Non-destructive measurement of lens refractive index based on point diffraction interferogram fringe frequency and numerical modeling

We propose a non-destructive method for measuring the refractive index (RI) of lenses, based on fiber point diffraction interferogram fringe frequency combined with numerical modeling. The interference fringes generated by the two fiber point light sources (PLSs) passing through the lens are enhanced by applying zero-padding to improve the apparent spatial frequency resolution. Spatial frequency information is extracted through a Fourier transform. A measurement model that takes the fringe frequency as input and the RI as output is then established using ray tracing combined with polynomial fitting. To enhance measurement accuracy, standard glass lenses with known RI and curvature radius are used to calibrate and correct the model. Experimental results indicate that the model achieves an RI measurement accuracy of 10⁻⁴ for lenses. This method requires only the extraction of frequency information from the interference fringes, eliminating the need for phase shifting and offering greater efficiency than conventional measurement techniques. The system features a minimalist optical layout, comprising only PLSs, the lens under test, positioning mechanism, and a linear-array camera, thereby providing enhanced robustness against external disturbances.