  We present a systematic study of the optical contrast of diselenide (NbSe2)
and molybdenum disulphide (MoS2) flakes deposited onto Si wafers with a
thermally grown SiO2 layer. We measure the optical contrast of flakes whose
thickness ranges from 200 layers down to a monolayer using different
illumination wavelengths in the visible spectrum. The refractive index of these
thin crystals has been obtained from the measured optical contrast dependence
on the flake thickness by using a simple model based on the Fresnel law. With
the refractive index of these NbSe2 and MoS2 crystallites, the optical
microscopy data can be quantitatively analyzed to determine the thickness of
the flakes in a fast and non-destructive way.
