normal incidence

https://doi.org/10.1351/goldbook.08647
Incidence of a beam of electromagnetic radiation when the angle of incidence θ=0 , i.e., the incident radiation beam is normal to the surface.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 756 [Terms] [Paper]