The Optical Technology Division of the National Institute of Standards and Technology has developed a new reference densitometer for measuring visual diffuse transmission densities using the diffuse influx mode. This densitometer is used to calibrate both x-ray and photographic film step tablet Standard Reference Materials. The design, characterization, and operation of the densitometer are detailed. The densitometer was fully characterized both to verify compliance with the applicable documentary standards and to determine the combined uncertainty in transmission density associated with the calibrations.