Application of optical coherence tomography to automated contact lens metrology

Optical coherence tomography (OCT) is a nondestructive imaging modality with the potential to make quantitative spatial measurements. OCT's noncontact nature, sensitivity to small refractive index mismatches, and micron-scale resolution make it attractive for contact lens metrology, specifically, measuring prism. Prism is defined as the maximum difference in thickness of the contact lens, measured over a full 360 deg of rotation, at a fixed distance from the contact lens edge. We develop and test a novel algorithm that automatically analyzes OCT images and calculates prism. Images are obtained using a Thorlabs OCT930SR OCT system. The OCT probe is fastened to an automated rotation stage that rotates 360 deg in small increments (typically 10 deg) to acquire OCT images of the edge of the contact lens around the entire circumference. The images are 1.6 mm in optical depth (512 pixels) and 2 mm wide (1000 pixels). Several sets of images are successfully analyzed. The prism measured for a toric lens is 42 μm, which is in line with design parameters. Thickness measurements are repeatable with a standard deviation of 0.5 μm and maximum range of 1.8 μm over ten image sets. This work demonstrates the possibility of using OCT to perform nondestructive contact lens metrology.