Mapping of optical surfaces with quarter wavelength fringes

A contactless interference surface testing method is described that is capable of creating fringe patterns with fringe separations corresponding to a surface deformation of a quarter-wavelength of the illumination light. Contrary to common Fizeau fringes where the interference pattern occurs by superposing two wavefronts, the described method is based upon a superposition of four wavefronts. These wavefronts are created by a diffraction transmission grating which is contactlessly placed over the surface to be tested. Basically, this technique provides a beat frequency pattern with adaptable fringe separation. Practical applications of this method to semiconductor silicon wafers are presented.