Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Optical excitation
Reexamination Certificate
1999-10-28
2003-02-11
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Optical excitation
C345S101000, C345S094000, C345S097000, C345S102000, C349S072000, C349S161000
Reexamination Certificate
active
06519012
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to liquid crystal-based light valve systems such as those used in video displays and in particular relates to contrast control in such light valve systems.
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 device at are small enough to be integrated into a helmet or a pair of glasses so that they an 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 replacement for the conventional cathode-ray tube (CRT) 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 Crevices that incorporate a light valve system that uses as its light control element one or more spatial light modulators based on liquid crystal material.
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 is preferably ferroelectric 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 spatial light modulators are similar in construction to transmissive 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 signal processing electronics that derive the drive signals for the pixel electrodes from the input video signal. A reflective spatial light modulator has the advantage that its pixel electrode drive circuits do not partially occlude the light modulated by the pixel. This enables a reflective spatial light modulator to have a greater light throughput than a similar-sized transmissive spatial light modulator and allows larger and more sophisticated drive circuits as well as the signal processing electronics 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) determines 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 substantially 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. Polarization analyzers are less than 100 percent efficient, however, absorbing a fraction of the light that should be transmitted in the 1 state and transmitting a fraction of the light that should be absorbed in the 0 state. The ratio of the intensity of light transmitted in the 1 state to the intensity of light transmitted in the 0 state is known as the “contrast ratio” or simply as “contrast.” A contrast ratio of at least 100:1 is required for excellent image quality and is usually found in CRT based displays.
To produce the grayscale required for conventional display devices, the apparent brightness of each pixel is varied. In nematic liquid crystal-based spatial light modulators, grayscale is achieved by changing the voltage of the drive pulse. Ferroelectric liquid crystal-based spatial light modulators, however, are digital devices switching between the 1 state and the 0 state almost independent of drive voltage. Grayscale in ferroelectric liquid crystal-based spatial light modulators is therefore achieved by temporally modulating the light transmitted 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.
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 occurs 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.
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 with a color separator. Typically three spatial light modulators are used, one illuminated with red light, one with blue light, and one with green light. Each of the spatial light modulators modulates the colored light that illuminates it to form a single-colored image, and the single-colored images from each of the spatial light modulators are combined into a single full-color image.
FIG. 1
shows part of a conventional display device
5
incorporating a conventional reflective light valve
10
that includes the reflective ferroelectric liquid crystal-based spatial light modulator
12
. Other principal components of the light valve are the polarizer
14
, the beam splitter
16
and the analyzer
18
. The light valve is illuminated with light from the light source
20
, the light from which is concentrated on the polarizer using a reflector
22
and collector optics
24
. The light output by the light valve passes to the imaging optics
26
that focus the light to form an image (not shown). The light valve
10
, light source
20
and imaging optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source
20
enters the light valve
10
by passing through the polarizer
14
. The polarizer polarizes the light output from the
Helbing Rene P.
Kuramoto Akinobu
Owen Geraint
Chung David
Hewlett--Packard Company
Parker Kenneth
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