Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-03-14
2003-01-14
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C349S025000
Reexamination Certificate
active
06507330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to liquid crystal devices and more specifically to schemes for driving a liquid crystal cell, such as a ferroelectric liquid crystal cell, both with and without requiring DC-balancing of the liquid crystal cell.
BACKGROUND OF THE INVENTION
In the field of image generators and especially those using spatial light modulators (SLMs), it is well known that stationary and moving images, either monochrome or color, may be sampled and both color-separated and gray-scale separated pixel by pixel. These pixelated separations may be digitized, forming digitized images that correspond to the given images. These digitized images are used by devices in this field to create visual images that can be used for a direct visual display, a projected display, a printer device, or for driving other devices that use visual images as their input.
One such novel image generator is disclosed in U.S. Pat. No. 5,748,164, entitled ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued May 5, 1998, which patent is incorporated herein by reference. An image generator of this type is further described in U.S. Pat. No. 5,808,800, entitled OPTICS ARRANGEMENTS INCLUDING LIGHT SOURCE ARRANGEMENTS FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, and issued Sep. 15, 1998, which patent is also incorporated herein by reference.
As described in detail in the above recited patents, the inventions disclosed contemplate the use of a liquid crystal material, such as a ferroelectric liquid crystal (FLC) material, as a preferred light modulating medium for the spatial light modulator of the disclosed inventions. This light modulation of liquid crystal material is accomplished by establishing and maintaining electric fields across the liquid crystal material in a controlled way in order to switch the light modulating characteristics of the material. As an example, in the case of an FLC material, an electric field is established in one direction across the FLC material in order to produce a first light modulating state, for example an ON state. An electric field is established in the opposite direction across the FLC material in order to produce a second light modulating state, for example an OFF state.
Because currently available liquid crystal materials manufactured using currently available manufacturing processes are not completely insulating, and because currently available assembly processes for manufacturing liquid crystal SLMs may introduce contaminants into the SLM assembly, this formation of electric fields across the liquid crystal material may cause leakage current to flow through the liquid crystal material while the electric fields are applied to the material. If these electric fields are not balanced, the unbalanced fields (or the unbalanced leakage current) are believed to cause the degradation of the electro-optic characteristics of the liquid crystal material, thereby dramatically reducing the effectiveness and useful life of the material as a light modulating medium.
The presence of unbalanced fields across the light modulating medium tends to polarize or bias the light modulating medium if the electric fields are not balanced over time. When the electric fields are not balanced, it is believed that the net electric field in one direction causes ionic charges to migrate through the light modulating medium and build up or stick on the sides of the light modulating medium. This sticking or build up of ionic charges tends to interfere with the electric fields subsequently applied to the light modulating medium and therefore interfere with the operation of the spatial light modulator. This interference typically results in image sticking that interferes with the proper operation of the display system. For purposes of this specification, image sticking is defined as unwanted image interference during a given frame that is caused by latent electrical effects caused by previous image frames. Traditionally, this problem of image sticking is avoided or reduced by DC-balancing the driving electric field applied to the FLC material. As mentioned above, 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. +2.5 VDC) and switched to the other state (i.e. OFF) by applying a different voltage through the material (i.e. −2.5 VDC). Because FLC materials respond differently to positive and negative voltages, it is not a trivial matter to simply DC balance them with a single signal 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. Then, a frame of the inverse image data is displayed (but made not visible) 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 active matrix image generating system or display, the image produced by the SLM during the time in which the frame is inverted for purposes of DC-balancing is not typically made available to the user. If the system were viewed during the inverted time without correcting for the inversion of the image, the image would be degraded. 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 flicker 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 illuminate the SLM is switched off or directed away from the SLM during the time when the frame is inverted. 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-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, the light from a given light source may only be utilized a maximum of 50% of the time.
In order to overcome this problem of not being able to view the system during the DC-balancing frame inversion time, compensator cells have been proposed for SLMs. For example, U.S. Pat. No. 6,100,945, entitled COMPENSATOR ARRANGEMENTS FOR A CONTINUOUSLY VIEWABLE, DC FIELD-BALANCED, REFLECTIVE, FERROELECTRIC LIQUID CRYSTAL DISPLAY SYSTEM, and issued Aug. 8, 2000, which patent is incorporated herein by reference, discloses several approaches to providing display systems that include compensator cells. These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted state, thereby allowing the display to be substantially continuously viewable. Although these compensator cell arrangements appear to work well, they increase the complexity and cost of the display system by requiring the use of a compensator cell and in many cases other additional components.
Much of the earliest work with FLC displays also encountered the DC balance problem and a class of solutions was found. The early work dealt with passive matrix displays, because the unique properties of FLCs were expected to enable much larger displays having many more rows and columns of pixels than were then allowed using passive matrix nematic displays. There is a large amount of patent and scientific literature associated with passive matrix FLC displays. However, U.S. Pat. No. 4,709,995 issued to Kuribayashi is typical of the approach to DC balance taken in almost all such work.
In a passive matrix FLC display, the pixels are defined as the intersection of a column electrode with a row electrode. The column electrodes are formed as long, narrow, and parallel conductors that run entirely across
Handschy Mark A.
Ji Lianhua
Xue Jiuzhi
Bell Paul A.
Chow Dennis-Doon
Crouch Robert G.
Displaytech, Inc.
Marsh & Fischmann & Breyfogle LLP
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