## 2 Image Quality Evaluation Results
### Image Quality Degradation Effects of Single Droplet Contamination  
Figure 3 shows a side view of a single droplet (3 \(\upmu\)L in volume) on hydrophobic and hydrophilic lens cover glass surfaces, respectively. On the hydrophobic surface, the droplet maintains a hemispherical shape, whereas, on the hydrophilic surface, the droplet appears more flattened. These differences in shape are due to the varying surface energies of the two materials. The wettability of each surface was measured using the sessile drop method. The hydrophobic surface, with lower surface energy, showed a droplet contact angle of 112.5\({}^{\circ}\), while the hydrophilic surface, with a relatively higher surface energy, exhibited a droplet contact angle of 66\({}^{\circ}\). These differences in the contact angle and droplet shape suggest that the refraction of light through the droplets varies between surfaces, potentially affecting image quality.  
Since most cameras capture their surroundings rather than the sky or ground, they are positioned vertically to the ground. Droplets on the cover glass that are vertical to the ground are affected by gravity, so droplets over a certain volume slide off the surface [29]. We observed that droplets larger than 10 \(\upmu\)L on the hydrophilic surface and droplets larger  
Figure 2: Experimental setup for analyzing image degradation from droplet contamination using a slanted-edge target: (**a**) Configuration of the MTF measurement apparatus; two contamination methods for the camera cover glass: (**b**) single droplet contamination and (**c**) multiple droplet contamination.  
than 5 \(\mu\)L on the hydrophobic surface slid off. Based on these observed results, droplets smaller than 10 \(\mu\)L (1, 3, 5, and 10 \(\mu\)L in volume) were selected for use as contaminants.  
Figure 4a-c show images of a slanted-edge target when the hydrophobic and hydrophilic cover glass were contaminated with a single droplet. The droplet on the surface caused image distortion, acting like a blur filter [35]. This distortion occurred on both types of cover glass. Specifically, for the same droplet volume, the droplet affected a larger area on the hydrophilic surface than on the hydrophobic surface, as illustrated in Figure 4b,c. Additionally, the droplet altered the color of certain parts in the image to yellow due to halogen lights that were used. These image distortions and color variations result from the droplets scattering and refracting light similarly to a lens [38].  
Figure 4d shows the measured \(\mathrm{MTF50_{loss}}\) when the cover glass was contaminated with a single droplet. The \(\mathrm{MTF50_{loss}}\) is the difference between the \(\mathrm{MTF50_{clean}}\) and the \(\mathrm{MTF50_{contamination}}\), as mentioned in Section 2.1. \(\mathrm{MTF50_{loss}}\) occurred on both hydrophobic and hydrophilic surfaces. Especially, the \(\mathrm{MTF50_{loss}}\) was larger on the hydrophilic surface compared to the hydrophobic surface. This indicates that droplets spreading widely on the hydrophilic surface have a greater negative impact on image sharpness. Furthermore, we observed that the \(\mathrm{MTF50_{loss}}\) tended to increase as the droplet volume increased.