Characterization of rotational symmetry and noise in multifocal contact lens power maps

Abstract

AbstractPurpose: Spherical multifocal contact lenses are typically designed with a rotationally symmetrical power profile. Imperfections in the raw material, manufacturing and measurement process can break this symmetry, altering the intended lens design. The purpose of this work is to develop a method to separate the intended symmetric design profile from the asymmetries and to give the spatial distribution of the measurement noise uncertainty in diopters.Methods: NIMO TR1504 (Lambda‐X) version V.2.13.2 has been used to measure the local power map of various multifocal contact lenses. From the map, a set of radial power profiles is obtained for different angles (0°, 180°), (0°, 360°). A symmetrical lens should show a constant radial profile for different angles, while asymmetries manifest as differences in the set of radial power profiles. They are analysed using a Principal Components Method (PCA) implemented in MATLAB.Results: The method displays a map of the symmetric radial profile of the lens, another map of the deviations with spatial structure indicating the angles where the deviations are stronger (rotational asymmetries), and noise with three separated components (spatially random variations). Asymmetries of the order of 0.17D (with maximums of 0.4D), and random noise values of 0.12D were found in some lenses.Conclusions: The characterization of multifocal contact lens prescription is important in visual optics. The prescription perceived by the patient depends on the distribution of the radial power profile on the pupil. That is the reason why asymmetries and imperfections estimation is important. The method developed in this work allows to calculate a diopter equivalent number for them.