Optical: systems and elements – Optical modulator
Reexamination Certificate
1999-07-29
2001-07-17
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
C359S315000, C359S566000, C359S888000
Reexamination Certificate
active
06262829
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for digital grayscale control in light valves such as those used in color video displays and in particular relates to methods of illuminating the light valve to provide improved grayscale control.
BACKGROUND OF THE INVENTION
A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a spatial light modulator. Spatial light modulators are typically based on liquid crystal material as described in U.S. Pat. No. 4,813,771, entitled “Electro-Optic Switching devices using Ferroelectric Liquid Crystals,” but may also be based on arrays of moveable mirrors as described in U.S. Pat. No. 4,954,789, entitled “Spatial Light Modulator.”
Liquid crystal-based spatial light modulators are available in either a transmissive form or in a reflective form. The transmissive spatial light modulator is composed of a layer of a liquid crystal material sandwiched between two transparent electrodes. The liquid crystal material can be either ferroelectric or nematic type. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the transmissive spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the transmissive spatial light modulator rotates the direction of polarization of light falling on the pixel. The transmissive spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90° so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
Reflective liquid crystal-based spatial light modulators are similar in construction to transmissive liquid crystal-based spatial light modulators, but use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, reflective spatial light modulators can be built on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. A reflective light valve has the advantage that its pixel electrode drive circuits do not partially include the light modulated by the pixel. This enables a reflective light valve to have a greater light throughput than a similar-sized transmissive light valve and allows larger and more sophisticated drive circuits to be incorporated.
As with the transmissive spatial light modulators, the direction of an electric field (in this case between the transparent electrode and the reflective electrode) determined whether or not the corresponding pixel of the reflective spatial light modulator rotates through 90° the direction of polarization of the light falling on (and reflected by) the pixel. Thus, the polarized light reflected by the pixels of the reflective spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
The resulting optical characteristics of each pixel of both the transmissive and reflective liquid crystal-based spatial light modulators are binary: each pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
Spatial light modulators based on arrays of moveable mirrors are typically arranged so that the mirror of each pixel has a resting position and a powered position. The resting position is the position the mirror takes when its control mechanism is unpowered. The powered position is the position the mirror takes when power is applied to its control system. When the mirror is in one of the resting position or powered position, it is configured so that light illuminating the mirror is reflected towards an output. In the other position, the mirror is configured so that light illuminating the mirror is reflected away from the output. The resulting optical characteristics of each pixel of moving mirror-based spatial light modulators are binary: each pixel either reflects light toward the output (its 1 state) or away from the output (its 0 state), and therefore appears light or dark, depending on the power condition to the control mechanism.
To produce the grayscale required for conventional display devices with either liquid crystal-based or moving mirror-bases spatial light modulators, several techniques are known in the art. These including time domain grayscale control, light source intensity grayscale control, and a hybrid of time domain and light source intensity domain grayscale control.
With time domain grayscale control, the apparent brightness of each pixel is varied by temporally modulating the 0 state and 1 state of each pixel. The level of gray is controlled by defining a basic time period that will be called the frame period of the spatial light modulator and controlling the duration of the 1 state relative to the duration of the 0 state during the frame period. This determines the apparent brightness, or grayscale, of the pixel.
With time domain control, the frame period for a given pixel is typically divided into time elements associated with a binary weighted value.
FIG. 1
depicts time domain control for a pixel given 4-bit grayscale data, corresponding to 16 levels of gray. The figure is a graph with relative light intensity shown on the Y-axis, and time in terms of a time period shown on the X-axis. The light source is depicted as having a constant relative intensity of {fraction (15/4)}ths. The frame period is divided into four time slices A, B, C and D, the relative duration of each period corresponding to the relative value of each of the four digits in a four-digit binary number, such as 1111. Thus, the relative durations correspond to the relative values of the binary numbers 1000, 0100, 0010, and 0001 or their decimal equivalents 8, 4, 2, and 1. Since the sum of these numbers is 15, the four time slices A-D have durations of {fraction (8/15)}, {fraction (4/15)}, {fraction (2/15)}, and {fraction (1/15)} of a frame period, respectively.
By selectively setting the pixel to either its 1 state or its 0 state during each of the four time slices, any of the 16 levels of gray can be selected as is shown in the following
TABLE 1
Decimal
Binary
Pixel State
Pixel State
Pixel State
Pixel State
Grayscale
Grayscale
First
Second
Third
Fourth
Level
Level
Period
Period
Period
Period
0 (black)
0000
0
0
0
0
1
0001
0
0
0
1
2
0010
0
0
1
0
3
0011
0
0
1
1
4
0100
0
1
0
0
5
0101
0
1
0
1
6
0110
0
1
1
0
7
0111
0
1
1
1
8
1000
1
0
0
0
9
1001
1
0
0
1
10
1010
1
0
1
0
11
1011
1
0
1
1
12
1100
1
1
0
0
13
1101
1
1
0
1
14
1110
1
1
1
0
15 (white)
1111
1
1
1
1
In practice, the frame period duration may be about {fraction (1/60)} second (approximately 16,640 &mgr;sec) which corresponds to a refresh rate of 60 Hz typically found in computer displays. In addition, grayscale is more typically defined by 8-bits of data than 4-bits of data allowing 256 levels of gray to be defined instead of 16 levels of gray. Using the time domain grayscale control as just described, the frame pe
Helbing Rene P.
Kuramoto Akinobu
Epps Georgia
Hewlett-Packard Co.
Thompson Tim
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