Liquid Crystal Birefringent Wavelength Filters

The operation of an LCTF may be understood by considering a simplified Lyot filter stack, in which (N +1) polarizers are separated by N layers of liquid crystals sandwiched between birefringent crystals. The optical retardation, Rnm, introduced by birefringent crystals is dependent on the thickness of the crystal, tfnm, and the difference between the refractive index of the ordinary ray, and the extraordinary ray, tie, at the wavelength of interest ... 

The optical path of the liquid-crystal tunable filter comprises a series of polarizers and liquid crystal elements whose birefringent properties are electronically controlled. These devices can select wavelengths ranging from the visible to 1100 nm, with a bandwidth as low as 5 nm. The wide, circular aperture makes it particularly attractive as a means to select wavelength regions for transmission to a near-IR camera. 

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