Defines a non-destructive method of determining the thickness of anodic oxidation coatings on aluminium and its alloys using a split-beam microscope. Applicable in most industrial cases, to anodic oxidation coatings above 10 m, or above 5 m when the surface is smooth. This 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.