## 4 Discussion  
The experiment is designed to measure the double emulsion droplets individually. This means, one after another the double emulsion droplets pass through the laser, which runs constantly during the measurement. This ensures that the measurement of one droplet is not influenced by another one. When light is irradiated into highly filled double emulsions, multiple scattering occurs at the interfaces. This results in a weak signal of the inner phase. Figure 6 shows that the light's ability to reach the inner water phase, represented by the signal of ammonium nitrate, depends on the refractive index matching. To interpret the results, we start from a single droplet. The light enters the drop, which acts as a converging lens when the refractive index of the outer phase is low. The more the refractive index of the outer phase increases, and thus the refractive index difference becomes smaller, the lower the reflectance at the phase interface. This can be calculated using Equations (1)-(4). Figure 7: Residues as a function of refractive index difference between the inner water and oil phase (**A**) and the outer water and oil phase (**B**). In the case of a double emulsion, the incident light is somewhat focused by the first phase boundary. In the oil droplet, it strikes the inner phase, whose refractive index relative to the oil phase was varied in the experiments. The experiments are designed so that the total amount of ammonium nitrate is constant. If the inner phase always allows all of the light to pass, the displayed ammonium nitrate signal should remain constant for all measurements. As expected, the ammonium nitrate signal increases with increasing ammonium nitrate concentration, respectively lowering the refractive index difference and reaching its maximum at 49% ammonium nitrate; the best matching. At 61% ammonium nitrate, the refractive index difference is larger, and respectively, the signal is smaller. A similar behavior is seen with the variation of the glycerol concentration. The measured values increase with increasing glycerol content, respectively decreasing the refractive index difference. However, they continue to increase above the best matching at 49% glycerol. To explain both observations, the reflectance during each experiment are considered. These are shown in Table 3. The reflectance at an incidence angle of 0\({}^{\circ}\) is calculated using Equation (4), and for 45\({}^{\circ}\) using Equation (3). To estimate the reflectance in the case of multiple scattering, the following equation, derived by the authors, is used. It is assumed that reflected radiation is lost and one phase interface is passed after the other. \[R_{ms}=1-(100\%-R)^{n_{phase\text{ }transitions}}. \tag{6}\]  
Each oil droplet contains numerous inner water droplets. Therefore, a correspondingly large number of phase passes of the incident light occur here. With only 1000 water droplets, the reflectance increases significantly. Table 3 shows that for 16% ammonium nitrate and 1000 phase passes, the reflectance is 24.08% (0\({}^{\circ}\)) and 44.64% (45\({}^{\circ}\)), respectively. In contrast, for 49% ammonium nitrate, the reflectance is 0.09% (0\({}^{\circ}\)) and 0.10% (45\({}^{\circ}\)). In other words, matching the refractive indices of the inner water and oil phases reduces the reflectance by a factor of 279.95 (0\({}^{\circ}\)) and 437.50 (45\({}^{\circ}\)). Therefore, the matching of the inner phase boundaries has a significant effect on the measurement signal. There is only one phase boundary between the outer water phase and the oil droplet. Accordingly, the reflectance is calculated for one phase transition. In the worst case, for 0% glycerol and a 45\({}^{\circ}\) angle of incidence, the reflectance is 0.12%. At 49%, practically no