The transmission of monochromatic light through the sandwich cell of MBBA (a typical n -type liquid crystal) with vertical alignment is investigated. When the cell is set in crossed Nicol and controlled by a.c. voltages, the transmissivity is zero at any voltage V below the threshold value V th . When V > V th , it shows nearly a sinusoidal variation with V . This phenomenon is observed even with fairly thick cells (50 µm). The threshold voltage V th is shown to be independent of the cell's thickness and about 4.0 V (r.m.s.) at 24.0°C for f <10 kHz. The orientation angles of molecules and the phase differences of the light beams are calculated theoretically by using the static continuum theory of liquid crystals. The experimental results agree well with the theoretical calculations.
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Ohtsu et al. (1974) studied this question.