The fact that organic binders, as used in paints, are transparent in the IR range 3.5-5.5 microns is utilized to detect and measure defects under the paint film of sheet metals. However, the method demands an elevated sample temperature to suppress reflections from the surrounding and is till now resorted to laboratory investigations. The IR system used is an AGEMA Thermovision 880 SWB. The maximum thickness of the paint film allowing evaluation is mainly governed by the absorption and the refraction caused by the paint pigments, depending upon pigment size and pigment material. To be detectable the defects have to be characterized by IR emissivities considerably different from the emissivity of the bare sheet metal as is found for metallic corrosion products. A quantitative determination of a corrosion attack is obtained by evaluating the IR image at a PC based image analyzer. As a standard today, such evaluations are performed by inspecting and comparing painted sheet metals with reference photographs. This method suffers both from the subjectiveness of the observer and the opaqueness of the paint film in the visible range.
Per Jernberg, Per Jernberg,
"Corrosion evaluation of coated sheet metal by means of thermography and image analysis", Proc. SPIE 1467, Thermosense XIII, (1 March 1991); doi: 10.1117/12.46443; https://doi.org/10.1117/12.46443