The Arges-45 is an integrated measurement system which uses the 45/0 degree measurement principle to measure colour of reflective surfaces.
When measuring the AR-coating of solar cells, the colour relates to the layer thickness as has been indicated in the below images.
Since the Arges-45 is a very sensitive colour measurement instrument, it can detect very small colour differences which in turn relate to layer thickness differences below 1nm.

AR-coating layer thickness versus colour. Represented in CIE L*a*b* colour space.