Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-10-29
2001-06-05
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C349S018000, C349S200000, C359S634000
Reexamination Certificate
active
06243065
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to reflective ferroelectric liquid crystal-based light valves such as those used in video displays and in particular relates to such light valves having a substantially increased light throughput.
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 surface-stabilized ferroelectric liquid crystal (SSFLC) material.
A SSFLC-based spatial light modulator is composed of a layer of a SSFLC material sandwiched between two transparent electrodes. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the 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 spatial light modulator rotates the direction of polarization of light falling on the pixel. The 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. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the 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 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. 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.
Recently, a need for reflective light valves based on reflective spatial light modulators has arisen. Reflective spatial light modulators use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, such 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.
FIG. 1
A shows part of a display device incorporating a conventional reflective light valve
10
that includes the reflective spatial light modulator
25
. Other principal components of the light valve are the polarizer
17
, the beam splitter
19
and the analyzer
21
. The light valve is illuminated with light from the light source
15
. The light output by the light valve passes to the output optics
23
that focus the light to form an image (not shown). The light valve, light source 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
15
passes through the polarizer
17
. The polarizer polarizes the light output from the light source. The beam splitter
19
reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator
25
. 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
21
.
The direction of an electric field in each pixel of the spatial light modulator
25
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 modulator passes through the beam splitter
19
and the analyzer
21
and is output from the light valve depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from the light valve
10
passes to the output optics
23
.
The light source
15
may be composed of LEDs. The LEDs are of three different colors in a color display. Other light-emitting devices whose output can be rapidly modulated may alternatively be used as the light source
15
. As a further alternative, a white light source and a light modulator (not shown) may be used. The light modulator modulates the amplitude of the light generated by the light source to define the illumination period and balance period of the spatial light modulator. In a light valve for use in a color display device, the light modulator additionally modulates the color of the light output from the light source.
The polarizer
17
polarizes the light generated by the light source
15
. The polarization is preferably linear polarization. The beam splitter
19
reflects the polarized light output from the polarizer towards the spatial light modulator
25
, and transmits to the analyzer
21
the polarized light reflected by the spatial light modulator. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
The spatial light modulator
25
is composed of the transparent electrode
33
deposited on the surface of the tr
Robrish Peter R.
Weber Andreas
Agilent Technologie,s Inc.
Hardcastle Ian
Mengistu Amare
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