## 4 Conclusions and Outlook  
The microscopic and macroscopic LIF and Mie signals of micrometric monodisperse fuel droplets doped with nile red were investigated and the suitability of this tracer for planar droplet sizing was discussed. The surrogate fuel Toliso, used to simulate commercial gasoline, and the ethanol-gasoline blend E20 were used as fuels. The spectral emissions of the investigated fuel-dye mixtures were analyzed in a specially designed dye cell in a preliminary study at varying temperatures. Excited at 532 nm, the fluorescence signal was in the visible wavelength region (400-750 nm). Both fuel-dye-mixtures showed a low temperature sensitivity of the fluorescence at moderate temperature variations (283-313 K), which is essential for reliable direct droplet sizing using the LIF/Mie approach. The ethanol admixture of E20 resulted in slightly larger fluorescence signals, and the fluorescence spectrum was shifted 50 nm towards higher wavelengths. This suggests that for each gasoline--ethanol mixture, an individual LIF/Mie-calibration curve is necessary. For calibration, monodisperse droplets were produced with a droplet generator and the respective 2D LIF and Mie signals were simultaneously analyzed with microscopic and macroscopic detection systems. The fitting curves for the LIF and Mie signals were deduced after adequate post processing according to the power law function \(I=a\,d^{b}\). The LIF and Mie signals of Toliso showed an approximate \(d^{3}\)- and \(d^{2}\)-dependence, respectively, in accordance with theory. The Mie signals showed an overall higher standard deviation in comparison to the LIF signal because of the typical Mie signal oscillations within the chosen detection angular aperture. For the macroscopic detection, a lower LIF/Mie ratio and higher respective standard deviations were observed in comparison to the microscopic detection. This is caused by the lower resolution and SNR of the macroscopic detection system. A comparison of Toliso and E20 led to similar LIF/Mie ratios for both fuels despite the spectral shift of the fluorescence. An analysis of the single droplets showed more distinct MDR effects and glare points for E20 in comparison to the base fuel Toliso. This was probably because of the different absorption and emission behaviors of the dye dissolved in E20 leading to an increase of the LIF exponent from \(\sim\)3.03 to \(\sim\)3.34 and of the Mie exponent from \(\sim\)1.87 to \(\sim\)2.02. Further investigations showed that the LIF/Mie ratio did not depend on laser power but directly increased with the dye concentration as expected. To avoid enrichment of the dye within the evaporating droplets of a spray, fuel evaporation had to be avoided (or corrected) so that a reliable droplet sizing was possible under moderate ambient temperatures. An investigation of the temperature dependence of the LIF/Mie ratio showed almost no temperature sensitivity under moderate ambient conditions (between 263 K and 293 K), but a distinct increase of several tens of percent for a temperature rise to 323 K. In summary, the suggested dye is suitable for planar droplet sizing in sprays at moderate temperatures based on the LIF/Mie ratio at which evaporation and distinct droplet heating or evaporation cooling can be neglected. Under conditions with larger droplet heating and increased evaporation rates, additional dye concentration measurements and droplet thermometry is required to avoid overestimation of droplet sizes. Finally, a detailed study of the MDR effects on the LIF/Mie ratio calibration curve and its measurement uncertainty will be performed in future. Afterwards, subsequent planar measurements in the spray will be conducted which require a suppression of multiple scattering, for example, using the structured laser illumination planar imaging (SLIPI) [20; 23; 47].