## 1 Introduction  
The characterization of sprays is of great importance for many technical applications. Prominent examples are fuel sprays in automotive and aircraft engines or spray drying in industrial processes. The knowledge of droplet size or evaporation rates are essential for the operation of such systems [1]. In this context, spray modelling and numerical predictions are important tools for the design process. The goal of numerous experimental investigations is the tuning of model parameters of semi-empirical submodels or validating submodels and overall simulations [2]. In conclusion, an effective comparison of spray measurements and numerical simulations is the key for a deeper understanding of sprays. Essential processes like atomisation, spray structure, penetration and evaporation may be affected significantly by temperature, pressure and flow velocity of the ambient gas. Hence, to perform the experimental investigations under typical operating conditions is of major importance. Since in most applications not only the droplet size but also the droplet velocity is of interest, an adequate and established measurement technique is the Phase Doppler Anemometry (PDA). A drawback of this technique is that beam steering may compromise measurements at high temperature conditions [2], which may limit the achievable accuracy or impede the task to obtain quantitative data completely. Nevertheless, successful measurements at technical relevant conditions were published [3]. An alternative to interferometric techniques are imaging diagnostics. In this context, the light sheet configuration in combination with structured illumination has become a prominent technique for the analysis of spray systems in recent years [4; 5]. A specific advantage of this configuration is the well-defined measurement volume due to a low contribution of out-of-focus light. However, the line-of-sight arrangement, with camera and illumination on the same optical axis, is known to be well-suited for recording the droplet size [6]. This configuration is often referred to as shadowgraphy and is proven to be a robust technique to obtain droplet size and velocity [7; 8]. High quality raw images are as important as a precise post-processing routine and a calibration of the optical setup in order to obtain quantitative droplet data. In several studies, a strong agreement to PDA results is demonstrated, which emphasizes the capabilities of direct imaging techniques [9; 10; 11]. In addition, imaging techniques can be extended for detecting the size and shape of nonspherical droplets, which is of particular interest for the investigation of the primary breakup close to the atomiser. The motivation for a high spatial resolution of the imaging system can be the investigation of microscopic spray structures or the detection of small droplets with a size in the range of \(10\,\mathrm{\SIUnitSymbolMicro m}\) with sufficient accuracy. A long working distance is favourable for the application of microscopic imaging to spray measurements in order to minimize contamination of optical components and enable the investigation inside pressurised flow channels. By an appropriate microscopic imaging setup, a resolution in the range of \(1\,\mathrm{\SIUnitSymbolMicro m}\) per pixel may be achieved, which has been reported in literature as a powerful tool for spray diagnostics [12; 13; 14]. At this point, it should be noted that such imaging systems may be applied close to their diffraction limit. Hence, a further increase of the resolution is only sensible by a reduction of the wavelength \(\lambda\) or an increase of the numerical aperture NA [15]. The latter was successfully applied by [PERSON] et al. [16] at the cost of a strong reduction of the working distance. An imperfect alignment of the main optical components, camera and illumination, can be tolerated for direct imaging techniques. This is an advantage for conducting measurements under high temperature conditions in comparison to PDA, where beam steering can compromise the data rate considerably.