## 2 Content of the Special Issue  
The special issue starts with an educational and comprehensive review article [11] written by [PERSON] (University of Bremen) which lays out the basics of light scattering, highlighting its application to imaging flow velocimetry. The article describes the different flow measurement principles, as well as the fundamental physical measurement limits. Furthermore, the progress, challenges and perspectives for high-speed imaging flow velocimetry are considered. The contributed articles discuss a large variety of methods and applications. [PERSON] et al. [12] describe the use of a passive technique--frame emission spectroscopy--for in situ measurements of alkali metals evaporated during the incineration of municipal solid waste (MSW). They succeeded in detecting sodium, potassium, and rubidium species in the flame of an industrial incinerator. This suggests that their method is suitable for monitoring technical facilities for biomass and waste combustion. In another gas phase study, [PERSON] and colleagues [13] demonstrate a non-linear optical spectroscopic technique, termed Femtosecond Adaptive Spectroscopic Technique Coherent Anti-Stokes Raman Scattering (FAST CARS),to quantitatively map species' concentrations. Using the example of molecular oxygen, they illustrate how CARS can be used for the visualization of a gas flow in a free-space configuration. This method is proposed to be applicable to performing gas flow imaging utilizing any Raman-active species. Liquid systems have been studied in a number of papers in this special issue as well. Collaborative activity by the Universities of Bremen and Manchester applied infrared spectroscopy to biosurfactants produced by microorganisms in a fermentation process [14]. Such biosurfactants represent amphiphilic compounds with polar and non-polar moieties and they can be used to stabilize emulsions, e.g., in the cosmetic and food sectors. They are highly viscous fluids and their structures may be affected when exposed to light and elevated temperatures. In this study, attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was applied to analyze the structure and aging of rhamnids as a representative of a complex biosurfactant. However, cell suspensions represent even more complicated systems that can be studied by optical techniques. [PERSON] et al. [15] combined Raman spectroscopy with a discriminant analysis data evaluation approach to successfully distinguish between eight different cancerous human cells. The comparison of two methods--Linear Discriminant Analysis (LDA) and Quadratic Discriminant Analysis (QDA)--revealed better performance of the QDA when applied to the Raman spectra. Further, fluorescence imaging is another technique that is frequently used to study biological samples. The evaluation of such images, however, is not always straightforward. For this purpose, [PERSON] et al. [16] propose a sparse-representation-based image fusion method. They combine principle component analysis (PCA) to initially extract geometric similarities and classify the images. In a second step, the constructed dictionary is used to convert the image patches to sparse coefficients by a simultaneous orthogonal matching pursuit (SOMP) algorithm. As proof-of-concept, the proposed method is successfully applied to fluorescence images of biological samples. The three remaining papers are concerned with fluid-solid interfaces that are widespread in nature, science, and engineering. [PERSON] et al. [17] propose an infrared spectroscopic method to study metal-liquid interfaces that are of interest in electrochemistry and catalysis. They utilize an attenuated total reflection (ATR) spectroscopy approach in which a thin film of fluid is placed in between the ATR crystal and a metal plate. They obtain IR spectra from aqueous salt and acid solutions and an aluminum plate to demonstrate that useful information about the molecular interactions at the metal-liquid interface can be deduced. [PERSON] et al.