Determination of the twist angle and the retardation properties of twisted nematic liquid crystal television by spectral measurements

We describe a method to determine the anisotropic param- eters of liquid crystal televisions (LCTVs) formed by twisted nematic liq- uid crystals, as a function of the wavelength over a broad spectral range. One of them is the twist angle F and the second, b, is related to the birefringence of the liquid crystal. The method is based on the concept of the channeled spectrum and applies whenever the user has no informa- tion about the structure of the cell and the molecular director. It is shown that the recording of three channeled spectra is sufficient to determine the twist angle and the retardation properties of the cell. The method is illustrated with the experimental characterization of red, green, and blue LCTVs from a video projector.