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

在工程、生物和医学的许多领域中，快速、非接触地表征表面水膜的性质，如膜厚、温度和溶解物质浓度，是一项具有挑战性的任务。因此，对于这样的应用，光学诊断是选择的方法。作为SCU 1369/16-1项目的延续，本项目建立在1.3至1.5微米波长范围内应用基于半导体激光的吸收传感器所取得的成果的基础上，该传感器可以检测水膜的局部性质，如膜厚、温度和溶解物质(盐、尿素)的浓度。到目前为止，该传感器仅在不太适合实际应用的传输模式下运行，将进行升级，以用于反向反射或后向散射几何形状的测量，并用于相应参数的动态测量。该测量是基于测量由选定的二极管激光器发射提供的几个波长的吸光度比。为了实现更高的灵敏度，传感器将在1.9微米处配备新的半导体激光器，在那里水的吸光度明显更大。水相薄膜的特征量将以毫秒的时间分辨率准同时测定。除上述参数外，还将对传感器进行测试，以确定薄膜的pH值。为了保持传感器的低复杂度，将采用时分复用技术。此外，还将研究该传感器在使用近红外LED作为半导体激光器的低成本替代品时的适用性。第二个重要目标是将诊断方案从点测量扩展到二维成像。具有高重复率的近红外相机允许顺序地测量各个波长上的透射率，从而以二维方式确定胶片属性。测量胶片参数的可达到的精密度和准确度将使用贝叶斯推理方法进行量化。

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.