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Fabrice Manns, Bianca Maceo, Arthur Ho, Jean-Marie Parel; Contribution of the Refractive Index Gradient to the Spherical Aberration of the Human Crystalline Lens. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3406.
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To determine how age-related changes in the refractive index gradient of the human crystalline lens affect changes in lens spherical aberration with age.
An age-dependent optical model of the relaxed crystalline lens with continuous index gradient was developed, using the following parameters (from Dubbelman et al, 2001 Vis Res): Anterior Radius (mm) = 12.9-0.057×Age; Anterior Asphericity = -4; Posterior Radius (mm) = -6.2+0.012×Age; Posterior Asphericity = -3; Thickness (mm) = 2.93+0.024×Age. The refractive index gradient is modeled as a set of aspheric iso-indicial surfaces, with radius of curvature R(z) and asphericity Q(z) that vary linearly from R=0 and Q=0 at the lens equator (z=0) to the value at the surface. The axial refractive index follows a power-dependence in each half of the lens: 1.421-0.035×(abs(z)/t)b, where t is the anterior or posterior half-thickness (measured from the equator) of the lens and abs(z) is the absolute value of z. The value of the coefficient b was calculated to produce an equivalent index equal to n=1.441-0.00039×Age. The contributions of the anterior and posterior surfaces and the differential contribution of each iso-indicial surface to Seidel primary spherical aberration was calculated analytically for a ray height of 1mm. The differential contribution was integrated numerically over the lens thickness to calculate the spherical aberration contribution of the gradient.
The power coefficient increases from b=3 at age 18 to b=10 at age 50, consistent with the formation of a refractive index plateau. The model predicts that the gradient contribution to spherical aberration is -0.23µm at age 18, it becomes progressively less negative with age, and is positive above age 30. The surface contribution decreases monotonically with age from 0.07µm at age 18 to 0.02µm at age 50.
The progressive formation of a refractive index plateau with age progressively changes the lens Seidel spherical aberration from negative to positive.Support: NEI Grants: 2R01EY14225, 5F31EY15395, P30EY14801 (Center Grant); the Florida Lions Eye Bank; the Henri and Flore Lesieur Foundation; Research to Prevent Blindness; Australian Federal Government CRC Program (Vision CRC).
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