Method to identify optical path transmittance by using cat’s-eye retroreflectors

In optical systems used in nuclear fusion reactors, the reflectance of in-vessel mirrors may change due to the deposition of impurities or corrosion. To address this issue, a methodology was proposed whereby ex-vessel light is used to determine the transmittance of an optical system by measuring the light reflected by a retroreflector that is temporarily placed at the tip of the optical system. However, because the reflectance of retroreflectors themselves is not always constant, a reliable method for absolute sensitivity calibration has yet to be established. In this study, we developed a method for measuring the reflectance of a cat’s-eye retroreflector and, consequently, the transmittance of the desired optical system. Developing this method entailed an analysis of the variation in number of reflections and reflection positions within the cat’s-eye retroreflector.