Lens Spherical Aberrations in Cynomolgus Monkeys: Comparison of Laser Ray Tracing Measurements and Reconstructed GRIN Model Predictions

Abstract

Purpose: To compare the spherical aberration of cynomolgus monkey lenses measured with a laser ray tracing (LRT) system and the spherical aberration estimated from numerical ray-trace through a model of the lens with the measured lens shape and a reconstructed gradient refractive index (GRIN).

Methods: Measurements: LRT measurements were performed on 5 cynomolgus monkey lenses from 4 donors (4.4-7.3 years, PMT= 25.2±13.6 hours). The tissue was mounted in a lens stretcher (Ehrmann et al, Clin Exp Optom, 2008) and measured in the unstretched (accommodated) and fully stretched (relaxed) state. The LRT delivered 51 equally-spaced parallel meridional rays over a 6mm diameter zone. The height of each individual ray was measured at 12 positions along the optical axis using a camera. The measured ray heights were plotted as a function of entrance ray height and fit with a 3rd order polynomial. The focus of the lens was determined from the fits and the Zernike spherical aberration coefficients were extracted. Model: The shape and GRIN of the 5 individual lenses were reconstructed from cross-sectional Optical Coherence Tomography images and measurements of the lens power (De Castro et al, IOVS, 2013) in the same stretching conditions. A numerical ray trace was performed through the reconstructed GRIN lens for a 6-mm pupil diameter (101 rays, ray spacing 30µm) to estimate the spot diagrams at the same axial positions as the LRT measurements. The spherical aberration was estimated from a calculation of the wavefront in the exit pupil plane of the reconstructed GRIN lens. The predicted and measured spot diagrams and spherical aberration were compared.

Results: The Zernike spherical aberration coefficients obtained with the LRT were -6.11±0.29µm in the unstretched position and -2.78±0.47µm in the stretched position. The Zernike spherical aberration coefficients estimated with the GRIN reconstruction were -5.84±1.10µm in the unstretched position and -2.34±0.64µm in the stretched position. There was a close match of measured and simulated spot diagrams.

Conclusions: The agreement between experimental data and the model predictions validates the method for reconstructing the GRIN from OCT images of crystalline lenses. Both experiment and model show that Zernike spherical aberration coefficients become more negative with simulated accommodation for cynomolgus monkey lenses.

Keywords: 404 accommodation • 626 aberrations • 630 optical properties