subsystems combines the output of fine gray levels and the output
of coarse gray levels to produce enhanced gray levels.
38. The color display system claimed in claim 33, wherein the first
and the second modulator array in each one of a plurality of color
subsystems modulate incident light on a linear scale such that a
separation between adjacent gray levels is a constant value.
39. The color display system claimed in claim 33, wherein for each
one of a plurality of color subsystems a ratio of the secondary
intensity value to the primary intensity value is greater than
1/2.sup.N and less than 1/2, wherein N is a number of linear bits
of a single modulator, such that there is overlap of the coarse and
the fine gray levels.
40. The image-forming system claimed in claim 2, wherein the beam
divider is an optical grating that receives the source beam and
divides the source beam unequally into the primary and the
secondary beam of light.
41. The image-forming system claimed in claim 1, wherein the first
and the second modulator array are located on a single
substrate.
42. A method for forming an image with enhanced gray levels,
comprising the steps of: a) providing a primary beam of light
having a primary intensity value; b) providing a secondary beam of
light having a secondary intensity value significantly less than
the primary intensity value; c) receiving the primary beam of light
at a first modulator array of discrete devices; d) producing an
output with coarse gray levels with the first modulator array of
discrete devices; e) receiving the secondary beam of light at a
second modulator array of discrete devices; f) producing an output
with fine gray levels with the second modulator array of discrete
devices; g) synchronously controlling the first and the second
modulator array; and h) combining the output with fine gray levels
and the output with coarse gray levels to form the image with
enhanced gray levels.
43. The method claimed in claim 42, wherein the step of
synchronously controlling the first and the second modulator array
includes the steps of: g1) receiving image data with enhanced gray
levels; and g2) separating the image data to produce the output
with the coarse gray levels and the output with the fine gray
levels.
44. The method claimed in claim 42, wherein the first and the
second modulator array modulate incident light on a linear scale
such that a separation between adjacent gray levels is a constant
value.
45. The method claimed in claim 42, wherein the coarse and the fine
gray levels are derived from pulse width modulation.
46. A method for forming an image with enhanced gray levels,
comprising the steps of: a) choosing a state of polarization of a
polarized light beam; b) dividing the polarized light beam
unequally, based on the chosen polarization, into a primary beam of
light having a primary intensity value, and a secondary beam of
light having a secondary intensity value significantly less than
the primary intensity value; c) receiving the primary beam of light
at a first modulator array of discrete devices; d) producing an
output with coarse gray levels with the first modulator array of
discrete devices; e) receiving the secondary beam of light at a
second modulator array of discrete devices; f) producing an output
with fine gray levels with the second modulator array of discrete
devices; g) synchronously controlling the first and the second
modulator array; and h) combining the output with fine gray levels
and the output with coarse gray levels to form the image with
enhanced gray levels.
47. The method claimed in claim 46, wherein the step of choosing
the state of polarization of a polarized light beam, further
includes the step of: a1) selecting an orientation of a
waveplate.
48. The method claimed in claim 46, wherein the step of
synchronously controlling the first and the second modulator array
includes the steps of: g1) receiving image data with enhanced gray
levels; and g2) separating the image data to produce the output
with coarse gray levels and the output with fine gray levels.
49. The method claimed in claim 46, wherein the first and the
second modulator array modulate incident light on a linear scale
such that a separation between adjacent gray levels is a constant
value.
50. The method claimed in claim 46, wherein the coarse and the fine
gray levels are derived from pulse width modulation.
Description
FIELD OF THE INVENTION
[0001] This invention relates to an image-forming system containing
spatial light modulators that generate a visible image from an
electronic signal. More particularly, the invention relates to a
projection display system having enhanced gray levels for each
pixel in the image.
BACKGROUND OF THE INVENTION
[0002] Many display systems, including projection and video type
display systems, employ a uniformly illuminated 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 electronics.
[0003] 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 coarse and fine
gray levels. The fine gray scale is obtained by attenuating the
illumination for some time during the display of a frame. The
coarse 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.
[0004] An alternative approach employs a pulsating light source
such as a pulsed laser to reduce speed requirements on the
electronics, as described in U.S. Pat. No. 5,668,611, issued to
Ernstoff 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 electronics to
load new image data bits into the spatial modulator.
