an interferometer which has the capability of measuring optical elements and systems with an accuracy of λ / 1000 where λ is the wavelength of visible light . whereas current interferometers employ a reference surface , which inherently limits the accuracy of the measurement to about λ / 50 , this interferometer uses an essentially perfect spherical reference wavefront generated by the fundamental process of diffraction . this interferometer is adjustable to give unity fringe visibility , which maximizes the signal - to - noise , and has the means to introduce a controlled prescribed relative phase shift between the reference wavefront and the wavefront from the optics under test , which permits analysis of the interference fringe pattern using standard phase extraction algorithms .