A method of determining a parameter of a liquid crystal cell is provided in which parameters, such as the thickness of the liquid crystal layer and the angle of the twist of liquid crystal molecule orientation in the liquid crystal cell, are accurately determined in a short time with a simple apparatus. Light from a light source 1 is transmitted through a polarizing plate 2 and incident to a liquid crystal cell 3. The light is transmitted by a polarizing plate 4 and the intensity of the transmitted light is measured by a photodetector 6. Then, the outputs of the photodetector 6 for the following cases are measured: when the polarization direction of the polarizing plate 4 is set (1) in the direction of the X axis, (2) in the direction of Y axis and (3) at 45 degrees to the X and Y axes and (4) when, with the polarization direction of the polarizing plate 4 being at 45 degrees to the X and Y axes, a quarter wavelength plate 5 is inserted between the polarizing plate 4 and the liquid crystal cell 3 so that its axial direction is tilted at 45 degrees to the polarization direction of the polarizing plate 4. Stokes parameters are obtained from the measurement values, and the thickness of the liquid crystal layer and the angle of twist of liquid crystal molecule orientation are calculated from the measured Stokes parameters.
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