Fig. 1
From: Hybrid refractive holographic single vision spectacle lenses
Illustration of two sample gaze directions (a) and (b) of an eye using a spectacle lens. The aim of optical design of spectacle lenses is to correct the refractive errors of the optics of the eye for all possible gaze directions. These refractive errors such as SPH or AST are defined using the back focal lengths of the lens with reference to the vertex sphere