Phase shifting Fizeau interferometry of front and back surfaces of optical flats

The interpretation of Fizeau interferograms of optical flats is not straightforward because they are composed of more than two reflections. This results in a confusing fringe pattern. There are three main contributions to the interferogram given by the reflections from the reference surface, the front and the rear surface of the sample. We present a new to the best of our knowledge solution to the problem. We use phase shifting measurements of the wave fields, which are reflected by and transmitted through the sample. This eliminates the need for the suppression of reflections by immersion or other methods. As an illustration of this method, several examples will also be presented.