simulated and in-situ collected hyperspectral data. First, we use a detailed virtual orchard and forest model constructed in a physically based ray-tracing environment using detailed sub-models for the description of tree geometry, leaf and soil bidirectional reflectance and diffuse illumination [@Stuckens2009; @VanderZande2010; @Tits2012]. The model has been thoroughly validated with field observations [@Stuckens2009], and more recently we could provide, based on a comparison with in-situ data, strong evidence that our ray tracing model realistically describes the spectral scattering and thus nonlinearity observed in vegetated areas [@Somers2014]. Second, we use data from an in-situ experiment in a commercial citrus orchard. This experiment comprised in-situ measured mixed pixel reflectance spectra, pixel specific endmember spectra and subpixel cover fraction distributions. This unique dataset of in-situ measured mixed pixel reflectance spectra has previously been used to study nonlinearity in fruit orchards [@Somers2009].

The paper is organized as follows. Section \[sec:models\] introduces the main nonlinear models that have been proposed in the literature to describe mixtures encountered in vegetated areas. The ray-tracer based simulated data and the in-situ measurements used to validate these models are described in Section \[sec:data\]. The experiment results obtained by using the previously introduced nonlinear models on the two sets of 