The retrieval model OptiSAIL is built around the established components 4SAILH (Scattering of Arbitrarily Inclined Leaves, with 4-stream extension and hot-spot), PROSPECT-D (simulation of leaf spectra, version D including senescence; Féret et al., 2017), TARTES (Two-streAm Radiative Transfer in Snow; Libois et al., 2013), with the addition of an empirical soil reflectance model, a semi-empirical soil moisture model (Philpot, 2010), the Ross-Thick-Li-Sparse BRDF model, and a cloud contamination simulation.