## 2 Image Quality Evaluation Results
### Image Quality Degradation Effects of Multiple Droplet Contamination  
In Section 2.3, we observed that image quality degrades in proportion to the volume of a single droplet. The volume of multiple droplets depends on the spray amount, so it is necessary to define a contamination level for quantitative evaluation. In this section, we define contamination levels as slight, moderate, and severe based on spray volume. At each level, the contaminated area was measured. Then, we quantitatively measured and comparatively analyzed the degradation of image quality for each contamination level. Figure 5a shows the experimental setup for measuring the contamination area. To precisely measure the area covered by droplets on the cover glass surface, the light source (FOK-100W, Fiber Optic Korea, Cheonan-City, Korea) and the light barrier were aligned as shown in Figure 5a. The droplet boundaries were clearly visible as black. The light barrier had a diameter of 30 mm and was positioned 100 mm away from the glass. The cover glass was maintained vertically to the ground, and droplets were sprayed using the spray module 15 cm in front of the glass. The glass surfaces were photographed with a complementary metal oxide semiconductor (CMOS) color camera (EO-1312C, Edmund Optics, Barrington, NJ, USA). Figure 3: Side view of droplet on the cover glass parallel to the ground: (**a**) hydrophobic surface; and (**b**) hydrophilic surface. Figure 5b,c show droplets sprayed on hydrophobic and hydrophilic cover glass surfaces, respectively. Spray volumes of 0.1, 0.5, and 1.0 mL were set for different contamination levels, and multiple droplets were generated with a spray of 0.05 mL per shot. As the spray volume increased, droplets merged and formed larger droplets. When the same amount of liquid was sprayed, droplets were more easily attached and had a larger size on the hydrophilic surface compared to the hydrophobic surface. When the spray volume exceeded 1.25 mL, the merged droplets generally slid along the surface due to gravity, removing other droplets. The droplets on the hydrophobic surface maintained circular contact areas, as shown in Figure 5b1-b3, while droplets on the hydrophilic surface had irregular contact areas, as shown in Figure 5c1-c3. Figure 4: Image of a slanted-edge target with (**a**) clean cover glass; (**b**) hydrophobic cover glass; (**c**) hydrophilic cover glass, each contaminated by single droplet of varying volumes (1, 3, 5, and 10 \(\mu\)L); and (**d**) MTF50\({}_{\text{loss}}\) due to single droplet contamination. To quantitatively evaluate the image quality under multiple droplet contamination, contamination levels should be defined with an objective metric. We analyzed the contaminated area for each contamination level using the ImageJ 1.54 program (National Institutes of Health). A method for measuring the percentage of contamination area is shown in Figure 6. The definition of contamination area is provided in Equation (2). A\({}_{\text{total}}\) represents the ROI with a diameter of 30 mm on the glass, and A\({}_{\text{droplet}}\) refers to the area occupied by droplets within the A\({}_{\text{total}}\). A higher contamination area indicates that droplets are covering a larger area of the glass surface. \[\text{Contamination area}=\left(\frac{\text{A}_{\text{droplet}}}{\text{A}_{ \text{total}}}\right)\times 100\;(\%) \tag{2}\]  
Figure 7 and Table 1 show the results of the contamination area measurement. The contamination area was calculated by taking five repeated measurements and averaging the results. As the contamination level becomes more serious from clean to severe, the contaminated area continuously expanded, reaching 44.12% at the hydrophobic surface and 54.90% at the hydrophilic surface. A larger area was contaminated on the hydrophilic surface compared to the hydrophobic surface at the same contamination level.