Effects of performance parameters on the measurement of back vertex power of soft contact lenses using phase-shifting Schlieren method

Abstract

Back vertex power is a fundamental parameter for evaluating the optical performance of contact lenses, and the accuracy of its measurement is directly related to the corrective efficacy of the lens. Traditional methods, such as lensometers, require physical contact and are prone to errors caused by contact pressure, lens deformation, or misalignment, which prevent fast, high-resolution non-contact measurement. To overcome these limitations, this study employed an interferometer that combines the schlieren principle with phase shifting technology. The technique converts light deflection into phase information and obtains quantitative measurements by calculating the wavefront gradient, i.e., this approach is known as the phase-shifting schlieren method. Compared with Shack-Hartmann wavefront sensors and Moiré deflectometry, which are inherently limited in spatial resolution, the phase-shifting schlieren method encodes both phase and intensity at each pixel, providing a wider dynamic range and higher spatial resolution. The phase-shifting schlieren method enables non-destructive and rapid measurement based on known lens parameters. To derive the back vertex power from the measured wavefront gradient, an optical model requires input parameters such as the liquid refractive index, the material refractive index, the base curve radius, the center thickness, and the measuring aperture. This study systematically investigates the influence of these parameters on the accuracy of the conversion process which directly affects the reliability of the final results. To enhance the generalizability of the findings, soft contact lenses with differing water contents were included in the experimental cohort. The research demonstrates that the sensitivity of back vertex power measurements to different input performance parameters exhibits marked variability. Specifically, back vertex power measurements are relatively insensitive to changes in base curve radius, center thickness, and measuring aperture, but highly sensitive to even small variations in the saline and lens refractive indices. Consequently, precise control and calibration of these critical parameters during measurement are essential to ensure accurate back vertex power results.