The ordinary and extraordinary complex refractive indices, n o - jκ o and n e - jκ e, of a nematic liquid crystal were measured in the infrared region at 3.0–11.5-µm wavelength. The complex refractive indices were evaluated in terms of the angular dependence of the reflectance. Semicylindrical CsI prisms and a goniometer were used for measurement of the reflectance in a wide incident-angle range and throughout the wide infrared spectral region. Refractive indices n o and n e changed notably near the absorption wavelength. Negative birefringence, i.e., n e < n o, was observed in the vicinity of 6.6 µm, where n e changed more than did n o.
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