U.S. patent application number 11/092419 was filed with the patent office on 2006-10-05 for spatial light modulation display system.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Roger S. Carver, Daniel J. Morgan.
Application Number | 20060221019 11/092419 |
Document ID | / |
Family ID | 37069792 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060221019 |
Kind Code |
A1 |
Morgan; Daniel J. ; et
al. |
October 5, 2006 |
Spatial light modulation display system
Abstract
An improved spatial light modulation display system includes
light sources for providing red, green, and blue light. A system
controller includes functionality for controlling a color balance
of the red, green, and blue light. A sensor is provided for sensing
light from each of the light sources. The controller can detect a
shift in color balance based on the intensity of light sensed by
the sensor. If the sensor output indicates that the sensor is
operating out of a desirable range, the spatial light modulator can
modulate the light in order change the brightness of light sensed
by the sensor. The modulation pattern can be varied until the
sensor output is a specified value or within a specified range of
values. In a preferred embodiment, the sensor is located to receive
off-state light from the spatial light modulator so as to avoid
obstruction of light used for displaying images.
Inventors: |
Morgan; Daniel J.; (Denton,
TX) ; Carver; Roger S.; (Richardson, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
Texas Instruments
Incorporated
|
Family ID: |
37069792 |
Appl. No.: |
11/092419 |
Filed: |
March 29, 2005 |
Current U.S.
Class: |
345/84 |
Current CPC
Class: |
G09G 2320/0242 20130101;
G09G 3/346 20130101; G02B 26/007 20130101; G09G 5/02 20130101; G09G
2320/0693 20130101; G02B 26/0816 20130101; G09G 2320/0666 20130101;
G09G 2360/141 20130101 |
Class at
Publication: |
345/084 |
International
Class: |
G09G 3/34 20060101
G09G003/34 |
Claims
1. A display system comprising: a plurality of light sources; a
spatial light modulator located to receive light from at least one
of the plurality of light sources, the spatial light modulator
including a plurality of independently controllable elements that
can be controlled to direct received light in one of a first
direction for display and a second direction for non-display; a
light sensor located to receive light directed in the second
direction from elements of the spatial light modulator and issue a
brightness signal indicative of the intensity of the received
light; and a processing device for detecting a color balance ratio
based at least in part on the brightness signal issued by the light
sensor.
2. A display system according to claim 1, further comprising an
analog to digital converter for converting the brightness signal
into digital brightness data and providing the digital brightness
data to the processing device.
3. A display system according to claim 1, further comprising a
light source controller for controlling activation of the plurality
of light sources according to instructional color data received
from the processing device.
4. A display system according to claim 1, wherein the plurality of
light sources comprises a first LED array for emitting blue light,
a second LED array for emitting green light, and a third LED array
for emitting red light.
5. A display system according to claim 1, wherein the plurality of
light sources comprises a first LED array for emitting red and blue
light and a second LED array for emitting green light.
6. A display system according to claim 1, wherein the plurality of
light sources include a light source for emitting red light, a
light source for emitting green light, and a light source for
emitting blue light, wherein the color balance ratio detected by
the processing device is a ratio of red, green, and blue.
7. A display system according to claim 1, a processing device
determines whether the detected color balance ratio is equal to a
specified color balance ratio.
8. A display system according to claim 7, wherein the processing
device adjusts an output of at least one of the plurality of light
sources if it is determined that the detected color balance ratio
is not equal to the specified color balance ratio.
9. A display system according to claim 1, wherein the spatial light
modulator comprises a digital micromirror device.
10. A display system according to claim 1, comprising a second
spatial light modulator located to receive light from another of
the plurality of light sources, the second spatial light modulator
including a plurality of independently controllable elements that
can be controlled to direct received light in one of a third
direction for display and a fourth direction for non-display.
11. A display system according to claim 10, wherein the light
sensor is located to also receive light directed in the fourth
direction from the controllable elements of the second spatial
light modulator.
12. A display system according to claim 10, further comprising a
second light sensor located to receive light directed in the fourth
direction from the controllable elements of the second spatial
light modulator.
13. A display system comprising: a plurality of light sources; a
spatial light modulator located to receive light from at least one
of the light sources, the spatial light modulator including a
plurality of independently controllable elements that can be
controlled to direct light in one of a first direction for display
and a second direction for non-display; a light sensor for
receiving light directed from elements of the spatial light
modulator and outputting a brightness signal based on the intensity
of the received light; and a processing device for controlling the
spatial light modulator to modulate the light received from at
least one of the light sources so that the brightness signal output
from the light sensor is within a specified range of values.
14. A display system according to claim 13, wherein the light
sensor is located to receive light directed in the second direction
from elements of the spatial light modulator.
15. A display system according to claim 13, wherein the processing
device controls the spatial light modulator to modulate the light
according to a blue-noise pattern.
16. A display system according to claim 13, wherein the processing
device determines whether the brightness signal is within the
specified range of values, and changes a modulation pattern used by
the spatial light modulator to modulate the light if the brightness
signal is not within the specified range of values.
17. A display system according to claim 13, wherein the processing
device controls the spatial light modulator to modulate the light
so that the brightness signal output from the light sensor is a
particular value within the specified range of values.
18. A method of controlling color balance comprising: controlling a
spatial light modulator to direct light from a light source in an
off-state direction for non-display; sensing an intensity of light
directed in the off-state direction from elements of the spatial
light modulator; and detecting a color balance ratio based at least
in part on the sensed intensity.
19. A method according to claim 18, further comprising converting
the brightness signal into digital brightness data and providing
the digital brightness data to a processing device.
20. A method according to claim 18, wherein the light source is one
of a plurality of light sources, wherein the plurality of light
sources comprise a light source for emitting red light, a light
source for emitting green light, and a light source for emitting
blue light, wherein the detecting of the color balance ratio
includes detecting a ratio of red, green, and blue.
21. A method according to claim 18, further comprising determining
whether the detected color balance ratio is equal to a specified
color balance ratio.
