Background:
To avoid unwished coloring of plastics (used e.g. in roofs or noise protection walls), they are coated with a thin (50-100 µm) UV absobing foil.
Measuring method:
To measure this coating a UV lamp radiating with 350-380nm wavelength is use to light at the sample. This yields the blue stripe.
Using PicEd Cora Videomicroscop the sample is placed so that the microscope looks orthogonal to the coating. For better focusing a common cold light source is added, so the plastics can be seen too.

To measure the thickness of metall coatings a metall ball (here with diamter of 20mm) is used to generate an impression on the surface.
The coating thickness D is evaluated as

D = (Length1*Length2) / Balldiameter

This example yields D = 1.5µm