Patent FR-2789836-A1 (FR)
Title: Method and device for voltage tuning of counter-electrode of liquid crystal display drivers, comprising sensing of luminous flux and control to minimize amplitude of alternating signal
Inventor: BERTIN JEAN PIERRE
IPC: G09G3/36
Filed: 1999-02-16
Abstract: Tuning the voltage applied to the counter-electrode of a liquid crystal valve or driver comprises directing the driver for the variation of luminous flux, sensing the luminous flux by a photodetector at the output of driver, conversion of luminous flux to an alternating signal and amplification, processing of the alternating signal in a feedback loop and memory storage of tuning values corresponding to an adjusted voltage on the counter-electrode. The method for tuning the voltage applied to the counter-electrode of a liquid crystal valve or driver comprises the following steps:  (i) directing the driver (2) for the variation of luminous flux;  (ii) sensing the luminous flux by a photodetector (6) at the output of driver;  (iii) conversion of luminous flux to an alternating signal and amplification;  (iv) processing of the alternating signal in a feedback loop comprising an analog-to-digital converter (8), a field programmable gate array (11), and an analog interface (12), for control of signal applied to the counter-electrode for minimizing the amplitude of alternating signal; and  (v) memory storage of tuning values corresponding to an adjusted voltage on the counter-electrode. The sensing of luminous flux is carried out by a fibre-optic element connected to the photodetector (6), which is g. a blue enhanced photo diode, converting the luminous flux to electric signal.  The variation of luminous flux is with respect to a signal wherein only odd rows are activated in odd periods with even rows remaining at black level, and vice versa. The corresponding frequency is the period frequency divided by two. The fibre-optic element is positioned at the limit of display field, and its extremity is cut at an angle, which is eg. 45 degrees. to have a total reflection of light. The sensing of luminous flux is by aid of a plate of glass positioned in the display field, with one of the edges carrying the photodetector. The edges, excepting the part receiving the photodetector, are covered with a layer of absorbent material. The signal from photodetector is processed by computation by use of a convergence algorithm. In the case of a color projector, the device comprises three liquid crystal valves or drivers, and the counter-electrode voltage of each driver is tuned in succession. The values stored in memory are used for tuning the counter-electrode voltage in the following period. The device comprises three liquid crystal drivers (1A,1B,1C), an illumination device, a luminous flux sensor connected to photodetector (6), an analog-to-digital converter (8), a computing unit (9) implementing the convergence algorithm for minimizing the amplitude of signal from the photodetector, a memory circuit (10) for storage of tuning values, and means (11,12) for adding the tuning values to the counter-electrode voltage. An amplifier (7) is connected between the photodetector (6) and the analog-to-digital converter (8).