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)