Method and apparatus for controlling common mode electrode voltage in LCOS/LCD

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.