Direct experimental determinations of the birefringences of quartz in the 0.6–2.6μ region were made by means of channeled spectra. Up to 2.0μ, the experimentally determined birefringences of quartz were found to agree within experimental error with values obtained by subtracting from each other independently measured refractive indices, n0 and ne, as listed in the International Critical Tables. Beyond 2.0μ, the experimentally determined birefringences gradually became appreciably smaller than the corresponding (ne—n0) values. The variation appears to be associated with the moderately strong absorption band of quartz, which occurs at about 3.0μ, and suggests the possible existence of two sets of birefringence values near such a band.
JOHN H. SHIELDS and JOSEPH W. ELLIS, "Dispersion of Birefringence of Quartz in the Near Infrared," J. Opt. Soc. Am. 46, 263-265 (1956)