Optical method for estimating the equivalent refractive index of the crystalline lens in vivo

In numerous studies it has been necessary to represent refractive indices of the crystalline lens by an equivalent refractive index (here denoted nL). Its traditional value of 1.416, based on in vitro measurements, may be in error by more than 0.025 in some cases. In vivo estimates are subject to recognized deficiencies of ultrasonography. This study describes and evaluates a new and exclusively optical technique. Using exact ray tracing, nL is found by combining routine optical section data with optical depths that may be measured by a ranging system termed OCDR. Gullstand's no. 1 eye is used to calculate the effects of errors in eye data on the accuracy of the technique. From pooled errors estimated for 22 parameters, a standard deviation of 0.0045 is anticipated for the estimate of nL, although allowance for additional uncertainty associated with the refractive index profile of the lens must await further research.