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Rejean Munger, Cynthia N. Nelson, IV; Uniformity Of Equatorial Grin Profiles Across 3 Mammalian Species. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3405.
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© ARVO (1962-2015); The Authors (2016-present)
The gradient of index of refraction of the crystalline lens (GRIN) in humans and other mammalian species is still not fully understood and cannot be measured directly and cannot be easily inferred from optical measurements because of its complex shape and lack of symmetry. One exception is the equatorial plane of the lens dividing the anterior and posterior segment of the lens where the GRIN has been shown to be rotationally symmetric. Any full GRIN lens model must this equatorial GRIN as a starting point. The purpose of this project was to compare the equatorial GRIN of different mammalian species to determine if a common mechanism is occurring across species.
A device was constructed which allowed multiple collimated pencils of light (1mm separation) to be incident along the equatorial plane of the crystalline lens. Images of the ray pattern following refraction through the lens were recorded in two planes and ray deviations calculated. The equatorial GRIN was then calculated for each lens using the well know solution for rotationally symmetric GRINS n(r)= nc [1- (nc2- ns2)/nc2)f(r)]1/2 where nc and ns are the central and surface refractive index of the lens respectively and f(r) is a unique function for each lens. As in previous work we used the form f(r)=a +b(r/R)2+c*(r/R)4 where R is the radius of the lens at the equator and r the distance from the lens center. Equatorial GRIN for porcine (n=17), rabbits (n=7) and bovine (n=6) lenses were obtained and the variances across species for each coefficient (a,b, and c) compared using a one way Anova.
Although lens equatorial diameters varied widely across species and more modestly within species it was found that there were no significant differences in the f(r) parameters across species (P= 0.52, 0.62, and 0.16 for a, b and c respectively). The parameters for rabbit, bovine and porcine lenses were respectively; a= [0.005 (0.005), 0.005 (0.005), 0.007 (0.009)], b= [0.40 (0.06), 0.43 (0.03), 0.41 (0.06)], c= [0.58 (0.06), 0.62 (0.05), 0.59 (0.04)], where values in brackets are standard deviations.
Within measurement accuracy of ray tracing we find that the normalized (to lens radius) equatorial GRIN function is the same across 3 mammalian species are not statistically different suggesting that a single equatorial GRIN function could be used across these 3 species (rabbit, cow and pigs). This suggests that it may be possible to describe the equatorial GRIN of most mammalian lenses, including humans, using a single GRIN profile.
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