Fringe localization in the Twyman-Green Interferometer using extended monochromatic sources

It is well known that coherent illumination causes dust diffraction patterns which limit the ultimate accuracy of phase measurements. By the use of an extended monochromatic light source the phase can considerably be smoothened allowing a better accuracy for phase measurements. Before the invention of the laser interferometry had to use extended monochromatic sources like spectrum lamps. For plane mirror interferometers illuminated with an extended monochromatic incoherent source the localization plane and the contrast distribution in space have been described in extenso in the literature (see e.g. G. Schulz). In the 40-th of last century G. Hansen derived the condition for high fringe contrast in the Twyman-Green interferometer by rectifying the Twyman-Green geometry of sphericity tests to a plane mirror interferometer. Experiments will be presented using an extended source in form of a laser spot on a rotating scatterer demonstrating the theoretical background. These experiments demonstrate the necessity to image the surface to be tested as well as the reference surface sharply onto the detector plane in order to maintain high contrast. Due to the extended source the fringes are smooth and represent a cos-type intensity distribution.