We report the temperature dependence of the 0.4 to 0.8 eV refractive index of intrinsic and doped hydrogenated amorphous Si (a-Si:H) prepared from silane plasmas. Quantitative spectral similarity for amorphous and crystal Si follows from the retention of chemical short range order in the amorphous state. Small differences with doping are observed. The results are used to calculate the expected modulated thermoreflectance for thin film a-Si:H and to point out that electroreflectance experiments may be really looking at the consequences of thermal modulation. © 1980.