Patent ID: 8913250

Claim:
A grazing incidence interferometer comprising: an interferometer main body having a light source configured to generate coherent original light, a light beam divider configured to divide the original light coming from the light source into measurement light and reference light, an illuminator configured to apply the measurement light obliquely to a measurement subject surface of a measurement subject, a light beam combining module configured to combine the measurement light reflected from the measurement subject surface with the reference light, and a photodetector configure to detect a combined light beam produced by the light beam combining module; a stage configured to hold the measurement subject; a moving mechanism mounted on the stage, and configured to support the interferometer main body and to be capable of moving the interferometer main body along the measurement subject; and an auxiliary reflector fixed to the stage and disposed on an extension of an axis of movement of the interferometer main body by the moving mechanism, wherein the interferometer main body has an auxiliary light beam separator configured to separate auxiliary light from the original light coming from the light source and to apply the separated auxiliary light to the auxiliary reflector, and an auxiliary photodetector configured to detect the auxiliary light reflected by the auxiliary reflector, wherein the auxiliary photodetector comprises a half mirror so as to be located on an optical path between the auxiliary light beam separator and the auxiliary reflector, and a planar photodetector configured to detect a combined light beam of auxiliary light coming from the auxiliary reflector and light reflected from the half mirror.