Determination of the effective diameter for the calculation of equivalent thickness of soft contact lenses.

This study determines the effective diameter of soft contact lenses for the calculation of their equivalent thickness by relating the physiological response to the wear of prescription and calibration (constant thickness) lenses in a group of subjects. First a relationship is derived between corneal swelling response and thickness of the calibration lenses. Then, by relating the swelling response obtained with the prescription lens to this relationship, it is possible to determine the equivalent thickness of the prescription lens. After determining the geometry of the prescription lenses it is possible to calculate by an iterative integration technique the diameter of the prescription lens to be used in the determination of equivalent thickness. This diameter is found to differ between plus and minus prescription lenses. The integration diameter for all lens powers was found to be 6.7 mm.