Method and apparatus for controlling common mode electrode voltage in lcos/lcd

• US 20030098835A1
• Filed: 07/30/2002
• Published: 05/29/2003
• Est. Priority Date: 11/30/2000
• Status: Active Grant

• Current Assignee: InterDigital Patent Holdings, Inc. (InterDigital, Inc.)
• Original Assignee: Thomson Licensing (Vantiva SA)
• Inventors: Hoffman, Brent William, OʼDonnell, Eugene Murphy
• Granted Patent: US 7,088,331 B2
• Time in Patent Office: Days
• US Class Current: 345/87
• CPC Class Codes: G09G 2320/0204 Compensation of DC component across the pixels in flat panels
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/002 to project the image of a two-dimensional display, such as an array of light emitting or modulating elements or a CRT
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general
• CPC Class Codes: G09G 3/3655 Details of drivers for counter electrodes, e.g. common electrodes for pixel capacitors or supplementary storage capacitors
• CPC Class Codes: G09G 3/3659 the addressing of the pixel involving the control of two or more scan electrodes or two or more data electrodes, e.g. pixel voltage dependant on signal of two data electrodes

Abstract

A device (10) for controlling common mode electrode voltage in a liquid crystal display includes at least a first sensor (12) for measuring flicker resulting from applying a video signal with a predetermined color and drive level to an imager. A detector (18) for determining a difference between a positive field detector voltage and a negative field detector voltage is used to provide a feedback loop for feeding back the difference to a controller to adjust the common mode electrode voltage.

Claims

1. A method of controlling common mode electrode voltage in a liquid crystal display, comprising the steps of:

applying a video signal with a predetermined color and drive level to an imager;

determining a difference between a positive field detector voltage and a negative field detector voltage; and

feeding back the difference to a controller to adjust the common mode electrode voltage.

2. The method of claim 1, wherein the step of determining further comprises the step of using at least one sensor in an overscan area of a picture.

3. The method of claim 1, wherein the method further comprises the step of readjusting the common mode electrode voltage dynamically to minimize image sticking.

4. The method of claim 2, wherein the step of determining further comprises the step of using a primary sensor, a secondary sensor, and a differential amplifier to subtract a common mode signal due to ambient room light.

5. The method of claim 1, wherein step of determining a difference further comprises the step of using a gated comparator whose output indicates the direction in which to change the common electrode voltage.

6. The method of claim 1, wherein the step of determining a difference further comprises the step of using sensors.

7. The method of claim 1, wherein the step of determining a difference comprises the step of measuring flicker resulting from applying the video signal.

8. A device for controlling common mode electrode voltage in a liquid crystal display, comprises:

a detector for determining a difference between a positive field detector voltage and a negative field detector voltage; and

a feed-back loop for feeding back the difference to a controller to adjust the common mode electrode voltage.

9. The device of claim 8, wherein the device comprises a primary photosensor and a secondary photosensor placed a short distance apart in an overscan area of a picture.

10. The device of claim 8, wherein the device comprises a primary photosensor and a secondary photosensor placed under a plurality of fold mirrors in a system without overscan.

11. The device of claim 8, wherein the device further comprises a pinhole in the display to allow sufficient light to reach said at least first sensor.

12. The device of claim 8, wherein the detector comprises a gated comparator whose output indicates the direction in which to change the common electrode voltage.

13. The device of claim 8, wherein the device further comprises a source of light having a predetermined intensity level for illuminating said at least first sensor based on a video input to an imager for the liquid crystal display.

14. The device of claim 8, wherein the detector for determining a difference comprises at least a first sensor for measuring flicker resulting from applying a video signal with a predetermined color and drive level to an imager.

15. A device for controlling common mode electrode voltage in a liquid crystal display, comprises:

a source of polarized light having a predetermined intensity level for illuminating at least a first sensor through a liquid crystal cell; and

a detector responsive to an output of said first sensor for providing a feedback signal to adjust the common mode electrode voltage for the liquid crystal cell.

16. The device of claim 15, wherein the device further comprises a mirror placed on a cover glass to reflect and direct said radiation towards said at least first sensor.

17. The device of claim 16, wherein said at least first sensor and detector is integrated into a back plane of the liquid crystal display.

18. The device of claim 16, wherein said at least first sensor is a photo-sensor integrated into a back plane of a LCOS display, the source of radiation is a light emitting diode, and the detector is integrated into the back plane.

19. The device of claim 15, wherein the source of radiation and said at least first sensor is tuned to an invisible wavelength.

20. The device of claim 15, wherein said at least first sensor is at least one photodiode placed on a top cover of a cover glass over an electronically unmodulated area of an LCOS to check a zero voltage (un-modulated) point and/or at least another photodiode placed on the top cover of the cover glass over a ring electrode to check a maximum voltage point on an electro-optical transfer function, wherein the detector uses the voltage from the photodiodes to choose a correct common mode electrode voltage.