Flicker measurement method to adjust the common electrode voltage of LCDs

By Dr. Pierre BOHER

Flicker in LCDs is a key quality parameter that must be minimize on each panel at the end of the production line. Conventional methods use a potentiometer to adjust common electrode voltage V_com. Made by an operator, this optimization is time consuming and the repeatability of the process is not fully ensured. In the proposed paper we present a new ELDIM instrument dedicated to precise and rapid flicker measurements. This instrument collects the light from one small region of the display and drives it on a high sensitive photo-multiplier tube. The signal is sampled by a fully programmable true 16bits acquisition board. A 4Mb buffer memory and a USB2.0 connection ensure fast, reliable and unlimited acquisitions. The signal is automatically processed and the flicker amplitude and frequency is deduced in real time. The software can be integrated easily in an automated process using an ActiveX component.

Precise V_com adjustment requires an excellent signal over noise ratio for the optical sensor because the optimum V_com window is very narrow. One example of flicker = f(V_com) dependence measured on a TFT-LCD is reported in figure 1. The optimum V_com value is obviously located around 60 but must be carefully adjusted to go below -35dB which is around the limit of human eye detection. So the sensor must be capable to measure very low flicker amplitudes with a good accuracy and within a short time. To qualify our instrument we have generated sinusoidal signals on a computer controlled LED source and made different measurements versus the sinusoidal amplitude. As shown in Figure 2, the instrument can measure flicker amplitude accurately down to -75dB. In the proposed paper, the instrument will be presented in details with different V_com adjustment examples and a strategy for automation will be developed.