There is great interest in using commercially available twisted-nematic liquid crystal displays as programmable spatial light modulators. Proper use of such displays, particularly as phase-only filters for optical correlators or as diffractive optical elements, requires knowledge of the physical parameters of the device including the twist angle, the birefringence, and the orientation of the director axes. A simple experimental technique was published for evaluation of these parameters. When we used this technique, however, we found a number of different sets of values that satisfied the experimental data. We demonstrate that additional measurements at different wavelengths can resolve these ambiguities and provide valuable information about the wavelength dependence of the birefringence. © 1999 Society of Photo-Optical Instrumentation Engineers.