[0005] Each of the above described inventions trade light source
efficiency for improved gray levels or reduced electronic speed
requirements. However, efficient use of the light source is needed
for theater-type projection displays in order to maximize
brightness and color saturation of the projected image.
[0006] The display systems described in the prior art achieve fine
gray level control by lowering the average optical power incident
on the spatial light modulator for some period of time, thus
generating multiple illumination levels corresponding to decreased
intensity. Multiple illumination levels reduce the speed
requirements of the spatial light modulator and its associated
electronics. However, a serious technical drawback to this approach
is that it wastes optical power that is available from the light
source during lower illumination intervals. There is a need,
therefore, for a display system having fine gray level control
while simultaneously making efficient use of available optical
power.
SUMMARY OF THE INVENTION
[0007] The above need is met according to the present invention by
providing an image-forming system with enhanced gray levels that
includes a first light source that can emit a primary beam of light
having a primary intensity value; a second light source that can
emit a secondary beam of light having a secondary intensity value
significantly less than the primary intensity value; a first
modulator array of discrete devices receiving the primary beam of
light and producing an output with coarse gray levels; a second
modulator array of discrete devices receiving the secondary beam of
light and producing an output with fine gray levels; a controller
for synchronously controlling the first and the second modulator
array; and optics that combine the output with fine gray levels and
the output with coarse gray levels to form an image with the
enhanced gray levels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram of an image-forming system with
two spatial light modulators, illuminated by two different
intensity levels I.sub.C and I.sub.F;
[0009] FIG. 2A illustrates the formation of image gray levels
I.sub.image(p,q) by combining the coarse light output levels from
array C with the fine levels from array F, when there is no overlap
between the coarse and fine levels;
[0010] FIG. 2B illustrates the formation of image gray levels
I.sub.image(p,q) when there is overlap between the coarse light
output levels from array C and the fine levels from array F;,
[0011] FIG. 3A illustrates the generation of coarse light output
levels C.sub.out(p) by array C through pulse width modulation;
[0012] FIG. 3B illustrates the generation of fine light output
levels F.sub.out(q) by array F through pulse width modulation;
[0013] FIG. 4 is a schematic illustrating an optical sub-system for
illuminating two spatial light modulators unequally and recombining
the modulated light output;
[0014] FIG. 5 is a schematic illustrating a line-scanned display
system with two linear arrays of electromechanical grating devices,
which are illuminated by two different intensity levels;
[0015] FIG. 6 shows a linear array of electromechanical grating
devices illuminated by a line of light;
[0016] FIG. 7 is a view of the projection screen that illustrates
the formation of a two-dimensional image by scanning a line image
across the screen;
[0017] FIG. 8 is a schematic illustrating a color line-scanned
display system with two linear arrays of electromechanical grating
devices for each color; and
[0018] FIG. 9 is a schematic illustrating a line-scanned display
system with two linear arrays of electromechanical grating devices
on the same substrate, illuminated by two different intensity
levels.
DETAILED DESCRIPTION OF THE INVENTION
[0019] FIG. 1 shows a block diagram of a system for forming an
image with very fine intensity levels or gray levels. This
image-forming system could be, for example, a digital cinema
projector, a rear-projection HDTV or a silver halide printer. The
system contains two spatial light modulators: modulator array C 16;
and modulator array F 18. Both modulator arrays 16, 18, are
illuminated unequally by a primary and a secondary beam having
intensities I.sub.C and I.sub.F, respectively. As a convention, if
I.sub.C is chosen to be greater than I.sub.F, modulator array C 16
generates coarse gray levels and modulator array F 18 generates
fine gray levels. To produce the primary and secondary illumination
beams, the source beam 13 produced from a light source 12 is
divided unequally by a beam divider 14, which may be a simple beam
splitter or a grating. The beam divider 14 could also consist of a
waveplate followed by a polarization beamsplitter. As will be
described later, polarization-based division allows the ratio of
the two intensities I.sub.C and I.sub.F to be tuned to a desired
value. A variety of different spatial light modulator technologies
may be used for the two modulator arrays, for example, liquid
crystal panels, micromirror arrays such as the digital mirror
device (DMD) from Texas Instruments or electromechanical grating
arrays. The spatial light modulators may modulate light through
pulse width modulation, amplitude modulation or any other approach
for attenuating the incident illumination to produce gray levels in
the image. A controller 20 sends code values to each of the two
spatial light modulators (modulator array C 16, and modulator array
F 18) to generate the desired pixel gray levels. The light output
from the coarse and fine modulators, C.sub.out(p) and F.sub.out(q),
is then collected by imaging optics 22 and directed to an image
plane 24, where an image with intensity
I.sub.image(p,q)=C.sub.out(p)+F.sub.out(q) is formed. The code
values p and q are used to select the intensity for a given pixel
in the image. (In this notation, the spatial dependence of
I.sub.image(p,q), C.sub.out(p) and F.sub.out(q) is implicit.) It
should be pointed out that, for coherent light sources,
interference can occur between the two beams in the image. In the
preferred embodiment, the interference effects are avoided by
having the light output from the two modulators orthogonally
polarized.
