  The electromagnetic properties of 2D materials are modeled either as single
sheets with a surface susceptibility or conductivity, or as thin films of
finite thickness with an effective permittivity. Their intrinsic anisotropy,
however, has to be fully described to reliably predict the optical response of
systems based on 2D materials or to unambiguously interpret experimental data.
In the present work, we compare the two approaches within the transfer matrix
formalism and provide analytical relations between them. We strongly emphasize
the consequences of the anisotropy. In particular, we demonstrate the crucial
role of the choice of the thin film's effective thickness compared with the
parameters of the single sheet approach and therefore the computed properties
of the 2D material under study. Indeed, if the isotropic thin film model with
very low thickness is similar to an anisotropic single sheet with no
out-of-plane response, with larger thickness it matches with a single sheet
with isotropic susceptibility, in the reasonable small phase condition. We
illustrate our conclusions on extensively studied experimental quantities such
as transmittance, ellipsometry and optical contrast, and we discuss
similarities and discrepancies reported in the literature when using single
sheet or thin film models.
