A system for obtaining a measurement representative of a thickness of a layer on a substrate includes a support to hold a substrate, an optical assembly to capture two color images with light impinging the substrate at different angles of incidence, and a controller. The controller is configured to store a function that provides a value representative of a thickness as a function of position along a predetermined path in a coordinate space of at least four dimensions. For a pixel in the two color images, the controller determines a coordinate in the coordinate space from the color data, determines a position of a point on the predetermined path that is closest to the coordinate, and calculates a value representative of a thickness from the function and the position of the point on the predetermined path.