Precision interferometric measurements of refractive index of polymers in air and liquid

We have developed a procedure for precise measurement of the group refractive index for materials in air and liquid environments, using a low coherence interferometer. For example, in manufacturing of soft contact lenses, the lenses are always kept hydrated in a saline solution. Knowing accurate refractive index of the lens is important to metrology and quality control purposes. The small refractive index difference between the liquid and the lens makes such tasks especially challenging. The developed procedure allows us to obtain measurement repeatability for group refractive index less than 1 x 10(-3) for materials with thicknesses on the order of 100 microns, when measured in liquid. The measurement repeatability further improves for measurements in air, or for thicker materials.