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Judith Birkenfeld, Alberto de Castro, Sergio Ortiz, Pablo Pérez-Merino, Enrique Gambra, Daniel Pascual, Susana Marcos; Relative Contribution Of Crystalline Lens Surface Shape And Gradient Index Distribution To Spherical Aberration. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2230.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the contribution of the crystalline lens surface shape and its gradient index (GRIN) distribution to the spherical aberration of the lens, using Optical Coherence Tomography (OCT) and Laser Ray Tracing on in vitro porcine lenses.
The 3D GRIN of ten different in vitro porcine lenses was estimated by means of an optimization method based on genetic algorithms (de Castro et al. OE 2010). The optical path differences were obtained from 3D OCT images taken from every lens in two orientations. The 3D shape of the anterior and posterior crystalline lens surfaces was estimated using surface segmentation and fitting to Zernike polynomials.As additional input parameter, power measurements were performed with a laser ray tracer to determine the focal length of the individual lens at two different diameters (2 and 4mm). The optimization method searched for the parameters of a 4-variable GRIN model that best fits the distorted posterior surface of the lens in 18 different meridians. The spherical aberration of the lenses was estimated by means of computational ray tracing (assuming a homogeneous index, or the estimated GRIN) on each of the lenses.
The mean value of the radius and the asphericity of every lens were calculated for the anterior and the posterior surface. The mean radii of curvature of anterior and posterior surfaces were 6.9mm±0.3 and 4.9mm±0.3, respectively. Differences of up to 0.3mm were found between meridians, consistent with the presence of astigmatism. The mean asphericity values were 1.4±0.4 and 0.8±0.3, respectively.The estimated mean value of the surface refractive index at 663nm was 1.361 and the mean nucleus phase refractive was 1.435, within an estimated error of 0.003 (based on five repetitions of the algorithm). The mean equivalent phase refractive index was 1.462. The mean axial exponential decay was 1.73 and the mean meridional exponential decay was 2.34. This last parameter changed in the same lens between meridians up to 1.7, indicating a contribution of the GRIN to the lens astigmatism. The average spherical aberration of the lenses (for a 6-mm pupil) was estimated to be -3.2 ± 1.7 µm, considering the calculated GRIN distribution. For a homogeneous lens with equivalent refractive index the spherical aberration was 4.9 ± 1.0 µm.
3D reconstructions of the porcine crystalline lens shape and GRIN in vitro, from OCT imaging, allowed evaluating the relative contributions of the surface shape and GRIN to the lens spherical aberration. GRIN plays a fundamental role in the shift of the spherical aberration of the lens towards negative values. While the effect was well known in the fish lens, this is a novel demonstration in a mammal lens in three dimensions.
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