Collective name for the real refractive index,
\(n\) and the
absorption index (imaginary refractive index),
\(k\).
Source:
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)