Testing aspheric surfaces using two-wavelength phase-shifting interferometry

Abstract

Two-wavelength holography and phase-shifting interferometry are combined to measure the phase contours of deep wave fronts and surfaces, like those produced by aspheres, with a variable sensitivity. When interference fringes are very closely spaced, the phase data contain high frequencies where 2π ambiguities cannot be resolved, and many fringes are likely to fall on a single detector element. To reduce the problem of averaging fringes in a single pixel, an array of pinholes is used to mask the detector array enabling point sampling of the single-wavelength fringes. In this technique, the phase of the wave front is calculated modulo 27π using phase-shifting techniques at each of two visible wavelengths. The difference between these two phase sets is the phase of the wave front as it would be measured at a synthesized equivalent wavelength λeq = λ1λ2/|λ1 − λ2|, assuming that 2π ambiguities can be removed. The integrated two-wavelength data are used to correct the 27π ambiguities in the single-wavelength data, which increases the measurement range and keeps single wavelength precision. This technique enables aspheric surfaces with hundreds of waves of optical path difference to be measured to λ/100 in the visible.