22. A method according to claim 21, wherein the light source is one
of a plurality of light sources, further comprising adjusting an
output of at least one of the plurality of light sources if it is
determined that the detected color balance ratio is not equal to
the specified color balance ratio.
23. A method of controlling color balance comprising: controlling a
spatial light modulator to direct light from a light source in at
least one of an on-state direction for display and an off-state
direction for non-display; sensing an intensity of light directed
from the spatial light modulator; outputting a brightness signal
based on the sensed intensity of the light; and controlling the
spatial light modulator to modulate the light directed in at least
one of the on-state direction and the off-state direction so that
the brightness signal is within a specified range of values.
24. A method according to claim 23, wherein, in the sensing of the
intensity of light, said light is directed from the spatial light
modulator in the off-state direction.
25. A method according to claim 23, wherein the controlling of the
spatial light modulator to modulate the light includes controlling
the spatial light modulator to modulate the light according to a
blue-noise pattern.
26. A method according to claim 23, further comprising determining
whether the brightness signal is within the specified range of
values, and changing a modulation pattern used by the spatial light
modulator to modulate the light if the brightness signal is not
within the specified range of values.
27. A method according to claim 23, further comprising controlling
the spatial light modulator to modulate the light so that the
brightness signal is a particular value within the specified range
of values.
28. A method of controlling color balance comprising: (a)
controlling a spatial light modulator to direct light from a first
light source of a first color in an off-state direction and to
direct light from a second light source of a second color in an
on-state direction; (b) sensing a first intensity level of the
light from the first light source directed in the off-state
direction; (c) controlling the spatial light modulator to direct
light from the second light source in the off-state direction and
to direct light from the first light source in the on-state
direction; (d) sensing a second intensity level of light from the
second light source directed in the off-state direction; (e)
determining an intensity ratio based at least in part on the thus
sensed first and second sensed intensity levels; and (f) adjusting
control of the first and second light sources as needed according
to the thus determined intensity ratio to attain a specified color
balance.
29. A method according to claim 28, further comprising: (g)
detecting whether at least one of the first and second intensity
levels is out of a specified range; and (h) if the first intensity
level is detected to be out of range, performing steps of: (h1)
controlling the spatial light modulator to direct only a portion of
light from the first light source in the off-state direction
according to a specified calibration pattern and to direct light
from the second light source of a second color in an on-state
direction, and (h2) sensing a third intensity level of the portion
of the light from the first light source directed in the off-state
direction; and (i) if the second intensity level is detected to be
out of range, performing steps of: (i1) controlling the spatial
light modulator to direct only a portion of light from the second
light source in the off-state direction according to the specified
calibration pattern and to direct light from the first light source
in an on-state direction, and (i2) sensing a fourth intensity level
of the portion of the light from the first light source directed in
the off-state direction.
30. A method according to claim 28, wherein: step (a) includes
controlling the spatial light modulator to direct light from a
third light source of a third color in the on-state direction, and
step (c) includes controlling the spatial light modulator to direct
light from a third light source of a third color in the on-state
direction; further comprising: (j) controlling the spatial light
modulator to direct light from the third light source in the
off-state direction and to direct light from the first and second
light sources in the on-state direction, and (k) sensing a third
intensity level of the light from the third light source directed
in the off-state direction; wherein: step (e) includes determining
said intensity ratio based at least in part on the thus sensed
first, second, and third sensed intensity levels, and step (f)
includes adjusting control of the third light source as needed
according to the thus determined intensity ratio to attain a
specified color balance.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. patent application Ser.
No. 10/951,995, entitled "Spatial Light Modulation Display System,"
filed on Sep. 27, 2004,which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates generally to display systems
that use a spatial light modulation device.
BACKGROUND
[0003] Spatial light modulation (SLM) display systems are display
systems that use light reflected or transmitted by individual
elements of a spatial light modulator to generate a display image.
One type of spatial light modulator is a digital micromirror device
(DMD). SLM display systems are known that incorporate a DMD, such
as those commercially available from Texas Instruments, Inc. under
the trademark DLP.RTM. (Digital Light Processing).
[0004] FIG. 1 shows an example of an SLM projection display system
10. The system 10 includes an arc lamp 11 that emits white light. A
first condenser lens 13 focuses the white light onto a color filter
wheel 15. A second condenser lens 17 receives the filtered light
and focuses it onto a DMD chip 19. The DMD chip 19 includes an
array of tiny mirror elements, which together modulate the light
and transmit the modulated light to projection lens 29, where it
can be focused for display on the screen 31.
[0005] FIG. 2 shows a portion of a DMD array 19 having mirror
elements 21 suspended over a substrate 23. Electrostatic attraction
between the mirror 21 and an address electrode 25 causes the mirror
21 to twist or pivot, in either of two directions, about an axis
formed by a pair of torsion beam hinges 27a and 27b. Typically, the
mirror 21 rotates about these hinges until the rotation is
mechanically stopped. The movable mirror 21 tilts into the on or
off states by electrostatic forces depending on the data written to
an associated memory cell (not shown). The tilt of the mirror 21
can be on the order of plus 10 degrees (on) or minus 10 degrees
(off) to modulate the light that is incident on the mirrored
surface. For additional details, see U.S. Pat. No. 5,061,049
entitled "Spatial Light Modulator" and U.S. Pat. No. 5,280,277
entitled "Field Updated Deformable Mirror Device," both to Larry J.
Hornbeck.
[0006] Referring again to FIG. 1, the color filter wheel 15
includes red (R), green (G), and blue (B) filter elements. The
filter wheel 15 is driven by a motor 16 to rotate so that the
different color filter elements sequentially filter the light
passing through the filter wheel 15. Thus, as the filter wheel 15
rotates, the color of light emanating from the filter wheel 15
changes according to the wheel position. Typically the filter wheel
15 rotates at least once per frame for display of a multi-color
image. The frequency of the rotation of the wheel 15 is controlled
by a sequencer 33 based on the frame rate of image data received
from an image source 35.
SUMMARY
[0007] Disclosed herein is an improved SLM display system that uses
light emitting diodes (LEDs) for red, green, and blue light
sources. Thus, red, green and blue light can be emitted from the
light source, eliminating the need for a color wheel assembly as
used in prior systems.
