Physical and Optical Properties of Rigid Gas Permeable Contact Lenses Measured with Optical Coherence Tomography

Abstract

Purpose: To calibrate the axial and lateral dimensions of spectral domain optical coherence tomography (SDOCT) images using rigid gas permeable (RGP) contact lenses. To quantify the physical and optical characteristics of RGP lenses using cross sectional OCT imaging.

Methods: The parameters of 5 spherical RGP lenses of the same design, index of refraction = 1.433, were manually verified. The posterior side of each lens was mounted on a flat surface and imaged using the Heidelberg Spectralis SDOCT system with Anterior Segment Module (Heidelberg Engineering, Carlsbad, CA). The 30 deg image included the entire cross section of the central lens and extended beyond the lens boundaries to included locations of the mounting platform only, which was used as an axial reference. The lateral and axial pixel dimensions were calculated and divided by the known diameter of the contact lens and center thickness. The index of refraction of the lens material was used to compensate for the thickness changes induced in the cross sectional imaging. Optical path length differences created by the thickness and index of refraction change of the lenses were confirmed by measuring by the displacement of the flat mounting surface under the lens, relative to the undeviated surface imaged outside of the lens boundaries.

Results: Lateral and axial characteristics of the OCT images could be successfully calibrated using the known parameters of the RGP lenses. The coefficient of variation for distance conversions of the 5 lenses was less than 1% for each of the axial and lateral dimensions. The displacement of the flat surface caused by the optical path length difference induced by the contact lens was linearly proportional to the thickness of the contact lens (r² = 0.9736).

Conclusions: RGP lenses can be used to calibrate cross sectional OCT images in both the lateral and axial dimensions. Physical characteristics of RGP lenses can be extracted from cross sectional OCT data. Optical properties of RGP lens can be assessed using optical path length differences induced by the RGP thickness and index of refraction changes.