Control of liquid crystal display visual properties to compensate for variation in the characteristics of the liquid crystal

A liquid crystal display comprising;

• a plurality of liquid crystal elements;
• means for applying voltages across the elements;
• photodetector means to detect the transmissivity of one of said plurality of elements;
• means to repetitively switch said one element at predetermined times between a first state in which it exhibits a first level of transmissivity and a second state in which it exhibits a second level of transmissivity lower than the first level;
• means coupled to the said photodetector means for obtaining a signal corresponding to the mean transmissivity of the first and second transitivities;
• means for comparing the obtained signal with a reference signal representative of a desired mean transmissivity;
• means for producing the reference signal;
• and
• means for adjusting the voltages applied across the plurality of elements in dependence upon the difference between the obtained and desired reference signals, whereby the mean transmissivity of the liquid crystal elements is maintained at the desired mean transmissivity.