[0008] Like with other types of light sources, LEDs lose brightness
as their operational time increases. However, as LEDs age, each of
the R, G, and B LEDs may lose brightness at different rates. This
will cause the white point (color balance) to shift as the
operational time of the LED display system increases. Thus, a
method is needed to detect this white point shift and then adjust
the brightness of R, G, and B colors so that the white point is
restored to the desired value.
[0009] Even new display systems coming out of production with new
LEDs can have variations in white point due to variations in the
brightness of light emitted from the LEDs. Therefore, a method is
needed for adjusting the white point to an acceptable value when
these display systems are turned on. Preferably, this can be done
without having to manually adjust (through hardware or firmware)
any parts in the display system.
[0010] Accordingly, disclosed herein is a method and apparatus for
detecting and adjusting the white point (color balance) in an SLM
display system. As disclosed herein, a sensor is used to detect the
color intensity of each of the R, G, and B LEDs. This information
can then be used to adjust the intensity of light emitted by the
LEDs to move the white point back to a prescribed value.
[0011] The sensor can be positioned to sense light directed in an
off-state light direction by the DMD as described in greater detail
below. Then, as a display system is powered-up (or every N
power-ups), the DMD can be turned off for one color and on for the
other two. This will allow just one color LED light to strike the
sensor. For example, if the DMD mirrors are turned on during the
time intervals for R and G LED light and off during the time
interval for B LED light, then the display screen will show yellow
and the sensor will see only blue light. The intensity of the blue
light can then be measured independently of the red and green
lights. This process can then be repeated to sense the R and G
light independently, and data can be collected for the intensity of
each of the R, G, and B lights. Alternatively, the light sensor can
be placed at any point along a path between the LEDs and a display
screen where the display system displays images, including
positions where the sensor collects light at the boundaries of the
light path so as to avoid interfering with light going to the
screen.
[0012] During this process, the sensor sees full-scale light for
each color, which may be too bright for the sensor to accurately
measure given LED and optical variations that can exist. To correct
for this problem, the DMD can be used to modulate the light that
the sensor detects. So, for example, if the system controller
receives a signal from the sensor and determines that the sensor is
at or near full-scale (or otherwise outside an optimal operating
range of the sensor) during the blue-light reading, then the system
controller can command that a calibration image (modulation
pattern) be applied that reduces the intensity of blue light
striking the sensor. In some embodiments, a calibration image can
be applied by default rather than allowing full-scale brightness of
the LED light by default. The patterns for the calibration image
can be blue-noise patterns where the density of the pattern
determines the color intensity that the sensor sees. The pixels in
the pattern can be either always on or always off throughout the
blue light time (or red or green light time if the light of either
of those colors is too bright).
[0013] The sensor can be positioned such that the light reflected
from the DMD mirrors in their off-state is not focused directly on
the sensor. This way, the light from the blue-noise patterns will
appear as a single intensity over the entire sensitive area of the
sensor. In other words, the sensor will not see individual pixels,
so the pixel-to-pixel modulation will have no impact on the sensor
other than reducing the average brightness level according to the
density of the blue-noise pattern.
[0014] Different blue-noise patterns (e.g., densities) can be tried
by the control system until the sensor is in a desirable operating
range for determining the light intensities. The controller can
read and store sensor data as well as record the density of the
blue-noise pattern used for the intensity of light associated with
the sensor data. These two parameters can then be used to determine
the total relative intensity of light from the R, G, and B
LEDs.
[0015] Each display color intensity is adjusted until a prescribed
color balance is restored. For example, it is often desirable that
the ratio of lumens intensity that the display screen sees be set
to 0.2 R/0.7 G/0.1 B. This means that it is not always simply a
matter of achieving the same lumens for each color LED for proper
white balance. Depending on which color standard is used, the exact
ratio of colors can vary.
[0016] In some embodiments, the density of the blue-noise patterns
can be varied so that the sensor is driven to output a same
brightness signal (e.g., same voltage level) for each color. In
other embodiments, the density of the blue-noise patterns can be
varied just enough to where the sensor is operating within an
acceptable operating range, and the relative intensities of the
LEDs can be derived from the brightness signals from the sensor
along with the density of the blue-noise patterns used.
[0017] Also, in some embodiments, patterns other than blue-noise
patterns can be used for modulating the off-state light. For
example, other patterns can include white-noise patterns or
structured patterns. An example of a structured pattern can be a
checkerboard pattern for 50% density.
[0018] In some embodiments, the relative intensities of light from
the LEDs can be adjusted by increasing or decreasing electrical
current to the LEDs. Alternatively, the LED currents can be left
the same and the relative intensity of the appropriate colors can
be turned down electronically via signal processing. For example,
the P7, or "7 Primary" algorithm disclosed in U.S. Pat. No.
6,594,387 to Greg Pettitt, the contents of which are hereby
incorporated by reference, can be used to turn down the colors
needed by adjusting the electronic gain on the appropriate color
channels. A benefit of using the P7 algorithm is that P7 gains the
appropriate colors down for color points near white, but as the
colors approach saturation the gain is returned to its normal
setting. The result is, that unlike reduced actual drive current to
the LED, the LEDs can all remain at their full brightness for
saturated colors and near-saturated colors. Thus, colors near or at
R, G, B, Y, C, and M remain at the maximum brightness possible.
While this can result in some distortion in the relative intensity
of some color shades, many viewers of images prefer the brighter
colors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Embodiments are illustrated by way of example in the
accompanying figures, in which like reference numbers indicate
similar parts, and in which:
[0020] FIG. 1 shows a block diagram of a conventional DMD-based
display system;
[0021] FIG. 2 shows a perspective view of an array of DMD
mirrors;
[0022] FIG. 3 shows an embodiment of a spatial light modulation
(SLM) display system;
[0023] FIG. 4 shows a block diagram of control elements of a
display system, such as the one shown in FIG. 3;
[0024] FIG. 5 shows a block diagram of a system controller of a
display system, such as the one shown in FIGS. 3 and 4;
[0025] FIG. 6 shows a flowchart of a process for detecting and
adjusting color balance in a display system;
[0026] FIG. 7 shows a timing diagram for an alternative timing
scheme for the process of detecting and adjusting color balance;
and
[0027] FIG. 8 shows an alternative embodiment of an SLM display
system.
