The measurement of the absorption of radiation in the ultraviolet and in the visible range is described by the Beer-Lambert law. It indicates the attenuation of light through the absorbing substance as a function of concentration c and path length δ . With knowledge of the intensity of the laser light before and after traversing the film, which is to be examined, a proportionality to the path length δ and thus to the layer thickness can be inferred. In order to make the measurement method robust, the Beer-Lambert law is extended by a second wavelength, so that wavelength-independent transmission losses do not affect the results. Furthermore, a preselection of the wavelengths ensures that cross-sensitivities to temperature and urea concentration have no influence on the measurement. A correlation between the attenuation of the laser light and the film thickness can then be established via calibration measurements. The design of the sensor is based on real SCR systems, which is why a monostatic transceiver design, which only requires a single optical access, has been selected. In addition, the sensor is very robust and compact. Within the scope of the collaborative research centre 150, the sensor was tested on a generic film generator of the Institute of Fluid Dynamics and Aerodynamics (SLA). A validation against the commercially available Chromatic Line Sensor (CLS) was successful. Figure 1 shows the sensor above the film generator as well as the test head of the CLS.
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