DC-balanced and non-DC-balanced drive schemes for liquid crystal devices

• US 6,507,330 B1
• Filed: 03/14/2001
• Issued: 01/14/2003
• Est. Priority Date: 09/01/1999
• Status: Expired due to Fees

• Current Assignee: Citizen Finetech Miyota Company Limited (Citizen Electronics Company Limited)
• Original Assignee: DT FLCO, Inc. (Micron Technology, Inc.)
• Inventors: Xue, Jiuzhi, Handschy, Mark A., Ji, Lianhua
• Primary Examiner(s): Chow, Dennis-Doon
• Assistant Examiner(s): Bell, Paul A.
• Application Number:
  US09/809,741
• Time in Patent Office: 671 Days
• Field of Search: 345/87, 345/88, 345/89, 345/90, 345/94, 345/95, 345/96, 345/97, 345/98, 345/99, 345/102, 349/25, 349/33, 349/34, 349/36, 349/37
• US Class Current: 345/94
• CPC Class Codes: G09G 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 2310/063 Waveforms for resetting the whole screen at once
• CPC Class Codes: G09G 2320/02 Improving the quality of display appearance
• CPC Class Codes: G09G 2320/0204 Compensation of DC component across the pixels in flat panels
• CPC Class Codes: G09G 2320/0257 Reduction of after-image effects
• CPC Class Codes: G09G 3/36 using liquid crystals
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general
• CPC Class Codes: G09G 3/3651 using multistable liquid crystals, e.g. ferroelectric liquid crystals
• CPC Class Codes: G09G 3/3655 Details of drivers for counter electrodes, e.g. common electrodes for pixel capacitors or supplementary storage capacitors

Abstract

A method of operating a liquid crystal cell includes DC-balancing by displaying an inverse image with electric fields of increased magnitude relative to the image producing electric fields. While the inverse image is displayed the image is prevented from being visible by either turning off the light source or re-directing or blocking the light from reaching the viewing area. The image producing electric fields and the inverse image producing electric fields are such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of liquid crystal cell. The time duration of the inverse image portion is shorter than the time duration of the image portion by an amount proportional to the increased magnitude of the additional electric fields. Because of the shorter time period when no image is visible, the system brightness is increased.

Claims

1. A method of operating a liquid crystal cell during a given period of time, the method using input image data to control how the cell is operated, the method comprising:

applying image producing electric fields of a first magnitude to the cell during a first portion of the given period of time, the image producing electric fields depending in a predetermined way upon the input image data; and

applying additional electric fields of a higher, second magnitude to the cell during a second portion of the given period of time, the image producing electric fields and the additional electric fields being such that the cumulative time integral of the electric fields that are present in one direction across the liquid crystal material is substantially equal to the cumulative time integral of the electric fields that are present in the opposite direction during the given period of time during the operation of the liquid crystal cell.

2. A method as defined in claim 1, wherein:

the image data is divided into frame image data corresponding to individual frames of image data;

the given period of time is a frame time associated with one frame of image data; and

the method is a method of operating the liquid crystal cell for a plurality of frame times at a certain frame rate.

3. A method as defined in claim 1, wherein the liquid crystal cell is a ferroelectric liquid crystal cell including ferroelectric liquid crystal material.

4. A method as defined in claim 3, wherein:

the ferroelectric liquid crystal cell is a ferroelectric liquid crystal spatial light modulator for modulating light directed into the spatial light modulator;

the ferroelectric liquid crystal material of the spatial light modulator is divided into a plurality of individually controllable pixels; and

the operation of applying image producing electric fields to the cell includes applying image producing electric fields to each of the individually controllable pixels during the first portion of the given period of time, thereby causing the individually controllable pixels to form a desired light modulating pattern for modulating light directed into the spatial light modulator.

5. A method as defined in claim 4, wherein:

the spatial light modulator is part of an overall display system that includes an illuminator for directing light into the spatial light modulator, and

the method includes causing the illuminator not to direct light into the spatial light modulator during the second portion of the given period of time during which the additional electric fields are being applied to the spatial light modulator.

