CaCO is a widely used birefringent material but its use in IR astronomical instruments may be limited by the fact that at room temperatures the crystal becomes opaque to the ordinary ray at 2.2 m. The transmittance of CaCO at cryogenic temperatures is not known (cf. the Introduction). To fill this gap we performed measurements of Calcite at 20 C and 77 K using the FT-IR Perkin-Elmer System 2000 spectrophotometer of the National Optical Institute of Florence (Perkins 1986).
The resulting external transmittance is plotted in Fig. 3 (click here) where the absorption bands at 2.0, 2.15, 2.35 and 2.5 m are narrower and much deeper than in the old measurements of Nysander (1909) which are reported in optical handbooks. This is also evident in Fig. 4 (click here) which compares the absorption coefficients derived here and those of Nysander (1909). Cooling the crystals slightly decreases the absorption at some wavelengths but the overall transmission of our relatively thick (9 mm) specimen remain quite low.
Figure 4: Absorption coefficient of Calcite for the ordinary (solid lines) and extraordinary (dashed lines) rays at room temperature (upper panel) and 77 K (lower panel). The thin lines show the data available in optical handbooks (e.g. Bennet 1995) and which are based on the old measurements of Nysader (1909)