Optical testing and inspection methodology for modern intraocular lenses

A method of in vitro optical performance evaluation of intraocular lenses (IOLs) is discussed. The IOLs are tested in the eye cell, which is designed to be optically equivalent to the theoretical eye model. The eye cell is a combination of a wet cell and the optical element (the "conjugation lens") that converges a beam of light, similar to the action of the cornea in the theoretical eye model. The eye cell is designed so the spatial frequency scale (modulation transfer frequency [MTF] testing), expressed in terms of line pairs per millimeter (linear scale), equals the scale produced by the theoretical eye model. The most common way to express spatial frequencies in visual science is in line pairs per degree or cycles per degree (angular scale). The problem with in vitro testing is that the corresponding angular scale varies with the dioptric power of the conjugation lens. This paper provides a simple method of mapping the angular scale of the eye cell onto the theoretical eye model, allowing a comparison with clinical results as well as with MTFs measured in different eye cells. All corresponding formula for calculating the eye cell optical characteristics are provided.