08653 IUPAC Gold Book - optical path difference 10.1351/goldbook.08653 08653 current initialism: OPD synonym: optical retardation 1 Path-length difference between the arms of a two-beam interferometer. The term optical retardation is derived from 'phase retardation', which refers to the change in the phase of the electromagnetic radiation in the interferometer. SI unit: \(\pu{m}\). Common unit: \(\pu{cm}\). Path-length https://goldbook.iupac.org//terms/view/08278 electromagnetic radiation https://goldbook.iupac.org//terms/view/08242 PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 758 (https://doi.org/10.1515/pac-2019-0203) https://goldbook.iupac.org/terms/view/08653/html https://goldbook.iupac.org/terms/view/08653/json https://goldbook.iupac.org/terms/view/08653/plain Citation: 'optical path difference' 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.08653 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. 2026-04-17T19:38:49+00:00