  To help address the need for predicting radiative heat transfer (RHT)
behavior of molten salts, we conducted a comprehensive review of methods and
data from infrared optical spectroscopic measurements on molten fluoride salts.
Transmittance, reflectance, and trans-reflectance experimental methods are
discussed, along with the corresponding data reduction methodology and the
limitations of each technique. Optical spectroscopy is a convenient indirect
probe for changes in structural parameters with temperature and composition.
Electronic and vibrational absorption data for transition-metal, lanthanide,
and actinide solutes and vibrational absorption data for alkali and alkaline
earth fluoride solvents are compiled, and the corresponding structural
interpretation is discussed and compared with other experimental and
theoretical work. We find that solvent and solute vibrational absorption can be
significant in the mid-infrared, resulting in near-infrared edges of
significance to RHT. Extrapolation and averaging of existing edge data leads to
estimated gray absorption coefficient values at 700 degrees C of 493 m^(-1) for
FLiBe and 148 m^(-1) for FLiNaK, both within the range of 1 to 6000 m^(-1)
identified to be of engineering relevance for radiative heat transfer analysis.
