attenuated total reflection 

Synonym: frustrated total internal reflection
Initialism: ATR
https://doi.org/10.1351/goldbook.08227
Internal reflection from an absorbing material at angles of incidence at or above the critical angle.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 652 [Terms] [Paper]