## 3 Results and Discussion  
### Analysis of Mean Surface Temperature of Subjects  
Figure 6 shows the variation of mean surface temperature (MST) of the driver according to the cabin and vent discharge air temperature under maximum and average solar radiations of 1251 and 516 W/m\({}^{2}\), respectively. A thermal-imaging camera was used to measure the surface temperature of the subject based on rainbow-based color coding. Red indicates a higher temperature, and purple represents a lower temperature. As shown in Figure 6b, when the cooling mode is not operated at a cabin temperature of 35 \({}^{\circ}\)C, the skin temperature of the subject shows a sky color, which represents a higher temperature. In particular, the surface temperature on the abdomen of the subject is highest because the solar light directly heats that part, and a wide area of red around the abdomen appears. Figure 6c shows the surface temperature of the subject when HVAC with a vent discharge air temperature of 12.5 \({}^{\circ}\)C was operated at the same cabin temperature of 35 \({}^{\circ}\)C. The surrounding colors of the subject are mainly sky-blue and blue, which means a decrease in the surrounding temperature of the subject. In addition, the color of the air near the vent is purple, which represents a low temperature. The abdomen where the radiation is concentrated and its surroundings appear yellow and green compared with that when the cooling vent is not operated. The surface temperature of the subject rapidly decreases when the vent cooling is operated. As the temperature of the cabin decreases, the ambient temperature and the surface temperature of the subject decreases, and they are mostly blue when the cabin and vent discharge air temperatures are 30.5 and 16.5 \({}^{\circ}\)C, respectively, as shown in Figure 6e. The yellow area decreases and the green area increases in the abdomen and body of the subject. Then, the ambient temperature and the surface temperature of the subject decrease continuously until the cabin and vent discharge air temperature of 22.5 \({}^{\circ}\)C. When the cabin and vent discharge air temperature is 22.5 \({}^{\circ}\)C, the surrounding air is purple, indicating the lowest temperature. The largest decrease in surface temperature is observed in the abdomen. This is because the cooling effect is the most effective in the abdomen because the cold air blowing from the vent outlet directly contacts the surface of the subject, and the surface temperature is the highest because of solar illumination. Figure 7 shows the variation in the average surface temperature on the left hand, right hand, forehead, and abdomen of the subjects by using the thermal-imaging camera during cooling in a car. At the common room temperature of 25 \({}^{\circ}\)C, the average surface temperature on the left hand, right hand, forehead, and abdomen of the subject were 28.6, 28.9, 31.5, and 30.1 \({}^{\circ}\)C, respectively. When the cabin temperature was 35 \({}^{\circ}\)C without the vent cooling operation, the average surface temperatures on the left hand, right hand, forehead, and abdomen increased significantly to 37.9, 36.6, 36.5, and 47.4 \({}^{\circ}\)C, respectively. In addition, the temperature on the abdomen was the highest compared with other surface areas because the solar light irradiated intensely on the abdomen under a cabin temperature of 35 \({}^{\circ}\)C; however, the surface temperatures of the subjects, except for the abdomen, were similar. When the vent discharge air temperature was 12.5 \({}^{\circ}\)C under a cabin air temperature of 35 \({}^{\circ}\)C, the average surface temperatures on the left hand, right hand, forehead, and abdomen of the subjects were 37.0, 34.5, 34.5, and 41.6 \({}^{\circ}\)C, respectively. By using vent cooling, the surface temperature of the forehead and abdomen of the subject during driving decreased by 3.0 and 11.5 \({}^{\circ}\)C, respectively.