## 1 Introduction
In addition, the release of encapsulated substances can be controlled precisely, which is particularly beneficial for encapsulated active ingredients in drugs [21]. Furthermore, WOW double emulsions are used to reduce the fat content in foods [22]. On an industrial scale, double emulsions are produced using classical methods, such as rotor-stator machines [18]. Here, the inner emulsion is produced under high energy input, resulting in very small W\({}_{1}\) droplets [23]. The second emulsification step is carried out at lower energy input so that the inner emulsion is affected as little as possible [24]. Industrial emulsification methods have the disadvantage of producing wide droplet size distributions [25]. Microfluidic systems offer the chance to provide nearly monodisperse droplet size distributions [26]. It uses miniscule channels to produce emulsions in laminar flow profiles [27]. For double emulsions, glass capillary devices have proven to be very useful [28]. However, the fabrication of a single device is costly and difficult to reproduce [29]. Therefore, an optimized glass capillary device has recently been developed [30]. It follows the same emulsification principle but is much easier to manufacture and can also be modified during operation [31]. It offers the possibility of producing double emulsions, having a wide range of internal and external droplets in terms of size and degree of filling, both parameters being independently variable [10]. Due to their complex structure, double emulsions tend to undergo various instability mechanisms [32]. Besides diffusional losses of encapsulated water or actives, inner as well as outer droplets may coalesce with each other, or inner droplets may coalesce with the outer water phase. Improving the stability of double emulsions is the focus of many research groups today [18]. However, research is limited as e.g., real-time measurement of structure-related parameters, as inner and outer droplet size distributions, and their filling degree, is extremely challenging. Usually, to measure those parameters, non-real-time measurement methods, such as dynamic and static scattered light methods [18], differential scanning calorimetry [33], or nuclear magnetic resonance NMR spectroscopy [34], are used. Those methods are usually performed offline and require sample preparation. Furthermore, the measured emulsion sample is usually unusable for other measurements. Therefore, there is a need for a new measurement method suitable for double emulsions, which can be performed in- or online to deliver real-time results without sample preparation and which does not affect the double emulsion [35]. Raman spectroscopy fulfils these requirements. Furthermore, because of its high selectivity and its suitability for water [36], it is expected that each double emulsion phase can be detected individually. The only challenge for Raman measurements of double emulsion, will be multiple light scattering, caused by different refractive indices of the three double emulsion phases. Thus, the scope of this work is to investigate whether the method of refractive index matching creates the possibility to suppress the cross-influences resulting from phase boundaries in Raman measurements of double emulsions and the effect of refractive index differences between the three double emulsion phases on the Raman signal strength. Therefore, the signal of the inner water phase is measured using Raman spectroscopy, while the refractive indices of the inner and outer water phases are varied. It is expected that the signal strength of the inner water phase increases as the differences between the refractive indices of the inner water phase, the oil phase and the outer water phase, decrease. Furthermore, the work investigates the possibility to compensate refractive index differences by mathematical corrections via simple functional relationships. We found one publication with a similar scope, which is about the molecular mapping of multiple emulsions using coherent anti-Stokes Raman scattering (CARS) microscopy [37].