Abstract: The present invention provides a method and apparatus for measuring the reflectivity of a substrate surface in situ. A transmitting assembly includes a light source positioned externally to a vacuum chamber and proximate an opening to transmit an optical beam into the vacuum chamber. An optical beam supplied by the light source is transmitted into the chamber via one or more optical devices such as fiber optics cables, lens and the like. Reflected portions of the optical beam are collected by a receiving assembly. A signal processing system coupled to the receiving assembly is programmed to determine the reflectivity of a substrate disposed in the vacuum chamber.