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

在工程、生物学和医学的许多领域，快速、非接触地原位表征水膜表面的特性，如膜厚度、温度和溶解物质的浓度，是一项具有挑战性的任务。因此，对于此类应用，光学诊断是首选的方法。作为SCHU 1369/16-1项目的延续，本项目建立在波长范围在1.3至1.5µm之间的二极管激光吸收传感器的应用所获得的结果之上，该传感器可以检测水膜的局部特性，如膜厚度、温度和溶解物质（盐、尿素）的浓度。到目前为止，该传感器仅以传输模式运行，不太适合实际应用，将对其进行升级，以进行反向反射或后向散射几何形状的测量，并用于各自参数的动态测量。测量是基于测量在几个波长的吸收比提供了选定的二极管激光器的发射。为了实现更高的灵敏度，传感器将在1.9µm处配备一个新的二极管激光器，在1.9µm处水的吸光度明显更大。水膜的特征量将以准同时和毫秒时间分辨率确定。除了上面列出的参数外，传感器还将用于测定薄膜的pH值。为了降低传感器的复杂度，将采用时分复用技术。此外，当使用近红外led作为二极管激光器的低成本替代品时，将研究传感器的适用性。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依次测量各自波长上的透射，从而以二维方式确定薄膜特性。所测薄膜参数的可实现精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.