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Arthur Ho, Fabrice Manns, Bianca Maceo, David Borja, Stephen Uhlhorn, Esdras Arrieta, Jean-Marie Parel; Estimation of Central Average Group Refractive Index of Crystalline Lens from Optical Thickness Measurements. Invest. Ophthalmol. Vis. Sci. 2011;52(14):815. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the central/axial average group refractive index (AGRI) of the crystalline lens for the purpose of converting optical thickness measurements to geometrical thickness values. The age-dependency of AGRI was also investigated.
Isolated baboon (n=56, 1.8-28 y/o) and human lenses (n=103, 6-94 y/o) were imaged with either a custom-built time-domain OCT system (24 baboon, 53 human), or a shadow-photographic system (35 baboon, 51 human) fitted with a digital camera. From the OCT images, the central/axial optical path length (OPL) was extracted while from shadow-photographic images, the central thickness (CT) of lenses was measured. The quotient of the OPL to CT represents the AGRI of the lens along its central axis.
The average AGRI was 1.414 and 1.450 for baboon and human lenses respectively. An age-dependency of AGRI was apparent in the lens of both species. The youngest age-range of the baboon lens returned an average AGRI of 1.362 while the oldest returned AGRI of 1.488. In human lenses, the 14-29 y/o group had AGRI of 1.443 while the oldest (60-69 y/o) had AGRI of 1.479. By algebraically combining two mathematical relationships obtained by linear regression between OPL and CT with age, age-dependency was found to be AGRI=-4.770×10-5 Age2 + 4.834×10-3 Age + 1.355 for baboons and AGRI=-6.852×10-6 Age2 + 1.826×10-3 Age +1.380 for human.
Unlike previous studies, we find that the average group refractive index of baboon and human lenses increases with age. This increase with age is consistent with the formation of a refractive index plateau.
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