Abstract
Purpose :
Measurement of the refractive index of the crystalline lens in the human eye in vivo has been a challenge for a long time. The anterior segment optical coherence topography (AS-OCT) and Scheimpflug imaging (SI) technique have been used to access the refractive index in vivo, but need assumption of the lens thickness. Theoretically, a combination of the AS-OCT and the SI could solve the lens refractive index and lens thickness simultaneously. The purpose of this study was to measure the refractive index of the human eye in vivo by using an AS-OCT equipped ray-tracing SI system (RTSI).
Methods :
An AS-OCT was constructed to be combined with a RTSI, and mounted on a slit lamp platform together. Both AS-OCT and RTSI shared the same vertically scanning light illumination (about 850nm). The right eyes of five subjects (24 to 62 yr old) were tested. The images were processed to trace the light ray from the cornea to the lens and to accurately solve the lens refractive index and the lens thickness by using a self developed MatLab program. The derived lens refractive index represents the equivalent refractive index in the optical axis direction.
Results :
During the image processing, the refractive index of aqueous was first obtained. For the five subjects, the mean aqueous refractive index was 1.3312±0.0013. The mean refractive index of the lens at the center was 1.4032±0.0177, with a range of 0.0366, which was more than 10 times of that for the aqueous (0.0034). The lens refractive index decreased as the distance from the lens center increased, and the rate of reduction in the lens refractive index varied from subject to subject. The mean central lens thickness was 3.93±0.49mm with a range of 1.10mm.
Conclusions :
A combination of AS-OCT with RTSI accurately measures the refractive index of the ocular media in the human eye in vivo, and the measurement of the refractive index of the aqueous provides a useful validation of this method. The lens refractive index changes from one eye to another, and reduces from the lens center to the periphery. The rate of the reduction of lens refractive index estimates the gradient refractive index of the lens. With the lens refractive index measured, accurate estimate of the posterior lens shape is achieved.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.