6. A method as defined in claim 4, wherein;

the ferroelectric liquid crystal material includes a top and a bottom surface, the top and bottom surfaces of the liquid crystal material being approximately coplanar;

the ferroelectric liquid crystal spatial light modulator includes a top electrode located adjacent to the top surface of the ferroelectric liquid crystal material and a plurality of pixel electrodes located adjacent to the bottom surface of the ferroelectric liquid crystal material, each of the plurality of pixel electrodes being associated with, and capable of controlling, one of the plurality of pixels; and

wherein applying the additional electric fields to the cell for the second portion of the given period of time includes (i) individually setting each pixel electrode to an electric potential related in a predetermined way to at least one of the electric fields applied to that pixel during the first portion of the given period of time during which the image producing electric fields are applied to each of the individually controllable pixels and (ii) applying a constant electric potential to the top electrode of the spatial light modulator for the second portion of the given period of time.

7. A method as defined in claim 6, wherein the setting of the pixel electrodes during the second portion of the given period of time includes inverting the polarity of the fields applied to the pixels and increasing the magnitude of the electric fields.

8. A method as defined in claim 7, wherein the setting of the pixel electrodes during the second portion of the given period of time includes shortening the time duration of the electric fields by an amount proportional to the increase in the magnitude of the electric fields.

9. A method as defined in claim 1, wherein the second portion of the given period of time is less than or equal to about forty-five percent of the duration of the given period of time.

10. A method for operating a liquid crystal display during a given period of time, the method using input image data to control how the display is operated, the display creating visible images at a viewing area, the method comprising:

applying a first series of voltage signals to the liquid crystal display during one portion of the period of time, the first series of voltage signals being arranged to produce an image as represented by the input image data;

allowing the display to be viewed at the viewing area, while the image is being produced by the first series of voltage signals applied to the display, by allowing illumination light to be directed to the display and from the display to the viewing area;

applying a second series of voltage signals to the liquid crystal display during another portion of the period of time, the second series of voltage signals being arranged to produce an inverse image, the second series of voltage signals being related to the first series as being inverted in polarity relative to the first series, having an increased magnitude relative to the first series, and having a shorter time duration than the first series; and

substantially preventing the display from being viewed at the viewing area, while the inverse image is being produced by the second series of voltage signals applied to the display, by substantially preventing illumination light from reaching the viewing area.

11. A method as defined in claim 10, wherein the display is made viewable or substantially not viewable by controlling the light emitted from a light source operatively associated with the liquid crystal display.

12. A method as defined in claim 10, wherein the image is made viewable or substantially not viewable by selectively allowing or substantially preventing light to pass from the light source to the liquid crystal display to the viewing area.

13. A method as defined in claim 10, wherein the one portion of time is a contiguous sub-period of the given period of time.

14. A method as defined in claim 10, wherein the one portion of time is divided into a plurality of sub-periods of the given period of time.

15. A method as defined in claim 10, wherein the second series of voltage signals has a magnitude that is at least 20% greater than the magnitude of the first series of voltage signals.

16. A method as defined in claim 10, wherein the second series of voltage signals has a magnitude that is at least 50% greater than the magnitude of the first series of voltage signals.

17. A method as defined in claim 10, wherein the second series of voltage signals has a magnitude that is at least 75% greater than the magnitude of the first series of voltage signals.

18. A method as defined in claim 10, wherein the second series of voltage signals has a magnitude that is at least twice as great as the magnitude of the first series of voltage signals.

19. A liquid crystal display system, comprising:

a liquid crystal spatial light modulator;

a data writing arrangement that that provides drive signal information to the spatial light modulator; and

a light source that selectively illuminates the spatial light modulator;

wherein the drive signal information provided to the spatial light modulator includes a first series of signals for producing an image during one portion of a period of time while the spatial light modulator is illuminated by the light source and a second series of signals for producing an inverse image during an other portion of the period of time while the spatial light modulator is not substantially illuminated by the light source, the second series of signals being related to the first series as being inverted in polarity relative to the first series, having an increased magnitude relative to the first series, and having a shorter time duration than the first series.

20. A system as defined in claim 19, wherein the one portion of time is a contiguous sub-period of the given period of time.

21. A system as defined in claim 19, wherein the other portion of the given period of time is less than or equal to about forty-five percent of the duration of the given period of time.

22. A system as defined in claim 19, wherein the spatial light modulator is a ferroelectric liquid crystal spatial light modulator including ferroelectric liquid crystal material.