Abstract
Purpose:
The relative contribution of crystalline lens geometry and gradient index (GRIN) to its spherical aberration (SA) and its age-related changes is unknown. We investigated these relationship using Optical Coherence Tomography (OCT) and Laser Ray Tracing (LRT).
Methods:
11 ex vivo human lenses (22-71 years) from an eye bank were imaged in 3D with a custom OCT to obtain optical path differences. The shape of the lens surfaces was extracted from the images using surface segmentation and Zernike polynomial fitting. The lens power was measured using LRT for 2 and 4-mm pupil diameters. The 3D GRIN was estimated by means of an optimization method based on genetic algorithms (de Castro et al. OE 2010), which searched for the parameters of a 4-variable GRIN model that best fits the distorted posterior lens surface of the lens in 18 different meridians. The SA of the lenses was estimated by computational ray tracing, assuming both a homogeneous index and the estimated GRIN.
Results:
Geometrical data of all lenses were reconstructed using 3D OCT images. Anterior radius of curvature and asphericity varied widely across lenses (6.1 to 11.3 mm, and -9.9 to 5.6, respectively), and were rather constant for the posterior surface (mean: 5 mm and -0.3, respectively). Lens power ranged from 34D (younger lens) to 24D (older lens). In 10/11 lens power decreased with pupil diameter, revealing a negative spherical aberration. Lens thickness (ranging from 3.8 to 5.2 mm) increased and mean group refractive index (ranging from 1.392 to 1.407) decreased slightly with age. The reconstructed GRIN showed surface refractive index values between 1.368 and 1.376, nucleus refractive index values between 1.403 and 1.415, and an exponential decay value ranging from 1.8 to 3.4 (axial) and from 1.9 to 5.8 (meridional). The estimated SA (from lens geometry and index) ranged from -0.8 to 0.3 µm for the equivalent refractive index, and from -2.1 to -0.3 µm for the estimated GRIN. SA shifted with age towards less negative values (slope=0.028 µm/yr and 0.022 µm/yr assuming equivalent index or GRIN, respectively).
Conclusions:
3D OCT data and experimental power data of human donor lenses of different ages allowed reconstruction of the lens GRIN and evaluation of external geometry and GRIN contribution to the lens spherical aberration. GRIN shifted the SA towards negative values in all cases and played a role in the age-related shift of SA.
Keywords: 414 aging: visual performance •
653 presbyopia