# The Circadian Effect Versus Mesopic Vision Effect in Road Lighting Applications
## 4 Discussion  
The circadian effect is a non-visual imaging effect, mainly caused by a third photoreceptor. The two types of visual photoreceptors, cones and rods, which are sensitive to luminance and color, also play important roles in circadian responses under specific situations. EML takes the melanopic sensitivity curve as the main influence of the circadian response, while CL\({}_{\rm A}\) (CS) is a comprehensive model dedicated to measuring the general circadian response. EML includes the melanopic sensitivity curve, which is close to the scotopic sensitivity curve. There is moderate correlation between EML and CCT and extremely high correlation between the EML and S/P ratio due to the similarity of the melanopic and scotopic sensitivity curves. It is practical to predict the EML value using the S/P ratio in road lighting. EML only includes the melatonin sensitivity curve, which might adapt to an ideal situation for the circadian response. However, all photoreceptors contribute to the circadian response. Therefore, EML may not be practical for some situations. At present, the International Well Building Institute [26] recommends the use of EML for circadian lighting evaluations by setting a threshold in various scenes and requires no more than 50 EML under nocturnal lighting. Figure 6 shows EML as a function of the S/P ratio at a 30 lx level, where the maximum EML is below 50. [PERSON] et al. [44] reported that two high CCT LEDs (\(\geq\)5200 K) might cause a small stimulating effect (namely, 12-15% nocturnal melatonin suppression) in the human circadian system after one hour of exposure at a 95 lx level under outdoor lighting, while low CCT light sources would not. These results are consistent with ours. Thus, in a mesopic vision state, the melanopic suppression level might not be a significant problem. Because CL\({}_{\rm A}\) and CS are piecewise functions, there are no significant correlations between the CL\({}_{\rm A}\) (or CS) and S/P ratio using the Spearman correlation test. We categorized these points into two groups according to their b-y relation. For example, at a 10 lx illuminance level, there is a high goodness-of-fit between the CS and S/P ratio of 0.955 for b-y \(<\) 0 and 0.842 for b-y \(>\) 0, as shown in Figure 7. However, in actual applications, this conclusion might be impractical due to the trouble of declaring the b-y value. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **HPS** & **LED 1** & **LED 2** \\ \hline \multirow{4}{*}{EML} & CCT (K) & 2238 & 2497 & 6060 \\  & S/P ratio & 0.8417 & 1.1642 & 2.0711 \\ \hline \multirow{4}{*}{EML} & 1 lx & 0.2807 & 0.4453 & 0.8781 \\  & 3 lx & 0.8421 & 1.3358 & 2.6342 \\  & 10 lx & 2.8069 & 4.4527 & 8.7808 \\  & 30 lx & 8.4206 & 13.3580 & 26.3424 \\  & 100 lx & 28.0687 & 44.5265 & 87.8080 \\ \hline \multirow{4}{*}{CL\({}_{\rm A}\)} & 1 lx & 0.5196 & 0.8330 & 1.1096 \\  & 3 lx & 1.5588 & 2.4989 & 3.3293 \\  & 10 lx & 5.1959 & 8.3298 & 11.1040 \\  & 30 lx & 15.5878 & 24.9893 & 33.3659 \\  & 100 lx & 51.9594 & 83.2978 & 111.8434 \\ \hline \multirow{4}{*}{CS} & 1 lx & 0.0005 & 0.0009 & 0.0012 \\  & 3 lx & 0.0018 & 0.0029 & 0.0040 \\ \cline{1-1}  & 10 lx & 0.0066 & 0.0110 & 0.0150 \\ \cline{1-1}  & 30 lx & 0.0216 & 0.0355 & 0.0480 \\ \cline{1-1}  & 100 lx & 0.0750 & 0.1175 & 0.1528 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The EML, CL\({}_{\rm A}\), and CS of HPS and the low CCT LED and high CCT LED at different photopic adaptation illuminance levels. A high CCT light source is likely to have high EML and CS. However, there is a moderate correlation between EML and CCT and a weak correlation between CS and CCT. Moreover, due to the divergence of CS caused by the b-y mechanism, at each varying CCT, there are great variations among the CS values. Therefore, CCT is not a good indicator for determining the circadian response levels. In the mesopic vision range, intensity might reach the threshold of suppressing melatonin.