REST2 is the High-performance solar radiation model for cloudless-sky irradiance, illuminance, and photosynthetically active radiation. NREL Software record: SWR-23-96

Christian A. Gueymard, "REST2: High-performance solar radiation model for cloudless-sky irradiance, illuminance, and photosynthetically active radiation - Validation with a benchmark dataset", Solar Energy, Volume 82, Issue 3, 2008, Pages 272-285, ISSN 0038-092X, https://doi.org/10.1016/j.solener.2007.04.008. Science Direct Link.