Patent ID: 8922790

Claim:
An optical film thickness measuring device for an optical thin film forming apparatus including a rotating base substrate holder, comprising: a projector that projects a measurement beam from one side of an axial line of a rotating shaft of the rotating base substrate holder toward an inside of the base substrate holder, a light receiver that receives the measurement beam from the projector, a plurality of inner beam splitters that is provided inside the base substrate holder so as to reflect the measurement beam projected from the projector to a base substrate, an inner optical reflector that is provided inside the base substrate holder so as to totally reflect a measurement beam from a closest inner beam splitter among the plurality of inner beam splitters, a plurality of external beam splitters that is provided outside the base substrate holder so as to reflect the measurement beam from the plurality of inner beam splitters toward the light receiver, and an outer optical reflector that is provided outside the base substrate holder so as to reflect the measurement beam from the inner optical reflector toward the light receiver; wherein the measurement beam reflected by the plurality of inner beam splitters and the inner optical reflector is passed through the base substrate and then reflected by the plurality of external beam splitters and the outer optical reflector so as to be guided to the light receiver, so that the measurement beam is received by the light receiver.