# The Circadian Effect Versus Mesopic Vision Effect in Road Lighting Applications
## 4 Discussion
Therefore, although blue light improves mesopic efficiency and suppresses melatonin, this might not be a significant issue according to present knowledge on circadian responses. Improving mesopic efficiency is still a good optimization strategy for road lighting. Previous work on optimizing light sources might still work in road lighting applications. However, some people are sensitive to light, and about 10 to 30 lx may cause 50% melatonin suppression after about one-hour exposure [37]. Thus, choosing light sources with relatively high S/P ratios and moderate EML and CS would be a better strategy. In some roadway activities, such as walking and cycling activities, the duration of exposure to artificial light might be less than one hour, so there might be a smaller suppression effect. By taking the road surface reflection rate as 0.3, we transformed photopic adaptation illuminance to mesopic illuminance for 318 light sources. Figure 8 shows the CS plot against mesopic illuminance at a photopic adaptation illuminance of 10 lx. When mesopic illuminance increases from 10 to 11 lx (i.e., a 10% improvement), although the CSs vary in a large range, there is still a group of light sources with very  
Figure 6: EML as a function of the S/P ratio at 30 lx. Figure 7: CS as a function of the S/P ratio at 10 lx. low CSs, as shown in the red dashed circle. It is possible to find ideal light sources with relatively high mesopic efficiency and low CS in application. The mesopic vision design of road lighting work is dedicated to providing a good brightness lighting environment for pedestrians and drivers. In the mesopic vision state, peripheral vision, which contains many rod cells, plays an important role and works for most dim light situations. However, [PERSON] et al. [17] noted that mesopic vision-designed light may not help improve visibility at night for high driving speeds. For relatively high driving speeds, foveal vision may be the dominant mode of detection. Therefore, for some road lighting that is used for people driving cars, rather than pedestrians or cyclists, road lighting strategy might not focus on mesopic vision design. There are several situations in which road lighting luminance levels are suitable for photopic vision, and some of the results for EML, \(\mathrm{CL_{A}}\), and CCT may still work. Both EML and \(\mathrm{CL_{A}}\) (under the condition of b-y \(<\) 0) are linear with illuminance levels, and their characteristics with varying CCTs are quite similar in photopic and mesopic vision. \(\mathrm{CL_{A}}\) under the condition b-y \(>\) 0 does not have a simple linear relationship with intensity, but the trend along with CCT similarly remains. The low/high CCT PC LED is about 1.5/3 times higher than the HPS in EML, 1.5/2.3 times higher in CS, and 40%/150% higher in the S/P ratio. These results are different from those of previous work [41] showing that high CCT LED suppresses melatonin secretion about 4 to 5 times more strongly than HPS. Therefore, in mesopic vision applications, we recommend using relatively high CCT LED (about 4000 to 5000 K) light sources to achieve high mesopic efficiency because both the EML and CS of light sources are below or around the threshold; in photopic vision applications, we recommend using HPS or low CCT light sources (2000 to 4000 K) in road lighting for low EML and CS levels. In a low-to-moderate photopic range, short-wavelength output enhances scene brightness [39; 45], which might indicate a partial contribution of cones and ipRGCs. This brightness mechanism is still being studied and might affect the optimization strategy for light source SPD values in outdoor lighting applications. Although we did not include this scene brightness enhancement mechanism in the present work, it deserves attention and further study.