Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-01-29
2001-11-06
Saras, Steven (Department: 2625)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000
Reexamination Certificate
active
06313820
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ferroelectric liquid crystal-based spatial light modulators such as those used in video displays and in particular relates to such spatial light modulators operated in a Non-DC-balanced mode.
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 based on a ferroelectric liquid crystal (FLC) material.
FLC-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 FLC material sandwiched between two transparent electrodes. The FLC material is preferably a surface-stabilized FLC material. 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 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 grey scale required for conventional display devices, the apparent brightness of each pixel is varied 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.
Ferroelectric liquid crystal-based spatial light modulators (both transmissive and reflective) 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 passing through it, the DC balance of the pixel must be restored or a condition called “pixel sticking” will eventually result. Pixel sticking is a condition where a pixel will not change states despite a change in the direction of an electric field applied between a pixel electrode and the other electrode. Mild pixel sticking is comparable to ghost images sometimes seen on CRT screens that have had one image displayed for too long.
When operated in the DC-balanced mode, pixel sticking is not problematic. This is 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 having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal 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 so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. 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. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
FIG. 1A
shows part of a display device incorporating a conventional reflective light valve
10
that includes the reflective 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 efficiency of which may be improved using a reflector
22
and collector optics
24
that concentrate the light towards the polarizer
14
. The light output by the light valve passes to the output optics
26
that focus the light to form an image (not shown). The light valve, light source (including reflector and collector optics) and output 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
passes through the polarizer
14
. The polarizer polarizes the light output from the light source. The beam splitter
16
reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator
12
. The spatial light modulator is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve. The beam splitter transmits a fraction of the light reflected by the spatial light modulator to the analyzer
18
.
The direction of an electric field in each pixel of the spatial light modulator
12
determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90° relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modula
Butterworth Mark M.
Helbing Rene P.
Alphonse Fritz
Hewlett-Packard Co.
Saras Steven
LandOfFree
Method of operating a ferroelectric liquid crystal spatial... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of operating a ferroelectric liquid crystal spatial..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of operating a ferroelectric liquid crystal spatial... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2574636