Interaction Between Primary and Secondary Spherical Aberrations and Its Impact on Retinal Image Quality Through Focus

Abstract

Purpose: : It has been proposed that spherical aberrations (SA) can improve retinal image quality for a greater depth of focus with a single optical element. This study was performed to understand how primary and secondary SA can affect the resultant retinal image quality (RIQ) through focus.

Methods: : Primary (Z₄⁰) and secondary (Z₆⁰) SA were modeled using a custom made algorithm in Matlab. The aberration interaction was calculated for a 4.5mm pupil in monochromatic light (0.632µm) from 0 to 3 diopters (D). The area under the visual Strehl optical transfer function (AVSOTF) was calculated to predict visual performance as a function of focal ranges (distance = 0 to 0.75D, intermediate = 1 to 1.75D and near = 2 to 3D). In this model, interactions were optimized for distance RIQ which was chosen as the maximum VSOTF value for each interaction.

Results: : With primary SA alone, intermediate and near RIQ began to dominate the AVSOTF at -0.2µm and -0.4 µm respectively with diminished distance RIQ at these points. On the contrary, both positive and negative values of secondary SA produced optimal intermediate and near RIQ. There was a strong positive correlation between primary SA and positive secondary SA (R² >0.97) when they interact with each other. The ratios of secondary SA to primary SA for the intermediate and near peaks were 0.35 ± 0.01. The difference in secondary SA between the intermediate and near peaks was constant (0.1 ± 0.01 µm). The equivalent RIQ for intermediate and near focal ranges was obtained at half the secondary SA difference between the peaks.

Keywords: intraocular lens • aberrations