16252 IUPAC Gold Book - numerical aperture 10.1351/goldbook.16252 16252 current initialism: NA 1 For an optical fibre, \(n_{\rm{o}}\) sin \(\theta_{\alpha}\), where \(n_{\rm{o}}\) is the refractive index of the exterior medium and \(\theta_{\alpha}\) is the maximum input half angle that can support propagation. PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 757 (https://doi.org/10.1515/pac-2019-0203) https://goldbook.iupac.org/terms/view/16252/html https://goldbook.iupac.org/terms/view/16252/json https://goldbook.iupac.org/terms/view/16252/plain Citation: 'numerical aperture ' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.16252 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-04-17T19:29:59+00:00