Real-time curvature radii measurements using diffraction edge waves

Abstract

A laser beam impinging at grazing incidence on a 3-D surface produces a diffraction edge wave whose obliquity factor contains, among other things, the information concerning the local radius of curvature of the surface in the plane of incidence. In this paper we show how to retrieve this curvature radius information from light irradiance measurements of the edge wave and from exact electromagnetic diffraction calculations. This new method of optical metrology gives the curvature radius of a surface in the incident plane and at a given point by a single measurement at that point, while other techniques depend on a scanning or a mapping in the vicinity of the point of interest. Moreover, a measuring system using this diffractional method is very easily implemented and can yield a precision in the 5–10-μm range. Experimental results are presented for six metallic circular cylinders and two metallic spheres.