[0020] FIG. 2A illustrates the formation of fine gray levels in the
image. For simplicity, in this illustration, the pixel intensity in
the image I.sub.image(p,q) has 8 linear bits or 256 levels, whereas
the modulator arrays F 18 and C 16 of FIG. 1 each have only 4
linear bits or 16 levels. The light output of coarse and fine
modulator arrays is linearly dependent on the code value p and q,
respectively. In this example, p and q can take on values between 0
and 15. In practice, 12 or more linear bits are required in a high
quality image and the modulation may be a non-linear function of
code value. The ratio of the fine and coarse illumination in FIG.
2a, where the number of linear bits equals 4, is chosen to be
I.sub.F/I.sub.C=1/16 so that there is no overlap between the two
scales or redundancy in I.sub.image(p,q) for any combination of
code values p and q.
[0021] More generally, by using the two unequally-illuminated
modulators to generate coarse and fine levels, it is possible to
increase the number of gray levels from 2.sup.N to 2.sup.2N, N
being the number of linear bits for a single modulator. The
illumination ratio is then chosen to be I.sub.F/I.sub.C=1/2.sup.N.
In a practical implementation, it is difficult to achieve and
maintain this exact ratio. Therefore, a better choice is
I.sub.F/I.sub.C>1/2.sup.N, which allows for some overlap between
the pixel intensities on the coarse and fine scales. The overlap
also provides some headroom for calibration between the two gray
scales.
[0022] FIG. 2B is an example similar to FIG. 2A except that the
ratio of the fine and coarse illumination is chosen to be
I.sub.F/I.sub.C=1/2.sup.- N-1=1/8, N=4, so that there is both
overlap and redundancy in I.sub.image(p,q) for combinations of code
values p and q. For example, I.sub.image(0,8)=I.sub.image(1,0) and,
more generally, I.sub.image(p,q)=I.sub.image(p+1,q-8). Because of
the redundancy, the pixel intensity in the image I.sub.image(p,q)
has 7 linear bits or 128 possible gray levels.
[0023] FIGS. 3A and 3B illustrate the formation of the coarse and
fine outputs C.sub.out(p) and F.sub.out(q), by a pair of spatial
light modulators that use center-weighted pulse width modulation to
produce gray levels. The modulators are on for an integer multiple
of the least significant bit time (T) determined by the code values
p and q. The upper limit on p and q is determined by the modulation
window. For area arrays, the modulation window is simply the time
available to display a single frame. For linear arrays, it is the
time available to generate a single line of an image during a scan
of multiple sequential lines to produce a single frame. The outputs
C.sub.out(p) and F.sub.out(q) are equal to the corresponding
integrated intensities within a modulation window.
[0024] FIG. 4 shows an optical sub-system 60 that can be used to
implement beam division, illuminate two modulator arrays, and
recombine the outputs of the modulator arrays. In this embodiment,
the source 70 is preferably a linearly polarized laser that emits a
narrow spectrum of light, commonly referred to as a light beam 71.
The light beam 71 is conditioned by first and second conditioning
lens 72 and 74, respectively, and passes through a half-wave plate
92, which can be rotated to adjust the state of polarization. A
polarization beam splitter 96 splits the beam into its horizontally
and vertically polarized components. These orthogonally polarized
components are the coarse light beam 26 and fine light beam 28,
with intensities I.sub.C and I.sub.F, respectively, needed to
illuminate the two respective modulator arrays 16 and 18. The
modulator arrays 16 and 18 used in this embodiment are
electromechanical grating devices that increase the light in
diffracted orders when activated and consist of enough array
elements to create an image. The ratio I.sub.F/I.sub.C is
determined by the orientation of the half-wave plate 92. A
quarter-wave plate 95 is inserted in each arm of the optical
sub-system 60 between the polarization beam splitter 96 and the two
modulator arrays 16 and 18. As is well known in optical
engineering, a polarization beam splitter with a quarter-wave plate
in front of a reflecting surface serves as an optical isolator.
