This International Standard specifies a non-destructive method for determining the thickness of anodic oxidation coatings on aluminium and its alloys using a split-beam microscope. The method is applicable, in most industrial cases, to anodic oxidation coatings above 10 μm, or above 5 μm when the surface is smooth. The use of the method specified is limited by the need for the two luminous lines described in Clause 3 to be visible and distinctly separated, i.e. not in the case of opaque or dark-coloured coatings. NOTE Problems can arise as a result of the roughness of the surface.