Remote measurement of liquid water film thickness, temperature, and composition using near-infrared absorption

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The rapid, non-contact in situ characterization of properties of aqueous films on surfaces such as film thickness, temperature, and concentration of dissolved substances, is a challenging task in many areas of engineering, biology, and medicine. Therefore, for such applications optical diagnostics is the method of choice. As a continuation of the project SCHU 1369/16-1, the present project builds on results obtained through application of a diode laser-based absorption sensor in the wavelength range between 1.3 and 1.5 µm, which allows the detection of local properties of aqueous films, such as film thickness, temperature, and the concentration of dissolved substances (salts, urea). The sensor, which so far has been operated in a transmission mode only which is less suitable for practical applications, will be upgraded for measurements in retro-reflection or backscattering geometry and used for dynamic measurements of the respective parameters. The measurement is based on measuring absorbance ratios at several wavelengths provided by the emission of selected diode lasers. To achieve higher sensitivity, the sensor will be equipped with a new diode laser at 1.9 µm where the absorbance of water is significantly larger. The characteristic quantities of aqueous films will be determined quasi-simultaneously and at millisecond temporal resolution. In addition to the parameters listed above, the sensor will also be tested for pH determination of the film. In order to keep the complexity of the sensor low, a time-division multiplexing technique will be applied. Furthermore, the suitability of the sensor when using near-infrared LEDs as a low-cost alternative to diode lasers will be investigated. A second important goal is the expansion of the diagnostics scheme from point measurements towards two-dimensional imaging. An NIR camera with high repetition rate allows to sequentially measure the transmission on the respective wavelengths and thus determine the film properties in a two-dimensional manner. The achievable precision and accuracies of measured film parameters will be quantified using Bayesian inference methods.