The absorption limit of 8.73 eV in Table 23 is close to the band gap of 8.8 eV for alumina. At energies lower than 8.73 eV, trace impurities and defects in the alumina lead to absorption tails as described in . Values of n and к at higher energies than
8.73 eV are given in Table 23 to three or four significant figures. Values from different research groups can vary substantially ; those in Table 23 are from . See  for n and к values at many more energies (wavelengths).
The values of к in Table 23 show a maximum at about 13 eV, which can be attributed to exitonic absorption ; other electronic processes in the ultra-violet spectral range are also described in .
Appreciable absorption begins in the infrared spectral range above a wavelength of 4.0 pm, as shown in Table 24; there are absorption peaks at 17.24 pm (580 cm-1) and
22.73 pm (440 cm-1), which result from lattice vibrations. For more details see . The optical anisotropy of alumina results in slightly different n and к values for the
ordinary and extraordinary ray, as shown in detail in  (see Table 22). This anisotropy is related to the hexagonal (rhombohedral) structure of the alumina.
Table 21 Refractive Index of sapphire at 25°C in the spectral range where к < 10 6, from [49, 50]
Table 22 Refractive index , constants at 20°C from 0.2 to 5.5 mm, from [49, 50]
The refractive index of alumina increases slightly with increasing temperature T in the visible and near-visible spectral range. The value of the temperature coefficient dnl dT of the refractive index from wavelengths 0.4-0.8 pm is about 13(10)-6°K-1 .