Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
1999-10-28
2001-03-06
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S199000, C345S097000, C345S101000
Reexamination Certificate
active
06198523
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ferroelectric liquid crystal-based switchable half-wave plates and in particular to switchable half-wave plates with controlled tilt angle for improved contrast in 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 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 one or more 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.
One way this shortcoming can be overcome is the use of a switchable half wave plate also known as a light doubler. In the first of the two states, the 0 state, the switchable half-wave plate leaves the sense of operation of the light valve relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator unchanged. In the second of the two states, the 1 state, the switchable half-wave plate inverts the sense of operation of the light valve relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator. The light doubler is typically operated in the 0 state during the illumination period and in the 1 state during the balance period, during which the light source is not modulated (or modulated for only as long as it takes to switch between the 0 state and the 1 state).
Thus, the pixel electrode is driven with a complementary drive signal during the balance period and the DC balance of the pixel is restored. Since the light source is not modulated, light is transmitted by the pixel and its sense of operation is inverted by the switchable half-wave plate in the 1 state. This results in a pixel with the same apparent brightness during a balance period as during a illumination period. The apparent brightness of the pixel operated in conjunction with a switchable half-wave plate is therefore doubled during each display period
FIG. 1
shows part of a conventional display device
5
incorporating a conventional reflective light valve system
10
that includes the reflective spatial light modulator
12
and a switchable half-wave plate
11
. 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 switchable half-wave plate
11
. 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 switchable half-wave plate
11
. The switchable half-wave plate
11
then transmits the light to the spatial light modulator
12
.
The spatial light modulator
12
is divided into a two-dimensional array of p
Chowdhury Tarifur R.
Hewlett-Packard Co.
Mayer Marc R.
Sikes William L.
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