Heterodyne Fizeau interferometer for testing flat surfaces

A heterodyne Fizeau interferometer, which uses a rotating radial grating to achieve the required optical frequency shift, is described. An analysis of the effects of grating ruling errors shows that they may be nearly eliminated by averaging the interferometer phase readings over integral numbers of grating revolutions. Experimental tests indicate that the interferometer is capable of measuring with a reproducibility of lambda/200 (lambda = 632.8 nm), limited by temperature effects.