Optical spectroscopy of molten fluorides: Methods, electronic and vibrational data, structural interpretation, and relevance to radiative heat transfer

•Electronic and vibrational absorption data for transition-metal, lanthanide, and actinide solutes.•Vibrational absorption data for alkali and alkaline earth fluoride salts.•Structural interpretation and quantification of radiative heat transfer effects.•Transmittance and reflectance experimental setups for optical studies of molten salts.•Estimated gray absorption coefficient at 700 ˚C: 546 m−1 for FLiBe and 276 m−1 for FLiNaK.