The optical properties of sputter deposited hafnium fluoride films and their use in fabricating a front surface, interference filter for thermophotovoltaic energy conversion were investigated. In particular, HfF4 films were being considered for the low index component in a multilayer interference filter that will selectively transmit photons in the wavelength range of 1.0-2.4 mum, and strongly reflect all other wavelengths. Processes were developed for sputter depositing HfF4 films with an index of refraction between 1.5 and 1.6, and an extinction coefficient below 0.001 in the 1.0-2.4 mum wavelength range. All HfF4 coatings were deposited by RF reactive magnetron sputtering with pressed powder targets. Chamber pressure during depositions ranged from 0.27 to 1.33 Pa, and RF power ranged from 50 W (0.9 1 W/cm(2)) to 150 W (2.73 W/cm(2)). Deposition rates depended primarily on target power and ranged from 0.03 to 0.1 nm/s. All depositions were performed with Ar as the sputtering gas. Resputtering effects due to plasma and adatom bombardment and neutral Ar atoms reflected off the target played an important role in sputter deposition of hafnium fluoride. Film thickness and composition varied strongly with substrate distance from the target axis. Resputtering of growing films by energetic argon atoms also resulted in strongly absorptive, fluorine-deficient films on the substrate platform near the target axis. However, films became increasingly more stoichiometric and transmissive as one moves away from the target axis. The films were found to be amorphous or weakly crystalline. Nine-layer interference filters were fabricated utilizing HfF4 and an Si:H alloy for the low- and high-index components, respectively. Physisorbed water in the HfF4 layers and Si-H bond stretching were the primary sources of absorption between 1 and 12 mum wavelengths.