Non-destructive spectral reflectance measurements of antireflection coating on optical lenses with small radius of curvature

Antireflection coatings are crucial for minimizing surface reflections, reducing glare, and enhancing the light throughput of optical components, including lenses, filters, beam splitters, and wave plates. After the coating process, the spectral reflectance of the coated sample is typically evaluated using a spectrophotometer. For transparent specimens, the backside of the sample must be roughened and black-coated to prevent unwanted reflections from the rear surface. Unfortunately, this backside coating is a destructive process. Moreover, measuring the spectral reflectance of specimens with a small radius of curvature is challenging due to the non-flat surface, which complicates standard spectrophotometric measurements. In this work, a modified fiber-optic chromatic confocal microscope was used to measure the spectral reflectance of an antireflective-coated 20-mm-diameter N-BK7 plano-convex lens, taking advantage of the micron-sized focal spot of the confocal probe. For theoretical validation, we measured the spectral reflectance of an uncoated 20-mm-diameter N-BK7 lens over the 400 – 800 nm wavelength range using the proposed setup. The mean absolute error of the proposed technique was 0.04 %R when compared to the Fresnel reflection calculation. This modified chromatic confocal microscopy technique offers a non-destructive method for routinely evaluating the spectral reflectance of antireflection coatings on flat and curved optical components during the production process, eliminating the need for destructive sample preparation.