## 4 Imaging System Characterization and Performance
### Sample Spray Experiments  
To demonstrate the capabilities and performance of the imaging system, we performed droplet size measurements. The droplet field was generated by an air-operated spray system (airbrush). Figure 8: Measured diameter comparisons with calibrated diameter for metal-printed opaque disks of various diameters ranging from 4 to 100 μm. These measurements were performed with the scene illuminated by the LED and VCSEL light sources. Figure 9: Depth of field measurements evaluated as the spatial frequency at half modulation as function of distance around the object plane. The modulation transfer function was measured via the knife edge method with both the LED and VCSEL light sources. An airbrush operates with a flow of compressed air passing through a venturi, which creates a low-pressure region, pulling liquid, in this case water, from a directly-connected reservoir. The high-velocity air flow contributes to the fine breakup of the liquid stream into small droplets, appropriate for the size range of the imaging system presented herein. The airbrush was operated with an air pressure of 1.70 atm and an air flow of 10 L/min. The measurements were performed on axis with the droplet stream coming out of the airbrush nozzle, at a distance of approximately 50 mm. Both illumination systems (LED and VCSEL) were used to compare their performance in practical conditions. The images were processed according to the image processing methods developed by the authors and described in detail in [23], and summarized in the following lines. A dynamic background correction based on a three-level temporal derivative scheme is applied first to compensate dynamic changes in intensity distribution, acting as a normalization scheme. The droplet detection method is based on a modified Moore Neighborhood algorithm with optimized stopping criteria for boundary tracing [24]. Detected droplets are kept with their surrounding area, and the local two-dimensional attenuation distribution is computed for each droplet. The attenuation distributions are analyzed to extract peak attenuation and edge gradient criteria. These two parameters are used simultaneously to limit the depth of field and associated out-of-focus sizing uncertainty to below 5% over the imaging system's size range. The results of the tests performed with the airbrush are shown in Figure 10. The left size histogram reported the droplet size distribution measured when LED illumination was employed, whereas the right plot shows the results with the VCSEL illumination system. The results are presented as probability density function, as opposed to droplet counts, to assist with the comparison. A single, 5-s long acquisition (~500 frames) was recorded in both experimental configurations, resulting in detecting and measuring approximately the same number of drops with both illumination systems: 4225 droplets for LED and 4177 for VCSEL. The two droplet size distributions reported in Figure 10 are visually very similar. They both present the typical log-normal distribution profile expected in flow-driven liquid breakup conditions. The smallest droplet measured by the imaging system with LED illumination was 5.6 \(\upmu\)m, whereas it was 6.0 \(\upmu\)m with VCSEL illumination. The largest was 114 \(\upmu\)m and 117 \(\upmu\)m for LED and VCSEL illumination, respectively. These droplet diameter differences are more likely related to a particular event difference than actual differences in instrumentation performance using one illumination system or another. Similarly, the d\({}_{10}\) arithmetic mean diameter was 21.1 \(\upmu\)m when using LED illumination, and 21.9 \(\upmu\)m when the VCSEL illumination system was employed.