DETAILED DESCRIPTION
[0028] FIG. 3 shows an optical construction for a spatial light
modulation (SLM) display system 100. The display system 100
includes a first light source 102, a second light source 103, and a
third light source 104. The display system 100 also includes, along
an optical axis AX, an illumination optical system IL, a DMD 106,
and a projection optical system PL for projecting an image onto a
projection surface 107. The light sources 102-104 and the DMD 106
operate according to instructions received from a system controller
120 (shown in FIGS. 4 and 5).
[0029] The light sources 102-104 each can include a single light
emitting diode (LED) or an array of LEDs for emitting a respective
one of three primary colors. In the present embodiment, the first
light source 102 includes an LED array for emitting blue light, the
second light source 103 includes an LED array for emitting green
light, and the third light source 104 includes an LED array for
emitting red light. However, other colors and arrangements of
colors can be used.
[0030] The light radiated from the light sources 102-104 is
directed through the illumination optical system IL to the DMD 106.
The illumination optical system IL comprises a plurality of optical
elements for directing and smoothing the light from the light
sources 102-104.
[0031] The illumination optical system IL includes collimating
lenses 108-110 for collimating light from the light sources
102-104. Specifically, the blue light from the first light source
102 is collimated by a collimating lens 108,the green light from
the second light source 103 is collimated by a collimating lens
109, and the red light from the third light source 104 is
collimated by a collimating lens 110.
[0032] The illumination optical system IL also includes a pair of
filter elements 112 and 113. The first filter element 112 passes
the blue light and reflects the green light. The second filter
element 113 passes the blue and green light and reflects the red
light. In some embodiments, the filter elements 112 and 113 can be
optical elements having a dichroic surface for filtering.
[0033] As mentioned above, the illumination optical system IL
performs a function of smoothing the light from the light sources
102-104. Smoothing the light makes it possible to minimize the
difference in brightness between axial and off-axial rays on the
display surface of the DMD 106 (i.e., it is possible to make the
brightness distribution uniform). This smoothing of illumination
light is achieved by an integrator rod 115.
[0034] The illumination optical system IL further includes a relay
lens unit RL for relaying light from the integrator rod 115 to the
DMD 106. In the present embodiment, the relay lens unit RL includes
a first relay lens 117 and a second relay lens 118. However, those
skilled in the art will appreciated that other configurations and
combinations of optical elements can be used as needed for the
relay lens unit RL.
[0035] The DMD 106 includes an array of independently controllable
mirror elements, which together modulate the light received from
the illumination optical system IL and transmit the modulated light
to the projection optical system PL, where it can be focused for
display on the projection surface 107, such as a screen. More
specifically, the DMD 106 is so constructed that each of its mirror
elements is in one of two differently inclined states, namely
either in an ON state or in an OFF state. Only mirror elements in
their ON state direct the illumination light along the optical axis
ON shown in FIG. 3, towards the projection optical system PL for
display. The mirror elements in their OFF state direct the
illumination light along the optical axis OFF shown in FIG. 3, in a
non-display direction away from the projection optical system.
[0036] The light directed along the optical axis ON in the display
direction passes through the projection optical system PL and
eventually forms a display image on the projection surface 107. The
projection optical system PL can include any number of optical
elements for projecting the image light modulated by the DMD 106
onto the projection surface 107 located at a predetermined distance
or within a predetermined range of distances.
[0037] The light directed along the optical axis OFF in the
non-display direction is sensed by a light sensor 119. The light
sensor 119 is preferably located for sensing an intensity of light
directed from the DMD 106 in the non-display direction. However,
other locations for the light sensor 119 can be used. The light
sensor 119 outputs a brightness signal indicative of the intensity
of received light. In some embodiments, the brightness signal is a
voltage having a voltage level indicative of the intensity of
received light.
[0038] The light sensor 119 outputs the brightness signal to a
system controller as shown in FIG. 4, which shows an overview of
control elements of the display system 100. The system controller
120 also receives image data from a video source 122. The system
controller 120 outputs control signals to the light sources 102-104
and to the DMD 106 according to the image data received from the
video source 122. The system controller 120 controls the intensity
of the output of the light sources 102-104 according to brightness
signals received from the light sensor 119 based on a prescribed
color balance or white point.
[0039] FIG. 5 shows a more detailed view of the system controller
120. The system controller 120 includes a processing device 124
that receives image data from the video source 122. The image data
can be digital RGB or YUV video or graphics data. The controller
can convert YUV data into RGB data as needed for writing data to
the DMD. The image data can originate from any of a number of
devices, including a computer, a set-top box for cable or satellite
television, a television antenna or many other sources.
[0040] The processing device 124 can include circuitry for
processing the image data to put it in the proper format and/or to
otherwise modify characteristics of the image to be displayed.
Specific examples of circuitry that can be included in the
processing device 124 include gamma circuitry, color hue correction
circuitry, blue-noise spatial temporal multiplexing (STM)
circuitry, and noise-free boundary dispersion circuitry. If the
video source 122 provides analog image data, then the processing
device 124 can also include an analog-to-digital converter. The
processing device 124 can also include data arrangement circuitry
for arranging the pixel data into proper patterns to be displayed
by the DMD 106.
[0041] The graphics RAM 126 comprises a memory capable of storing
one or more complete video frames. The video frames may be stored,
for example, in the graphics RAM 126 as a set of bit-planes. For
example, where the pixel data has an 8-bit per color format, the
data can be stored as 24 bit-planes. Each bit plane corresponds to
one bit of an eight bit value representing the intensity of one of
the three primary colors of light. Because there are three
eight-bit values, 24 bit planes are stored. Alternative forms of
storage are readily contemplated and the pixel data can
alternatively be stored as groups of bits, rather than as
individual bit planes. The graphics RAM 126 can be a dynamic random
access memory array, for example a double data rate synchronous
random access memory. The graphics RAM 126 can additionally serve
as a general purpose processing RAM.
