Quasi-parallel glass plate measurements using Fizeau interferometer

The surface flatness of transparent plates is frequently tested in a conventional Fizeau interferometer. In case of quasi-parallel plates, however, a common problem is the additional reflection from the plate rear surface. Unwanted parasitic intensity distribution modulates the two-beam interferogram of the plate front surface and makes the application of phase methods for automatic fringe pattern analysis inefficient. On the other hand parasitic fringes contain the information on the light double passage through the plate (i.e., optical thickness variations). Several methods to suppress unwanted fringe modulations are available. However, these methods require either modification of a sample or sophisticated equipment and complicated data analysis. In this paper we present our proposal of processing the three-beam interferograms obtained in a Fizeau interferometer when testing quasi-parallel optical plates. The modulation distribution of acquired pattern encodes the information of the plate optical thickness variations, whereas the phase distribution contains the information about the sum of profiles of both surfaces (uniform refractive index distribution is assumed). Both maps can be derived using a combination of different interferogram analysis techniques such as Temporal or Spatial Carrier Phase Shifting and Vortex Transform. As the result separate information about both surfaces from a single measurement can be obtained.