Contouring aspheric surfaces using two - wavelength phase - shifting interferometry †

Two-wavelength holography and phase-shifting interferometry are combined to measure the phase contours of deep wavefronts and surfaces, such as those produced by aspherics, with a variable sensitivity. When interference fringes are very closely spaced, the phase data contain high frequencies where 2π ambiguities cannot be resolved. In this technique, the surface is tested at a synthesized longer equivalent wavelength. The phase of the wavefront is calculated modulo 2π using phase-shifting techniques at each of two visible wavelengths. The difference between these two phase sets is the phase of the wavefront as it would be measured at λ eq =λ1λ2/|λ1−λ2|, assuming that 2π ambiguities can be removed at λ eq. This technique enables surfaces to be contoured to an accuracy of λ eq/l00.