Spatially resolved height response of phase-shifting interferometers measured using a patterned mirror with varying spatial frequency

In the performance evaluation of phase-shifting interferometers for figure metrology, the height response, or height transfer function, is rarely taken into consideration, because in most applications smooth surfaces are measured and only the lowest spatial frequencies are of interest. For measurements with low uncertainty it is important to understand the height response as a function of the spatial-frequency content of a surface under test, in particular when it contains form-error components with frequencies at the high end of an interferometer's spatial-frequency passband. A mirror with a patterned area of 140-mm diameter, consisting of several subpatterns with varying spatial frequency, was used to evaluate the spectral response. Our goal was to develop a method for efficient mapping of the spectral response over the circular field of view of a phase-shifting interferometer. A new way of representing the dependence of the spectral response on the field of view of an interferometer is described.