To illustrate this, we can compare optical microscope images of WSe2 flakes deposited onto three different substrates: PDMS, 90 nm SiO2/Si, and gold. Flakes deposited onto PDMS show positive optical contrast, i.e. they appear brighter than surrounding substrate. In case of WSe2 deposited onto SiO2/Si, the contrast is negative for few-layer regions, but becomes positive with increasing thickness. Finally, WSe2 flakes on gold show negative optical contrast and a distinctive change in their colour with increasing thickness. Despite the difference in value and sign of optical contrast, all three samples show a systematic change in their appearance with increasing number of layers, which can be used to identify monolayers and estimate the thickness of few-layer regions.