Sagitta (SAG)

Sagitta (SAG) of any arc is defined as the distance from the center of the arc to the center of its base. The base can be thought of as the cord connecting the end points of the arc.

In optics, SAG is used for the prescription of lenses and mirrors as show in the figure. Occasionally, SAG is used in relation with the radius of curvature of a wave front.

For a spherical surface, SAG can be calculated based on the diameter D and the radius of curvature RoC: $$SAG = RoC - \sqrt{RoC^2 - \left(\frac{D}{2}\right)^2}.$$