  Here, we propose a method to determine the thickness of the most common
transition metal dichalcogenides (TMDCs) placed on the surface of transparent
stamps, used for the deterministic placement of two-dimensional materials, by
analyzing the red, green and blue channels of transmission-mode optical
microscopy images of the samples. In particular, the blue channel transmittance
shows a large and monotonic thickness dependence, making it a very convenient
probe of the flake thickness. The method proved to be robust given the small
flake-to-flake variation and the insensitivity to doping changes of MoS2. We
also tested the method for MoSe2, WS2 and WSe2. These results provide a
reference guide to identify the number of layers of this family of materials on
transparent substrates only using optical microscopy.
