Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1999-04-27
2002-12-24
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C353S034000, C349S008000, C349S172000
Reexamination Certificate
active
06498632
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 for forming color images and having a substantially increased light throughput.
BACKGROUND OF THE INVENTION
A need exists for various types of color video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature color 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 color 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 three reflective spatial light modulators, each based on a ferroelectric liquid crystal (FLC) material.
A FLC-based spatial light modulator is composed of a layer of a FLC material, preferably a surface-stabilized FLC material, sandwiched between a transparent electrode and a reflective electrode that is segmented into an array of pixel electrodes to define the picture elements (pixels) of the spatial light modulator. The reflective electrode is located on the surface of a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from an input video signal.
The direction of an electric field applied between each pixel electrode and the transparent electrode determines whether or not the corresponding pixel of the spatial light modulator rotates the direction of polarization of light reflected by the pixel. The reflective spatial light modulator is constructed as a quarter-wave plate so that the polarized light reflected by the pixels of the spatial light modulator is either rotated by 90° or not depending on the direction of the electric field applied to each pixel. A polarization analyzer is in the optical path of the light reflected by the spatial light modulator. The polarization analyzer is aligned to either: 1) transmit the polarized light which has rotated and absorb the polarized light which as not been rotated; or 2) transmit the polarized light which as not rotated and to absorb the polarized light which has been rotated. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the light reflected by the pixel either is transmitted through the polarization analyzer (its 1 state) or is absorbed by the polarization analyzer (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 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 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, 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. 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 conventional display device
5
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 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 light source. Alternatively, a polarized light source (not shown) can be used and the need for the polarizer
14
would be eliminated. The beam splitter
16
then reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator
12
. The beam splitter can additionally or alternatively be a polarizing beam splitter configured to reflect light having a direction of polarization parallel to the direction of polarization of the polarizer
14
towards the spatial light modulator
12
. The spatial light modulator
12
is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve
10
. Light reflected from the spatial light modulator can pass to the beam splitter
16
which transmits a fraction of the reflected light to the analyzer
18
. If the beam splitter is a polarizing beam splitter, however, only light having a direction of polarization orthogonal to the direction of polarization imparted by the polarizer will be transmitted and the need for an independent analyzer would be eliminated.
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 modulator passes through the beam splitter
16
and the analyzer
18
and is output from the light valve
10
through the imaging optics
26
depending on whether or not its direction of polarization was rotated by the spatial light modulator.
More specifically, the polarizer
14
polarizes the light generated by the light source
20
that passes through the collector optics
24
either directly or after reflecting off reflector
22
. The polarization is preferably linear polarization. The beam splitter
16
reflects the polarized light output from the polarizer towards spatial light modulator
12
, and the polarized light reflected from the spatial light modulator transmits to the analyzer
18
through the bea
Butterworth Mark M.
Helbing Rene P.
Hewlett--Packard Company
Parker Kenneth
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