Evaluation of optical parameters of quasi-parallel plates with single-frame interferogram analysis methods and eliminating the influence of camera parasitic fringes

Abstract

The surface flatness of transparent plates is frequently tested in Fizeau and Twyman-Green interferometers. In case of quasi-parallel plates, however, a common problem is the additional reflection from the plate rear surface and the occurence of three-beam interference. Conventional methods of interferogram analysis such as temporal phase shifting or Fourier transform fail when the three overlapping fringe sets are present in the image. Our method of deriving optical parameters of the plate requires recording two interferograms: a two-beam interferogram without a reference beam and the three-beam interference one. The images are processed using single-frame techniques only and information about shape of both surfaces and optical thickness variations of the plate is retrieved. Unwanted parasitic fringes introduced by the glass plate protecting the CCD matrix in the camera are also handled using recently developed special smoothing technique. The proposed method is based on algorithmic solution and does not require modification of a sample or the optical setup. The measurement procedure and the detailed image processing path will be presented on the example of quasi-parallel plate interferograms recorded in the Twyman-Green setup.