Surface Figure and Form

Abstract

Surface figure and form tolerances are low spatial frequency errors in an optical surface that directly contribute to the wavefront error on an optical surface. For surfaces sufficiently near the stop or a pupil conjugate, these errors are scaled and applied to the wavefront error of the system. Surfaces that are nearer than other surfaces to the object or an image conjugate will have the illuminated region affecting the wavefront. Such field lens and flattener types of optics can also have slope requirements depending on the required field flatness, distortion, and piston. These types of form errors are critical in defining the expected tolerances on an optical surface and may drive the tolerance budget for the entire optical system. Surface figure errors can be measured using numerous methods such as profilometry and interferometry. Data is assessed using different performance metrics such as power, irregularity, wavefront error [root mean square (RMS), peak-to-valley (PV), or robust peak-to-valley (PVr)]. Data can be analyzed in other various ways including decomposition into Zernike polynomials. In general, the measurement of precision optics’ surface figure errors is often performed with an interferometer. Because there are multiple ways to describe the surface figure error for an optical surface there are many ways to describe the surface figure tolerances with ISO 10110.