[0042] At the appropriate time, the processing device 124 fetches
output video pixel data stored in the graphics RAM 126, reformats
the pixel data as is appropriate for DMD data, and then transfers
the data to the DMD 106 for display. As explained below, the light
sources 102-104 are controlled to provide the blue, green, and red
light, respectively, to the DMD 106, allowing the DMD 106 to
produce a color video image composed of a plurality of colors
created by combining the primary colors of light supplied by the
light sources 102-104.
[0043] The processing device 124 controls the timing of transfers
of output video pixel data to the DMD 106, the location in the
graphics RAM 126 where the data is transferred from, the position
on the DMD 106 where the output pixel data is displayed, and the
timing necessary to display the data. The processing device 124
also provides instructional color data to an LED controller 130 for
controlling activation of the light sources 102-104 according to
colors associated with the output video pixel data. The processing
device 124 generates timing information based on the image data
received from the video source 122 with a frequency equivalent to
the frequency of the video frame that is to be displayed on the DMD
106. The processing device 124 uses this timing information for
controlling the transfer of video frames from the graphics memory
126 to the DMD 106. The sequence of timing instructions needed to
generate the addresses and timing signals necessary to display an
entire frame of video data can be stored in a timing memory (not
shown) internal to the processing device 124, or alternatively in a
separate external memory such as a Flash memory. The processing
device 124 outputs instructional color data for activation of the
light sources 102-104 sequentially in time, activating each color
for a portion of each frame. For example, the light sources 102-104
can be activated such that the red light source 104 is first
activated for 14% of the frame period, then the green light source
103 is activated for 60% of the frame period, and then the blue
light source is activated for the remaining 26% of the frame
period. It should be noted that this is just one of many possible
sequences and percentages that can be used.
[0044] The LED controller 130 includes logic for decoding the
instructional color data received from the processing device 124.
The LED controller 130 provides control signals for activating the
light sources 102-104 according to the instructional color
data.
[0045] The processing device 124 receives brightness data from the
sensor 119. In some embodiments, an optional analog-to-digital
converter 128 can be used to convert analog brightness signals from
the sensor 119 to digital brightness data. The processing device
124 receives the brightness data and performs a process for
detecting a shift in color balance as discussed in greater detail
below in connection with FIG. 6. In some embodiments, the
processing device 124 can then issue instructional data to the LED
controller 130 to adjust the current supplied to the light sources
102-104 as necessary to restore proper white balance. Another
method of adjusting the power would be to alter the duty cycles to
the LEDs while holding the current level to each LED constant. In
other embodiments, the processing device can use the P7, or "7
Primary" algorithm discussed above to turn down the colors as
needed by adjusting the electronic gain on the appropriate color
channels.
[0046] The processing device 124 can detect whether the sensor 119
is operating within a specified operating range based on the
brightness signal output from the sensor 119. For example, if the
light detected by the sensor 119 is too bright, the sensor 119
output is clipped. For other reasons discussed below, it can be
desirable for the sensor. 119 to output brightness signals having a
specific value or having a value that is within a specified range
of values. In any case, if the output of the sensor 119 is outside
of a desired range, the processing device 124 can control the DMD
106 to modulate the light being directed towards the sensor (along
the optical axis OFF in the embodiment shown in FIGS. 3 and 4). It
should be noted that, in some embodiments, a calibration image can
be applied by default rather than allowing full-scale brightness of
the LED light by default. By modulating the light, the brightness
of the light can be controlled so as to control the intensity of
light striking the sensor. The patterns for the calibration image
are preferably blue-noise patterns where the density of the pattern
determines the intensity of light directed in the direction of the
sensor 119. Alternatively, patterns other than blue-noise patterns
can be used for modulating the light. For example, other patterns
can include white-noise patterns or structured patterns. An example
of a structured pattern can be a checkerboard pattern for 50%
density.
[0047] The system controller 120 has been illustrated to include
four functional blocks. It is understood, however, that the
delineation of particular functions is somewhat arbitrary and that
each of these functions could be performed in one or more different
integrated circuits that operate according to circuit design and/or
software control. The functional blocks are labeled here for
purposes of illustration and several of the functions can be
combined of separated in various circuits or other functional
units. The system controller 120 is not limited to use with the
display system 100; rather it can be used with a variety of
illumination systems, particularly those including or one or more
light source(s) and one or more spatial modulation device(s). For
example, it is contemplated that a single light source could be
used to selectively emit two or more different colors of light.
[0048] FIG. 6 shows an example of a process that the processing
device can perform for detecting a shift in color balance. The
process shown in FIG. 6 is but one of several possible embodiments
and can be modified in many ways without departing from the
concepts being conveyed.
[0049] The process shown in FIG. 6 is initiated at step S10. This
process can be initiated at system startup, system shutdown, every
N startups or shutdowns, can be invoked by a user (e.g., in
response to a user command) or system command at any time. Since an
entire color time is used each time the sensor is read, the display
screen will be disturbed. Therefore, in some embodiments the LED
white point correction process can be run only at power down when
brief flashes to the display screen are allowable. The sensor data
collected can then be used at the next power-up. In such
embodiments, provisions can be made to account for a change in room
temperature at t,he next power-up. A change in room temperature can
affect the accuracy of the sensor data collected at the last power
down since the brightness of an LED can be affected by temperature.
Appropriate correction can be made by including a temperature
sensor in the system and reading temperature data any time the LED
white point correction is to be applied. Any brightness errors due
to temperature can be predicted by using a look-up-table in the
processing device that correlates LED brightness loss to room
temperature. This then allows the sensor data gathered at power
down to still be used with very accurate results.
