During the process of optical system design and layout, it is often necessary to determine the depth, or sag, of an optical surface at some specific height (aperture radius). For example, consider the configuration of a simple lens cell, as shown in Figure G.1(a). In order to protect the glass from physical damage by assuring that the lens vertex does not extend outside the lens cell, it is necessary to determine the sag of the convex lens surface at the height designated Y.