Direct Phase Measurement Of Aspheric Surface Contours

Two-wavelength holography and phase-shifting interferometry are combined to measure aspheric surface contours with a variable sensitivity. In this technique, the surface is effectively tested at a synthesized longer equivalent wavelength λeq = λaλb|λa - λb| using measurements made at wavelengths λa and λb where the difference of the phases measured for λa and λb yields the modulo 2π phase at λeq. A mask of point apertures is placed over the detector array in order to resolve closely spaced fringes. This technique has an rms repeatability of λeq/100. Limits to this technique are discussed and results are shown.