Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Optical excitation
Reexamination Certificate
2001-09-24
2002-07-30
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Optical excitation
C349S009000, C349S117000, C349S096000, C349S100000, C349S172000
Reexamination Certificate
active
06426783
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to image generating systems including a reflective type, ferroelectric liquid crystal (FLC) spatial light modulator (SLM). More specifically, the invention relates to an optics arrangement including an FLC compensator cell for allowing the system to generate a substantially continuously viewable image while DC-balancing the FLC material of both the SLM and the compensator cell.
FLC materials may be used to provide a low voltage, low power reflective spatial light modulator due to their switching stability and their high birefringence. However, a problem with FLC materials, and nematic liquid crystal materials, is that the liquid crystal material may degrade over time if the material is subjected to an unbalanced DC electric field for an extended period of time. In order to prevent this degradation, liquid crystal spatial light modulators (SLMs) must be DC field-balanced.
Nematic liquid crystal materials respond to positive or negative voltages in a similar manner regardless of the sign of the voltage. Therefore, nematic liquid crystals are typically switched ON by applying either a positive or negative voltage through the liquid crystal material. Nematic liquid crystal materials are typically switched OFF by not applying any voltage through the material. Because nematic liquid crystal materials respond to voltages of either sign in a similar manner, DC balancing for nematic liquid crystal materials may be accomplished by simply applying an AC signal to create the voltage through the material. The use of an AC signal automatically DC balances the electric field created through the liquid crystal material by regularly reversing the direction of the electric field created through the liquid crystal material at the frequency of the AC signal.
In the case of FLC materials, the materials are switched to one state (i.e. ON) by applying a particular voltage through the material (i.e. +5 VDC) and switched to the other state (i.e. OFF) by applying a different voltage through the material (i.e. −5 VDC). Because FLC materials respond differently to positive and negative voltages, they cannot be DC-balanced in situations where it is desired to vary the ratio of ON time to OFF time arbitrarily. Therefore, DC field-balancing for FLC SLMs is most often accomplished by displaying a frame of image data for a certain period of time, and then displaying a frame of the inverse image data for an equal period of time in order to obtain an average DC field of zero for each pixel making up the SLMs.
In the case of an image generating system or display, the image produced by the SLM during the time in which the frame is inverted for purposes of DC field-balancing may not typically be viewed. If the system is viewed during the inverted time without correcting for the inversion of the image, the image would be distorted. In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of the pixels would appear to be half on. In the case in which the image is inverted at a frequency slower than the critical clicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image. Neither of these situations would provide a usable display.
In one approach to solving this problem, the light source used to illuminated the SLM is switched off or directed away from the SLM during the time when the frame is inverted. This type of system is described in copending U.S. patent application Ser. No. 08-361,775, filed Dec. 22, 1994, entitled DC FIELD-BALANCING TECHNIQUE FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, which is incorporated herein by reference. However, this approach substantially limits the brightness and efficiency of the system. In the case where the magnitude of the electric field during the DC field-balancing and the time when the frame is inverted is equal to the magnitude of the electric field and the time when the frame is viewed, only a maximum of 50% of the light from a given light source may be utilized. This is illustrated in
FIG. 1
a
which is a timing diagram showing the relationship between the switching on and off of the light source and the switching of the SLM image data.
As shown in
FIG. 1
a
, the light source is switched on for a period of time indicated by T
1
. During this time T
1
, the SLM is switched to form a desired image. In order to DC balance the SLM, the SLM is switched to form the inverse of the desired image during a time period T
2
. In order to prevent this inverse image from distorting the desired image, the light source is switched off during the time T
2
as shown in
FIG. 1
a.
In order to establish a convention to be used throughout this description, the operation of a given pixel
10
of a reflective type FLC SLM using the above mentioned approach of switching off the light source during the time the frame is inverted will be described with reference to
FIGS. 1
b-d
.
FIG. 1
b
shows pixel
10
when it is in its bright state and
FIG. 1
c
shows pixel
10
when it is in its dark state. As illustrated in both
FIGS. 1
b
and
1
c
, a light source
12
directs light, indicated by arrow
14
, into a polarizer
16
. Polarizer
16
is arranged to allow, for example, horizontally linearly polorized light, indicated by the reference letter H and by arrow
18
, to pass through polarizer
16
. However, polarizer
16
blocks any vertically linearly polarized component of the light and thereby directs only horizontally linearly polarized light into pixel
10
. This arrangement insures that only horizontally linearly polarized light is used to illuminate pixel
10
. For purposes of clarity throughout this description, the various configurations will be described using horizontally linearly polarized light as the initial input light for each of the various configurations.
As also illustrated in
FIGS. 1
b
and
1
c
, pixel
10
includes a reflective backplane
22
and a layer of FLC material
24
which is supported in front of reflective backplane
22
and which acts as the light modulating medium. The various components would typically be positioned adjacent one another, however, for illustrative purposes, the spacing between the various components is provided. In this example, the FLC material has a thickness and a birefringence which cause the material to act as a quarter wave plate for a given wavelength. In this example, the FLC material is typical of those readily available and has a birefringence of 0.142. Therefore a thickness of 900 nm causes the SLM to act as a quarter wave plate for a wavelength of approximately 510 nm.
FLC material
22
has accompanying alignment layers (not shown) at the surfaces which have a buff axis or alignment axis that controls the alignment of the molecules of the FLC material. For this example of a reflective mode SLM, the SLM is oriented such that the alignment axis is rotated 22.5 degrees relative to the polarization of the horizontally linearly polarized light being directed into the SLM. The FLC also has a tilt angle of 22.5 degrees associated with the average optic axis of the molecules making up the FLC material. Therefore, when FLC material
24
of the pixel is switched to its first state, in this case by applying a +5 VDC electric field across the pixel, the optic axis is rotated to a 45 degree angle relative to the horizontally linearly polarized light. This causes the pixel to act as a quarter wave plate for horizontally linearly polarized light at 510 nm. Alternatively, when the pixel is switched to its second state, in this case by applying a −5 VDC electric field across the pixel, the optic axis is rotated to a zero degree angle relative to the horizontally linearly polarized light. This causes the pixel to have no effect on the horizontally linearly polarized light directed into the pixel. In other words, the tilt angle is the angle that the FLC optic axis is rotated one si
Chowdhury Tarifur R.
Crouch Robert G.
Displaytech, Inc.
Marsh Fischmann & Breyfogle
Sikes William L.
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