reference flux density

symbol: $\phi_{\rm{r}}(\lambda)$
https://doi.org/10.1351/goldbook.08538
Intensity transmitted by the reference medium in a double-beam spectrometer.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 731 (https://doi.org/10.1515/pac-2019-0203)