Linearly polarized light transmitted (reflected) by the interface
of the polarization beam splitter 96 will be reflected
(transmitted) by the interface if the polarization is rotated 90
degrees. Double passage of light through the quarter-wave plate 95
produces the needed 90-degree polarization rotation. In FIG. 4,
this approach is used to combine the outputs of the coarse and fine
modulators 16 and 18 at the interface of the polarization beam
splitter 96. Depending on the state of each array element, the
light exiting the polarization beam splitter 96 is either blocked
by a stop 97 or, when the array elements are activated, transmitted
through imaging optics to the image plane (not shown). Interference
artifacts are avoided because the coarse light beam 26 and the fine
light beam 28 from the coarse and fine modulators 16 and 18,
respectively, are orthogonally polarized.
[0025] The modulators of FIG. 4 can be micromirror arrays,
electromechanical grating arrays, or any spatial light modulators
that do not substantially alter the polarization.
[0026] FIG. 5 shows a digital projection display that incorporates
the optical sub-system 60 from FIG. 4. The two modulator arrays are
now linear arrays of electromechanical grating devices (referred to
hereinafter as first modulator array 85 and second modulator array
86), such as the conformal Grating ElectroMechanical System (GEMS)
made by Eastman Kodak Company (see U.S. patent application Ser. No.
09/491,354, by Kowarz, filed Jan. 26, 2000, entitled, "SPATIAL
LIGHT MODULATOR WITH CONFORMAL GRATING ELEMENTS," and U.S. patent
application Ser. No. 09/867,927 by Kowarz et al., filed May 30,
2001, entitled, "METHOD FOR MANUFACTURING A MECHANICAL CONFORMING
GRATING DEVICE"), or the grating light-valve made by Silicon Light
Machines (see U.S. Pat. No. 5,982,553 issued to Bloom et al. on
Nov. 9, 1999, entitled, "DISPLAY DEVICE INCORPORATING
ONE-DIMENSIONAL GRATING LIGHT-VALVE ARRAY.") Modulator array 85 and
86 may be of the conformal type and may be linear. The controller
20 in FIG. 5 actuates the first modulator array 85 and the second
modulator array 86 to obtain the desired pixel pattern for a given
line of a two-dimensional image. The scheme illustrated in FIGS.
2A, 2B, 3A, and 3B can be used to obtain the required gray levels
on the screen 90 for conformal GEMS devices designed for
pulse-width modulation. If a particular conformal GEMS device is
not actuated, it diffracts the incident light beam primarily into
the 0th order light beam (i.e., reflecting the incident light beam
78), which is blocked by a stop 97. If the device is actuated, it
diffracts the incident light beams 78 primarily into +2.sup.nd,
+1.sup.st, -1.sup.st and -2.sup.nd order light beams. These
diffracted light beams 79 pass around the stop 97 and are projected
on the screen 90 by the projection lens system 75. The scanning
mirror 77 sweeps the line image across the screen 90 to form the
two-dimensional image. The controller 20 provides synchronization
between the sweep of the scanning mirror 77 and data for each line.
The scanning mirror 77 is preferably placed near the Fourier plane
of the projection lens system 75 to minimize its size and mass.
[0027] FIG. 6 depicts a first modulator array 85 of a conformal
GEMS device 5 illuminated by a line of light 88. In practice, there
would be hundreds or thousands of such devices. FIG. 7 is a view
facing the screen 90 showing the formation of a two-dimensional
image from a series of 1,920 sequential line scans.
[0028] The display system in FIG. 5 could be either monochromatic
or color-sequential. In a color-sequential system, the controller
20 also synchronizes the color that illuminates the first modulator
array 85 and the second modulator array 86, and the associated data
for each line. Better utilization of available optical power and
better image quality may be obtained by combining three optical
sub-systems 60r, 60g and 60b into a system capable of displaying
RGB simultaneously. This approach is shown in FIG. 8 where a color
combining cube 100 (X-cube) is used to direct the red, green and
blue outputs, 81, 83, and 84, respectively, through the projection
lens system 75 and onto a screen (not shown).
