Broad wavelength range infrared lens refractive index measurement using confocal tomography

A new infrared confocal refractive index measurement (IR-CRIM) method with high precision is proposed for a lens. Based on the property that the maximum point of a confocal axial intensity response curve accurately corresponds to the converging focus of the measuring beam, IR-CRIM can precisely identify the front and back vertices of the test lens, and obtain the optical thickness d of the test lens, and resulting calculate the lens refractive index n using the ray-tracing algorithm. The broadband refractive index dispersion of the test lens can be acquired by the Cauchy dispersion law and the n obtained at several wavelengths. Preliminary experimental results and theoretical analyses indicate that IR-CRIM achieves an accuracy of 6 × 10⁻⁴ in the wavelength range of 500–1700 nm. It provides a novel approach for the high-precision and direct measurement of the refractive index of a lens in visible and near infrared wavelength range.