The overall shape of the experimental absorption coefficient is in rather good agreement with the computational results. However, the experimental absorption coefficient is larger than the calculated one by approximately a factor of 10 over the whole spectral range where indirect transitions dominate (Figure ). This discrepancy is particularly detrimental for applications of CaCuP as a transparent conductor, as it leads to very low optical transmission in the visible. By measuring the absorption coefficient as a function of composition (Figure ) we observe that an additional increase in absorption (by around a factor of 2) occurs when moving from slightly P-poor to a slightly P-rich stoichiometry. On the other hand, the dependence of the absorption coefficient on the Cu/Ca ratio is weaker, at least in the vicinity of the stoichiometric point.