[0050] At step S12, COLOR is initialized to "1". The value of COLOR
dictates which of the colors (e.g., R, G, or B) is to be directed
along the optical axis OFF in the non-display direction. For
example, COLOR=1 can correspond to red light, COLOR=2 can
correspond to green light, and COLOR=3 can correspond to blue
light. Thus, in the case of COLOR=1, red light will be directed
along the optical axis OFF in the non-display direction, while
green and blue light is directed along the optical axis ON in the
display direction.
[0051] At step S14, MOD_PATT is initialized to "0". The value of
MOD_PATT dictates the modulation pattern to be used by the DMD 106
for modulation of the off-state light. For example, MOD_PATT=0 can
correspond to no modulation of the off-state light. In this case,
the sensor 119 sees full-scale light for each color, which may be
too bright for the sensor 119 to accurately measure. Alternatively,
MOD_PATT=0 can correspond to any modulation pattern or density. To
correct for this problem, the value for MOD_PATT can be changed
causing the DMD 106 to modulate the light that is directed towards
the sensor 119 as discussed in greater detail below.
[0052] At step S16, the light sources 102-104 are sequentially
activated. The DMD 106 is controlled to direct light in the
non-display direction while the light source of light sources
102-104 emitting a color corresponding to the present value of
COLOR is activated. Otherwise, the DMD 106 is controlled to direct
light in the display direction. For example, for COLOR=1
corresponding to R, the DMD 106 is controlled to direct light in
the non-display direction while the red light source 104 is
activated and the DMD 106 is controlled to direct light in the
display direction while the blue and green light sources 102-103
are activated.
[0053] At step S18, the processing device 124 detects a brightness
value based on a brightness signal output from the sensor 119. In a
preferred embodiment, the sensor 119 outputs a voltage having a
voltage level Vout that varies linearly based on the brightness of
the sensed light. The voltage from the sensor 119, therefore,
constitutes a brightness signal. The analog-to-digital converter
128 receives the voltage Vout from the sensor 119, and converts it
to a digital brightness signal. The analog-to-digital converter 128
then forwards the digital brightness signal to the processing
device 124. It will be appreciated that the analog-to-digital
converter. 128 is optional, for example in cases where the sensor
119 outputs a digital signal rather than an analog signal.
[0054] At step S20, a determination is made as to whether the
brightness value received from the sensor 119 is acceptable. This
can ensure that an output of the sensor 119 is at least within an
acceptable operating range (e.g., not clipped). In some
embodiments, step S20 includes checking whether the brightness
value is within a specified operating range of the sensor 119. In
some embodiments, depending on how the relative intensities are
calculated at step S26, step S20 can include checking whether the
brightness value is equal to a specified value. If the brightness
value is not acceptable ("NO" at step S20), the process continues
to step S21. Otherwise, if the brightness value is acceptable
("YES" at step S20, the process continues to step S22.
[0055] At step S21, a value of MOD_PATT is incremented. In other
words, the modulation pattern to be used by the DMD 106 for
modulation of the off-state light is changed. If the brightness
value received at step S18 is determined at step S20 to be
representative of light that is too bright, then MOD_PATT can be
changed to a value associated with a modulation pattern that will
reduce the amount of light that the DMD 106 directs in the
non-display direction. For example, MOD_PATT can be changed from
MOD_PATT=0, where none of the light associated with the present
value of COLOR is modulated away from the non-display direction, to
an alternative value for MOD_PATT where some amount of the light
associated with the present value of COLOR is modulated away from
the non-display direction. It will be appreciated that the value of
MOD_PATT can be any value for which a modulation pattern or
calibration image can be used to achieve the desired modulation
density. For example, in some embodiments the modulation patterns
can include blue-noise patterns that can be controlled to vary
amoung 256 grayscale levels (e.g., 0=black, 255=full-scale). The
sequence of steps from step S16 to step S21 continue to be repeated
until the brightness value at step S20 is determined to be a
specified value or within a specified range of values.
[0056] At step S22, the brightness value determined to be the
specified value or within the specified range of values at step
S20, as well as the value of MOD_PATT associated with the
brightness value, are stored for later use. These values of
brightness and MOD_PATT are stored such that they are associated
with the present value of COLOR. Specifically, the brightness and
MOD_PATT values will be stored for each value of COLOR and used at
step S26 for calculation of the relative intensities of the colors
of light.
[0057] At step S24, a determination is made as to whether steps
S16, S18, S20, and S22 have been performed for light associated
with each color. Specifically, the value of COLOR is checked. If
the value of COLOR does not equal "3" ("NO" at step S24), then the
process continues to step S25 where the value of COLOR is
incremented (e.g., from COLOR=1 to COLOR=2 or from COLOR=2 to
COLOR=3) and steps S16, S18, S20, and S22 (and S21 if necessary)
are repeated. Otherwise, if the value of COLOR equals "3" ("YES" at
step S24), then the process continues to step S26.
[0058] At step S26, the processing device 124 calculates the
relative intensities of the colors of light using the brightness
values and MOD_PATT values stored at step S22. In some embodiments,
the density of the modulation patterns is varied during steps S
16-S20 until the sensor is driven to a prescribed brightness signal
value (e.g., same voltage level) for each color, in which case the
relative intensities can be determined directly from the relative
modulation densities necessary to achieve a same brightness signal
value. In other embodiments, the density of the modulation patterns
is varied just enough to where the sensor 119 is operating within
an acceptable operating range for steps S16-S20, in which case step
S26 can include deriving the relative modulation densities at which
a same brightness signal value could be expected.
[0059] An example of the calculations performed in step S26 will
now be described for illustration purposes. In an exemplary
embodiment, the sensor 119 has a linear output (where the Vout is
directly proportional to the intensity of sensed light) with a
valid output range of 0.1V-0.9V, most accurate in a range of
0.4V-0.6V, and an ideal output being 0.5V. The sensor 119 output
varies depending on the wavelength of color being sensed according
to a color balance ratio of 1.0 R/0.8 G/0.6 B. The modulation
patterns are blue-noise patterns that vary among 256 grayscale
levels (0=black, 255=full-scale).
