Process for the control of an optical characteristic of a material by signals of increasing time periods

Claim 1

A control process of an optical characteristic of a material for obtaining a display of a plurality of grey levels, said material having a plurality of zones distributed in a matrix-like manner and interposed between a first group of p lines of parallel electrodes and a second group of q columns of parallel electrodes, the lines and columns being crossed, a zone x i y j being defined by the region of the material covered by line x i , in which i is an integer such that 1≦i≦p, and by column y j , in which j is an integer such that 1≦j≦q, wherein, for controlling the zone x i y j of the material, the process includes: applying a first series of signals with a zero mean value to a line x i , said first series being divided into n different time fractions of different durations; and applying a second series of signals with a zero mean value to the column y j , said second series also being divided into n different time fractions of different durations such that the kth time fraction of the second series of signals is of the same duration as the kth time fraction of the first series of signals, k being an integer such that 1≦k≦n, said second series of signals being such that for each time fraction, the signal of the second series is either in phase opposition or in phase with the corresponding signal of the first series.