Contribution of shape and gradient refractive index to the spherical aberration of isolated human lenses

Abstract

Purpose: To estimate the contribution of 3-dimensional (3D) lens geometry and gradient refractive index (GRIN) to the lens spherical aberration (SA) with age.

Methods: A total of 35 donor human lenses (19-71 years) were imaged with 3D-spectral optical coherence tomography (sOCT). Paraxial and nonparaxial back focal length were measured with laser ray tracing (LRT). The parameters of a 4-variable 3D GRIN model were reconstructed from the data using a global search algorithm. Spherical aberration was calculated for GRIN lenses and their corresponding homogeneous lenses.

Results: Lens thickness and radii of curvature increased significantly with age. Negative anterior conic constant shifted toward more-positive values (slope: 0.228, P < 0.001), whereas posterior values remained almost constant (slope: 0.0275, P = 0.002). We found a minor decrease and a small significant increase of nucleus and surface refractive index, respectively. The GRIN meridional power exponent had a tendency to increase, indicating a flattening of GRIN distribution, whereas the axial exponent remained almost constant. We did not find a significant age-dependence of the equivalent index. The back focal length had a significant increase with age (P < 0.001). The SA shifted toward less-negative values (slope: 0.0249, P < 0.001) at higher rates when considering the reconstructed GRIN (slope: 0.041, P < 0.001).

Conclusions: Three-dimensional sOCT and LRT allowed reconstruction of lens geometry and GRIN in isolated lenses. The constancy of the GRIN axial power exponent, and the opposite slopes of surface and nucleus indices with age, explain the minor variations of the average index. Both geometrical changes and increase in the GRIN meridional power exponent contribute to the age-dependent shift of negative SA.

Keywords: crystalline lens; gradient refractive index; spherical aberration.