Liquid crystal twisted nematic polarization rotator (TN cell) is very useful when one wants to rotate the orientation of a linear polarization by a fixed amount of typically 45° or 90°. In optical systems the polarization is often rotated by quartz retardation plates (l/2 or l/4 plates). Quartz plates shows high quality and good transmission performances especially in the UV region. However, such plates present some disadvantages: They function only for a narrow spectral bandwidth and have a small incidence angle acceptance (field of view less then 2°). The liquid crystal nematic cells have a large acceptance angle, function over a very large spectral range from VIS to NIR (if they are thick enough).
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