Refractive index of Germanium

[astromam]

Find the right reference to compute the refractive index of Germanium at T=60K and wave=11.2 um.

So far, two references: Barnes & Piltch (1979) and Li et al. (1980).
Li et al. (1980) give refractive index for a temperature range from 100K to 1200K and wavelength range from 1.9um to 18.0um.
Barnes & Piltch (1979) give refractive index for a temperature range from 50K to 300K and wavelength range from 2.5um to 14.0um. Barnes & Piltch (1979) seem to have underestimated refractive index with respect to Li et al. (1980) and Frey et al. (2006).