[0060] Steps S16-S22 are performed for each of red, blue, and green
light yielding the following results. For red light, a first pass
(through steps S16-S21) using a blue-noise pattern at full-scale
results in an output of 1.0V from the sensor 119, which is just at
the edge of clipping. Thus, a second pass for red is performed
using a blue-noise pattern at 25% density (allowing only 25% of the
red light to be directed in the non-display direction) resulting in
an output of 0.25V from the sensor 119. For green light, a first
pass using a blue-noise pattern at full-scale results in a clipped
output from the sensor 119. Thus, a second pass for green is
performed using a blue-noise pattern at 25% density, which results
in an output of 0.75V from the sensor 119. For blue light, a first
pass using a blue-noise pattern at full-scale results in an output
of 0.55V. Thus, a second pass for blue light is not necessary.
Thus, at step S22, data for each color of light is stored as
follows: TABLE-US-00001 Color Brightness signal (Vout) Modulation
density Red 0.25 V 25% Green 0.75 V 25% Blue 0.55 V 100%
[0061] Next, at step S26, the processing device 124 uses the data
stored in step S22 to calculate the present (uncorrected) color
balance. In this example, it is desired that the color balance for
the display system 100 be maintained at two parts red, seven parts
green, and three parts blue (0.20 R/0.70 G/0.30 B). First, the
processing device 124 calculates a modulation density for each of
the colors of light at which the output of the sensor 119 could be
expected to be approximately 0.5V (e.g. 0.5V.+-.0.01V). The
calculation is made for each color based on simple ratios as
follows: R: D.sub.R/25%=0.5V/0.25V.fwdarw.D.sub.R=50% G:
D.sub.G/25%=0.5V/0.75V.fwdarw.D.sub.G=16.67% B:
D.sub.B/100%=0.5V/0.55V.fwdarw.D.sub.B=91% where D.sub.R, D.sub.G,
and D.sub.B are the respective densities at which an output of the
sensor 119 could be expected to be approximately 0.5V. In other
embodiments, the steps S16-S21 can be repeated until the desired
output of the sensor 119 is achieved rather than performing the
above calculation.
[0062] In this example, the sensor 119 output varies depending on
the wavelength of color being sensed according to a color balance
ratio of 1.0 R/0.8 G/0.6 B. Thus, a calculation of the present
color balance ratios must account for this sensor characteristic.
The lumens ratio for the present color balance can be expressed as
L.sub.R/L.sub.G/L.sub.B where L.sub.R is the percentage of red,
L.sub.G is the percentage of green, and L.sub.B is the percentage
of blue. Each component L.sub..lamda. (where .lamda.=R, G, or B) of
the lumens ratio can be calculated based on the following
expression: L .lamda. = ( 1 D .lamda. ) .times. ( 1 L S .times.
.times. .lamda. ) ##EQU1## where L.sub.s.lamda. is the
corresponding component of the color balance ratio of the sensor
119. Accordingly, the present color-balance ratios can be
calculated as follows: L R = ( 1 0.50 ) .times. ( 1 1.0 ) .apprxeq.
2.00 ##EQU2## L G = ( 1 0.1667 ) .times. ( 1 0.8 ) .apprxeq. 7.50
##EQU2.2## L B = ( 1 0.91 ) .times. ( 1 0.6 ) .apprxeq. 1.83
##EQU2.3## The above-calculated values are then normalized for 0.70
G (recall that, in this example, the target color balance ratio is
0.20 R/0.70 G/0.30 B) giving a present color balance ratio of 0.187
R/0.700 G/0.171 B. Thus, in this example, the present color balance
ratio requires correction.
[0063] Referring once again to FIG. 6, once the present color
balance ratio has been calculated at step S26, the process
continues to step S28. At step S28, a determination is made as to
whether the present color balance ratio is acceptable. If not, as
in the above example, the process continues to step S30 where the
color balance is adjusted. Otherwise, the process ends at step
S32.
[0064] At step S30, the relative intensities of the light colors R,
G, and B can be adjusted according to any known method. In some
embodiments, the relative intensities of light from the LEDs can be
adjusted by increasing or decreasing electrical current to the
LEDs. Alternatively, the LED currents can be left the same and the
relative intensity of the appropriate colors can be turned down
electronically via signal processing. For example, the P7, or "7
Primary" algorithm discussed above can be used to turn down the
colors needed by adjusting the electronic gain on the appropriate
color channels.
[0065] In the example above, where the uncorrected color balance
was calculated to be a ratio of 0.187 R/0.700 G/0.171 B, the gains
required for color balance correction can be calculated as follows.
First the weakest color is selected. In this example, R is weakest,
G is second weakest, and B is strongest. Next, the second weakest
color (G) is reduced by a certain gain amount until the ratio of
the two weakest colors (R and G) is corrected. Thus, the gain for G
can be calculated as follows: GAIN G = L NR L TR = 0.187 0.200
.apprxeq. 0.935 ##EQU3## where L.sub.NR is the red component of the
normalized, uncorrected color balance ratio, and L.sub.TR is the
red component of the target color balance ratio. The gain
GAIN.sub.G is then used to calculate the gain for B that will
achieve the target color balance: GAIN B = GAIN G .times. L TB L NB
= 0.935 .times. 0.10 0.171 .apprxeq. 0.547 ##EQU4## where L.sub.NB
is the blue component of the normalized, uncorrected color balance
ratio, and L.sub.TB is the blue component of the target color
balance ratio. The processing device 124 can now make adjustments
to the G and B color light sources according to the above
calculated gains GAIN.sub.G and GAIN.sub.B, respectively. Each
component L.sub.C.lamda. (where .lamda.=R, G, or B) of the
corrected lumens ratio can be calculated as follows: L.sub.CR=0.187
L.sub.CG=GAIN.sub.G=L.sub.NR=0.935.times.0.700.apprxeq.0.655
L.sub.CB=GAIN.sub.B.times.L.sub.NB=0.547.times.0.171.apprxeq.0.094
The above-calculated values are then normalized once again for 0.70
G (recall that, in this example, the target color balance ratio is
0.20 R/0.70 G/0.30 B) giving a corrected color balance ratio of
0.20 R/0.70 G/0.10 B. Thus, the color balance ratio can been
corrected to the desired target ratio by adjusting the outputs of
the green and blue light sources 102 and 103 according to the
above-calculated gain values GAIN.sub.G and GAIN.sub.B,
respectively.
