  We have implemented three different optical methods to quantitatively assess
the thickness of thin GaSe flakes transferred on both transparent substrates,
like Gel-Film, or SiO2/Si substrates. We show how their apparent color can be
an efficient way to make a quick rough estimation of the thickness of the
flakes. This method is more effective for SiO2/Si substrates as the thickness
dependent color change is more pronounced on these substrates than on
transparent substrates. On the other hand, for transparent substrates, the
transmittance of the flakes in the blue region of the visible spectrum can be
used to estimate the thickness. We find that the transmittance of flakes in the
blue part of the spectrum decreases at a rate of 1.2%/nm. On SiO2/Si, the
thickness of the flakes can be accurately determined by fitting optical
contrast spectra to a Fresnel law-based model. Finally, we also show how the
quantitative analysis of transmission mode optical microscopy images can be a
powerful method to quickly probe the environmental degradation of GaSe flakes
exposed to aging conditions.
