The definition of thickness for a lens

The thickness of a lens depends upon the direction of the measuring vector. The thickness as commonly used from the sagittal height formula does not give the path length for movement of gas, water, and heat through a lens. Furthermore, the amount of gas, water, or heat moving through a lens depends upon an areal average thickness, not central thickness. A formula is derived for an approximate areal averaged sagittal height thickness. This average thickness has the advantage of being easily calculated from a sagittal height table. The equation for the areal average thickness along a radius of curvature is also derived. Sample calculations are made to show how this average thickness is a function of lens dimension. It is recommended that an areal average thickness be used when oxygen transmissibility of a lens is calculated from oxygen permeability of the lens material.