Contrast and Contrast Measurement in Passive LCDs

A critical parameter in the evaluation and optimization of LCDs is the contrast ratio. If the contrast is too low, the image appears pale and washed out. However, if the contrast is set too high by increasing the VLCD, shadows or streaks may appear, the image becomes overdriven, and persistent burn-in effects can occur.

Generally, a higher contrast ratio results in a better visual appearance. However, beyond a certain point, the difference in increasingly higher contrast ratios becomes imperceptible to the human eye. Therefore, it is advisable to define minimum contrast values to establish a lower threshold; maximum values are not essential.

Contrast Measurement

The contrast increases with the VLCD voltage of the LCD until it reaches a peak and then declines. This optimum point must be identified, and the display adjusted accordingly.

Measurements are performed using suitable measuring instruments equipped with brightness sensors, preferably in a darkened room or with the display covered. The values provided in the examples refer to the respective measuring instruments and are not absolute values. Values may vary depending on the measurement tool used. In cases where minimum values are stipulated, the measuring instruments employed must also be defined and regularly calibrated.

Influence of Illumination

The influence of the backlight on contrast is very limited. While brighter illumination enhances the light area, an LCD with inherently poor contrast will retain this characteristic. In cases of fundamentally poor contrast performance of an LCD, the cell itself must be improved.

Further Information: Sensor Partners, Heise Online