Coating systems
The photo thermal coating thickness measurement is based on the physical principle of heat dissipation in different media.

With the pulsed photo-thermal measuring method, a short laser pulse is directed onto the coating which is then warmed up. The short-wavelength radiation is transformed into long-wavelength radiation, so-called heat radiation, when it hit the coating. The heat is then slowly conducted from the surface to lower coating areas until the substrate is reached. As long as energy is supplied by the laser pulse the temperature rises. After the energy supply stops, the coating temperature is reduced by rapid the head conduction via the substrate. The receiving optics register the time function of the temperature changes on the coating surface. Thinner coatings can be heated more rapidly by the laser than thick coatings. The time function is used as a measure for the coating thickness. The measuring method significantly depend on the coating and the substrate having different thermal conductivities.

In order to answer the question: which coatings or substrates can be measured, the heat dissipation properties of the media involved have to be considered.

Coating

In principle, all organic and pigmented coatings can be measured. The heat dissipation within these coatings is very slow and easily measured. Dry coatings can be measured in the range of 5 µm to 250 µm. With excessively thin coatings below 5 µm, the heat is dissipated too rapidly and thus cannot be recorded properly. A further problem is that thin coatings often cannot be thermally exited by the laser, because even a pigmented coating will be optically transparent, and a large part of the laser energy will be dissipated or reflected by the substrate. Similar effects occur when unpigmented coatings are to be measured. For this reason, a single unpigmented coating layer on metal cannot be measured. However, the situation is different if there is a multi-layer structure, with the unpigmented coating being the final coating. Then the unpigmented coating can be indirectly heated via the coatings underneath, provided these are dark. Metallic coatings cannot be measured, because they cannot be thermally exited by the laser and simply reflect the radiation. So-called mixed coatings containing metal particles can be measured, as long as there is a sufficient component of organic, pigmented substances. Coatings above 250 µm can, with the photo-thermal measuring method, only be reliably measured when very long measuring times are used, which make it much more difficult to take a measurement during motion.

With measurements of multi-layer structures, it must be observed that only the total thickness can be measured. If individual layers are to be determined, a differential measurement must be carried out after every coating has been applied. This also applies to the measurement of a moist coating on top of a dry multi-layer structure. For this, the coating thickness of the dry multi-coating structure must be known so that it can be subtracted from the total thickness. The coating thickness of the dry multi-layer structure does not need, however, be determined at the same location as the measurement. A statistical mean value for an area is adequate, because the usually thinner, dry multi-layer structure compared to the thick, moist coating only contributes a little to the total thickness. Measuring tolerances and faults in the dry coating structure have for this reason a much lesser effect.

In some exceptional cases, the measuring of a two-layer structure, e.g. a thin unpigmented coating, on top of thick dry coatings is possible with a single measurement, if the thickness of the two coatings is very different.

The pulsed photo-thermal measuring method can be used for the following coatings:
The following coatings cannot be measured:
As described above, heat transmitted to the coating via the substrate is re-dissipated. The heat dissipation in the substrate should be much faster than within the coating, so that the coating and the substrate can be clearly distinguished. According to our experience, all metals, ceramics and glass fulfil this condition. Pre-treated metals on which a metallic protection layer has been applied behave like a single metal layer and do not affect the measurement. They do not contribute to the total thickness measured. For all substrates, attention must be paid that the substrate is sufficiently thick. The thickness of the substrate and the existing volume determine how well the heat can be dissipated. Even a good heat conductor, such as metal, can only dissipate heat if an adequate material cross-section is available. Substrates made from metal should be at least 0.5 mm thick. The adhesion of the coating onto the substrate is also important. Inadequate adhesion reduces the heat transport and makes it more difficult to determine the coating thickness.

Polymers and fibre compounds can only be measured within certain limitations. Often a test measurement needs to be made for the polymer combination with the coating in order to investigate the heat dissipation properties and to gain information about the measuring precision and reproducibility. The large number of coating/substrate combinations unfortunately does not allow a general statement to be made, and each individual case needs to be tested. However, there are a number of empirical values which might be used. As a rule, harder polymers have better thermal conductivity than softer polymers.

Substrates tested by us are: