Dual fiber point diffraction interferometry for non-destructive measurement of refractive index and Abbe number in spectacle lens

This paper proposes a non-destructive method for measuring the refractive index and Abbe number of spectacle lens based on dual fiber point diffraction lateral interferometry. Based on ray tracing principles, we establish an optical path difference (OPD) model describing the propagation of dual fiber point light sources (FPLSs) through a spectacle lens to the imaging plane. Numerical simulations are employed to derive the functional relationships between the refractive index (or Abbe number) of the spectacle lens and the OPD, thereby providing a novel high precision measurement model. Experimental results demonstrate that this method efficiently measures the refractive indices of various spectacle lenses at multiple wavelengths. By fitting the Cauchy dispersion formula, the refractive indices at the F, d, and C spectral lines are accurately determined, achieving errors below 5 × 10^-3 for refractive indices (n_d) and under 1.5 % for the Abbe number (V_d). The hardware system features a simplified architecture comprising only dual FPLSs and a linear array camera, significantly reducing complexity and cost. Notably, the method enables non-destructive testing of spherical spectacle lenses with arbitrary edge contours, diopters, and thicknesses while maintaining insensitivity to adjusting errors. A distinctive advantage lies in its common-mode error suppression capability for Abbe number determination, substantially enhancing measurement accuracy. Compared with existing techniques, this methodology demonstrates superior performance in simultaneous refractive index and Abbe number measurements, offering a solution for spectacle lens inspection in ophthalmic industries.