Accurate measurements of phase refractive index of soft contact lenses

We describe a measurement approach for the bulk phase refractive index of hydrogel and silicone hydrogel (soft) contact lenses in solution at multiple wavelengths with an accuracy of +/- 0.001, and sensitivity to change of 0.0002. The method uses time-domain low-coherence interferometry to obtain group refractive index (GRI) for contact lenses between 530 to 670 nm in 10 nm increments. The measured GRI dispersion curve is then mathematically converted to the phase refractive index values. The approach is based on a point measurement using focused beam, and therefore does not require flattening of the lens for the measurements. We discuss practical implications of the method for the quality control in manufacturing of ophthalmic optics.