Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-01-07
2002-04-23
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000
Reexamination Certificate
active
06377237
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to display devices, and, more particularly, to a method and system in which the electro-optical material of the spatial light modulator used in a display device is illuminated with pulses of light delayed by a time equal to the measured response time of the electro-optical material to linearize the grey scale of the display device.
BACKGROUND OF THE INVENTION
Recently, display devices based on electro-optical materials such as ferroelectric liquid crystal materials have been introduced. Such display devices can form part of a miniature, wearable display, sometimes called an eyeglass display, and also can form part of a front- or rear-projection display.
FIG. 1
shows an example of a display device
1
. The display device is composed of reflective light valve
10
, light source
15
, which generates light that illuminates the light valve, and output optics
23
that focus the light to form an image (not shown). The light valve is composed of reflective spatial light modulator
25
, polarizer
17
, beam splitter
19
and analyzer
21
. The display device shown may form part of a miniature, wearable display, a projection display or other types of displays.
In display device
1
, light generated by light source
15
passes through polarizer
17
. The polarizer polarizes the light output from the light source. Beam splitter
19
reflects a fraction of the polarized light output from the polarizer towards spatial light modulator
25
. The spatial light modulator is divided into a two-dimensional array of picture elements (pixels) that define the pixels of the display device. In this disclosure, the term display pixel X will be used as a abbreviation for the term pixel of the display device defined by pixel X of spatial light modulator. The beam splitter transmits a fraction of the light reflected by the spatial light modulator to analyzer
21
.
The direction of an electric field in each pixel of 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 beam splitter
19
and 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 light valve
10
passes to output optics
23
.
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 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.
Polarizer
17
polarizes the light generated by light source
15
. The polarization is preferably linear polarization. Beam splitter
19
reflects the polarized light output from the polarizer towards spatial light modulator
25
, and transmits the polarized light reflected by the spatial light modulator to analyzer
21
. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
Spatial light modulator
25
is composed of transparent electrode
33
deposited on the surface of transparent cover
37
, reflective electrode
35
located on the surface of semiconductor substrate
39
, and layer
31
of electro-optical material sandwiched between the transparent electrode and the reflective electrode. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially-reduced number of pixel electrodes is shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600×1200 pixel electrodes. An exemplary pixel electrode is shown at
41
. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards beam splitter
19
.
A pixel drive circuit applies a pixel drive signal to the pixel electrode of each pixel of spatial light modulator
25
. Pixel drive circuit
44
of exemplary pixel
42
is shown in this example as being located in semiconductor substrate
39
. The pixel drive signal alternates between two different voltage levels, a high state and a low state. When a liquid crystal material is used as the electro-optical material of layer
31
, transparent electrode
33
is maintained at a fixed potential mid-way between the voltage levels of the pixel drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of liquid crystal layer
31
between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
When display device
1
forms part of a miniature, wearable display, output optics
23
are composed of an eyepiece that receives the light reflected by reflective electrode
35
and forms a virtual image at a predetermined distance in front of the viewer (not shown). In a cathode-ray tube replacement or in a projection display, the output optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of analyzer
21
is orthogonal to the direction of polarization defined by polarizer
17
, light whose direction of polarization has been rotated through 90° by a pixel of spatial light modulator
25
will pass through the analyzer and be output from light valve
10
whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to output optics
23
light whose direction of polarization has been rotated by pixels of the spatial light modulator. The direction of the electric field applied to each pixel of the spatial light modulator determines whether the corresponding display pixel will appear bright or dark. When a display pixel appears bright, it will be said to be ON, and when the display pixel appears dark, it will be said to be OFF.
The direction of maximum transmission of analyzer
21
can alternatively be arranged parallel to that of polarizer
17
, and a non-polarizing beam splitter can be used as beam splitter
19
. In this case, spatial light modulator
25
operates in the opposite sense to that just described.
To produce the grey scale required by to display an image notwithstanding the binary optical characteristics of the display pixels, the apparent brightness of each display pixel is varied by temporally modulating the direction of polarization of the light reflected by the corresponding pixel of spatial light modulator
25
. This, in turn, temporally modulates the light output by the corresponding display 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 the pixel drive signal that switches the pixel from ON to OFF. The fraction of the illumination period in which the pixel is in its ON state determines the apparent brightness of the display pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the pixel drive signal
Agilent Technologie,s Inc.
Wu Xiao
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