[0029] It is instructive to compare the implications of the
invention for a digital cinema system that contains a single linear
array of conformal GEMS devices per color with the system shown in
FIG. 8 that has a pair of linear arrays per color, which generate
coarse and fine gray levels. In this example, the system is chosen
to have HDTV resolution with 1,920 scanned lines (1,080 by 1,920
pixels), a frame rate of 60 Hz and a gray scale capability of 13
linear bits per color per frame (8,192 gray levels). The gray scale
is obtained by pulse width modulation as explained in the
description of FIGS. 3A and 3B. For the case of a single linear
array per color, the least significant bit time must be somewhat
less than 1/(1,920*60*8,192) seconds=1.06 nanoseconds to allow for
some scanning overhead. The digital electronics in the controller
must, therefore, be capable of generating a clock for the pulse
width modulation operation of approximately 1 GHz. This clock
frequency can be reduced substantially by implementing the system
with two linear arrays per color, while maintaining the final
system specifications. Specifically, a system having 13 linear bits
per color per frame can be obtained with two linear arrays, each
having 7 linear bits per color per frame. The ratio of the fine and
coarse illumination is chosen to be
I.sub.F/I.sub.C=1/2.sup.7-1=1/64 so that there is both overlap and
redundancy in the image gray levels. The requirement on T increases
to 1/(1,920*60*128) seconds=67.8 nanoseconds and the clock
requirement for the pulse width modulation operation drops to a
very reasonable 14.7 MHz. The increased optical complexity of the
display system in FIG. 8, therefore, significantly reduces the
clock speed for the digital electronics.
[0030] In the above embodiments, the two spatial light modulators
for generating coarse and fine gray levels are physically separated
in the system. A more compact system has both modulators on the
same substrate. This configuration is illustrated in FIG. 9 for a
display system similar to the system of FIG. 5. The two modulators
are linear modulator arrays 85 and 86, respectively, of high-speed
devices such as conformal GEMS. The conditioning lenses 72 and 74
produce a converging beam. a turning mirror 82 redirects the beam
toward the beam divider 98. The beam divider 98 splits an input
light beam 71 from the laser 70 into two output beams 73 and 76
that are slightly separated and have a desired ratio of
intensities. The beams 73 and 76 illuminate the first modulator
array 85 and the second modulator array 86, respectively, that
generate the coarse and fine gray levels. For a linearly polarized
input beam, this beam divider 98 could be a polarization beam
splitter plate made from a uniaxial crystal. The desired intensity
ratio is then obtained by appropriately rotating the linear
polarization of the input light beam 71. The diffracted light beam
79 from the modulator arrays 85 and 86 return through the beam
divider 98 and become collinear. Interference artifacts that might
result from combining coherent optical light beams are avoided
because the diffracted light beams 79 exiting from modulator arrays
85 and 86 are orthogonally polarized.
[0031] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
PARTS LIST
[0032] conformal GEMS device
[0033] 12 light source
[0034] 13 source beam
[0035] 14 beam divider
[0036] 16 coarse modulator array C
[0037] 18 fine modulator array F
[0038] 20 controller
[0039] 22 imaging optics
[0040] 24 image plane
[0041] 26 coarse light beam
[0042] 28 fine light beam
[0043] 60 optical sub-system
[0044] 60r red optical sub-system
[0045] 60b blue optical sub-system
[0046] 60g green optical sub-system
[0047] 70 laser
[0048] 71 light beam
[0049] 72 first conditioning lens
[0050] 73 output beam
[0051] 74 second conditioning lens
[0052] 75 projection lens system
[0053] 76 output beam
[0054] 77 scanning mirror
[0055] 78 incident light beam
[0056] 79 diffracted light beams
[0057] 81 red output
[0058] 82 turning mirror
[0059] 83 green output
[0060] 84 blue output
[0061] 85 first modulator array
[0062] 86 second modulator array
[0063] Parts List--Continued
[0064] 88 line of light
[0065] 90 screen
[0066] 92 half-wave plate
[0067] 95 quarter-wave plate
[0068] 96 polarization beam splitter
[0069] 97 stop
[0070] 98 beam divider
[0071] 100 color combining cube
* * * * *