The method is generally applicable to sufficiently smooth surfaces, coated with anodic oxide with a thickness greater than 5 [xmi. It is not applicable in the following cases:
——opaque or dark colored anodic oxide layers - oxide layers covering very rough surfaces.
UNI 6500-1969 history
1969UNI 6500-1969 Anodic oxidation lining of aluminum and its alloys. Measuring lining thickness by optical microscope