We have directly measured the retardance versus temperature for single-crystal quartz (SiO2) and magnesium fluoride (MgF2) at wavelengths of 633, 788, 1318, and 1539 nm and over a temperature range of 24–80 °C. To our knowledge, the temperature dependence of retardance for these two materials has not been directly measured. We compared our direct measurements of the normalized temperature derivative of the retardance γ with derived values from previously reported indirect measurements and found our results to be in agreement and our measurement uncertainties to be typically a factor of 4 smaller. Our overall mean value for γSiO2 is -1.23 × 10-4 with a combined standard uncertainty of 0.02 × 10-4 and little wavelength dependence over the 633–1539-nm range. Our overall mean value for γMgF2 is -5.37 × 10-5 with a combined standard uncertainty of 0.17 × 10-5 and with a small wavelength dependence over the 633–1539-nm range.
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