[0066] FIG. 7 shows a timing diagram illustrating a variant of the
timing to apply the light to the sensor 119. As shown in the timing
diagram, LEDs of red, green, and blue are sequentially turned on
for a certain color time and then turned off according to
respective signals R_EN, G_E, and B_EN. Instead of directing the
light from these LEDs to the sensor 119 throughout a complete color
time, the lightcan be directed just briefly to it, for example at
the end of a color time as shown by the sensing interval (square
portion) 170 of the off-state color pulses. In this case, the
sensing interval 170 can be relatively short compared to the color
time, for example in a range of 10 us to 20 us, for example 20 us.
The advantage of this is that the displayed image has almost no
alteration. The sensing interval 170 can be a dedicated interval
that always exists even if the LED calibration process is not being
performed. Thus, the LED white point correction can be run
real-time, for example over a span of 10 seconds. This is desirable
because any temperature drifts of the system, which might impact
LED brightness, can be corrected, and no look-up tables are needed
to correct for brightness drifts with temperature.
[0067] Blue-noise, or other modulation patterns, can still be used
during the sensing interval 170. If a 100% density pattern is used,
there is then no impact to the displayed image. If <100%
patterns are used, this will mean that some light is directed to
the display screen during the sensing interval 170, which will
slightly affect the displayed image. However, this is still much
less of a disturbance to the displayed image than allocating an
entire color time to collect sensor data.
[0068] It should be noted that the above-described system
controller 120 and method of detecting and correcting a shift in
color balance can be used in conjunction with display systems other
than display system 100. For example, FIG. 8 shows an optical
construction for an alternative spatial light modulation (SLM)
display system 150. The display system 150 includes a first light
source 152 and a second light source 153. The display system 150
also includes, along an optical axis AX, an illumination optical
system IL, a DMD 106, and a projection optical system PL for
receiving light directed in a display direction (along the optical
axis ON) and projecting the light onto a projection surface 107 for
display. The display system 150 also includes a sensor 119 for
sensing light directed in an non-display direction (along the
optical axis OFF) by the DMD 106. The sensor 119 outputs brightness
signals to the system controller 120, and the light sources 152,
153 and the DMD 106 operate according to instructions received from
the system controller 120, as discussed above.
[0069] Differences between the alternative display device 150 and
the display device 100 include a difference in the number of light
sources and a variation in the optics used for the relay lens unit
RL. Specifically, the alternative display device 150 includes a
first light source 152 for emitting red or blue light (e.g., an
array of light elements including red and blue light-emitting
elements such as an R/B LED package) and a second light source 153
that can be controlled to emit green light. As a result, the
filtering characteristics of the filter element 155 allows for
passing light in the red and blue color ranges, and reflecting
light in the green color range. Also, in the alternative display
device 150 the relay lens unit RL includes optical elements for
changing a direction of the optical axis AX, for example through
the use of prism and/or mirror elements according to known
techniques. Thus, it will be appreciated that the system controller
120 used in conjunction with the alternative display device 150 is
adapted to send control signals to two light sources rather than
three.
[0070] Still further embodiments can include display systems that
utilize color-light emitting light sources, such as LEDs, and
multiple spatial light modulators. For example, U.S. Pat. No.
6,587,159 to Dewald, the contents of which are hereby incorporated
by reference, discloses a projection system that includes three
DMDs, where each DMD can be used for modulating a respective
primary color. In some embodiments, such a three-DMD arrangement
can be used in combination with color-light emitting light sources,
for example red, green, and blue LEDs. In
multiple-spatial-light-modulator embodiments, one or more sensors
can be used for detecting and adjusting color balance. For example,
three sensors can be used, one for each color of light. As another
example, one sensor can be positioned such that it can receive
light from multiple spatial light modulators, for example placed in
a dump light path of a prism that three DMDs are mounted to. In
some embodiments, light can be applied to each DMD all the time (DC
operation of LEDs) during normal operation, for example such that
red light is constantly applied to a first DMD, blue light to a
second DMD, and green light to a third DMD. In this case, the
sensor would sense all three colors. So, in this case the
color-balance detection process can include cycling the LEDs off
two at a time so that the sensorsees just one color at a time.
Another option is to use a sensor in combination with R, G, and B
miniature dichroic filters, such that separate R, G, and B sensors
are essentially combined in one package.
[0071] While various embodiments in accordance with the principles
disclosed herein have been described above, it should be understood
that they have been presented by way of example only, and are not
limiting. Thus, the breadth and scope of the invention(s) should
not be limited by any of the above-described exemplary embodiments,
but should be defined only in accordance with the claims and their
equivalents issuing from this disclosure. Furthermore, the above
advantages and features are provided in described embodiments, but
shall not limit the application of such issued claims to processes
and structures accomplishing any or all of the above
advantages.
[0072] Additionally, the section headings herein are provided for
consistency with the suggestions under 37 CFR 1.77 or otherwise to
provide organizational cues. These headings shall not limit or
characterize the invention(s) set out in any claims that may issue
from this disclosure. Specifically and by way of example, although
the headings refer to a "Technical Field," such claims should not
be limited by the language chosen under this heading to describe
the so-called technical field. Further, a description of a
technology in the "Background" is not to be construed as an
admission that technology is prior art to any invention(s) in this
disclosure. Neither is the "Brief Summary" to be considered as a
characterization of the invention(s) set forth in issued claims.
Furthermore, any reference in this disclosure to "invention" in the
singular should not be used to argue that there is only a single
point of novelty in this disclosure. Multiple inventions may be set
forth according to the limitations of the multiple claims issuing
from this disclosure, and such claims accordingly define the
invention(s), and their equivalents, that are protected thereby. In
all instances, the scope of such claims shall be considered on
their own merits in light of this disclosure, but should not be
constrained by the headings set forth herein.
* * * * *