Reflectometer for precise measurement of absolute specular reflectance at normal incidence

In this paper we describe a simple and useful apparatus used to measure the absolute reflectance of a surface at normal incidence. A rotating beam splitter provides measurement of the intensity reflected by the sample and the intensity of the direct beam which has been previously compensated by a plate identical to the beam splitter, so that no correction for the transmittance of the beam splitter is necessary, which may even be unknown. The instrument can be used with or without a standard plate.