Perceived prescription for multifocal contact lenses

Abstract

AbstractPurpose: Multifocal contact lenses have a variable power map that is typically radial. The prescription of these lenses is given as a Sphere plus Addition values in diopters. They are calculated by assigning different radial rings of the lens to different mean power values (near and far vision zones). Depth of focus (DOF) should be high in order to focus both images on the retina. This calculation usually depends on each manufacturer and design type. The purpose of this work is to give a simple common criterion for perceived prescription, based on visual criteria and patient's pupil.Methods: The radial power profile of different multifocal contact lens designs was measured with a NIMO TR1504 (Lambda‐X). Using geometrical optics and MATLAB, ray tracing is performed and the position of the smallest spot size plane, noted as Df, is recorded. Spot size in that plane is recorded too. From Df, graphs of perceived diopters (inverse of Df in meters) versus different diameter portions of the lens are obtained.Results: The method provides the sphere and addition values for lenses with zonal designs based on the maximum and minimum effective power seen by the patient as a function of pupil size. The main perceived diopter grows or decrease smoothly depending on centre‐near or centre‐distance designs. The calculation of the approximate size of the spot with the diameter also allows estimating the DOF too. In Extended Depth of Focus (EDOF) lenses the method is also applicable without changes of criteria.Conclusions: The determination of the prescription perceived by the patient with multifocal contact lenses is important and there are no uniform methods for the different designs. The inclusion of the diameter used by each patient allows the inclusion of visual criteria for calculation of main perceived diopter and the use of a common method.