Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-07-29
2002-04-23
Zimmerman, Mark (Department: 2671)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S048000, C345S063000, C345S084000, C345S097000
Reexamination Certificate
active
06377236
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for illuminating light valves such as those used in color video displays and in particular relates to methods of illuminating such light valves with improved throughput and adjustable color balance.
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, but may also be based on arrays of moveable mirrors.
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 occlude 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) 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 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.
To produce the grayscale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted/reflected by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light either transmitted/reflected by it, the DC balance of the pixel must be restored. This is typically done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal (reverse representation) having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal (positive representation) during the illumination period. The illumination period and the balance period collectively constitute a display period.
To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated, either directly or with a shutter, so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period, as depicted in FIG.
1
. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness for ferroelectric liquid crystal-based spatial light modulators. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
To produce color output required for conventional display devices, a single spatial light modulator may be used or multiple spatial light modulators may be used. In order to produce a color output from a single spatial light modulator, the spatial light modulator is illuminated sequentially with light of different colors, typically red, blue, and green. This sequential illumination may be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a “white” light source with sequential color filtering. For purposes of this description a “white” light source is one that emits light over a broad portion of the visible light spectrum. In either case, each of the sequential colors is modulated individually by the spatial light modulator to produce three sequential single-color images. If the sequence of single-color images occur quickly enough, a viewer of the sequential single-color images will be unable to distinguish the sequential single-color images from a full-color image.
When the single spatial light modulator used to produce color output is a ferroelectric liquid crystal-based spatial light modulator, DC balance must be restored, as previously discussed. Typically, DC balance is restored after each of the sequential colored illuminations as depicted in FIG.
2
. Modulating the light source in this manner reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary.
To produce color output using multiple spatial light modulators, each of the spatial light modulators is simultaneously illuminated with a different colored light. This can be accomplished using multiple light sources, each having one of the desired illumination colors, or by using a “white” light source wi
Hewlett--Packard Company
Monestime Mackly
Zimmerman Mark
LandOfFree
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