# Effects of Sidewall Brightness on LED Lighting Environment and Visual Performance in Road Tunnels
## 4 Visual Characteristics of Human Eyes in Tunnel  
### Visible Distance for Small Object  
Figure 8 shows the comparison of the lighting environment in the tunnel decorated with different materials (energy-storage and luminescent coating and gray cloth) illuminated by LED lamps with different color temperatures (only the situations for electric power of LED lamps of 10 W are listed). The color temperatures of the LED light sources can be divided into warm color (less than 3300 K, and majority of the light is red light),  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{3}{*}{\begin{tabular}{c} **Color** \\ **Temperature** \\ **(K)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Tunnel Sidewalls Decorated with** \\ **Energy-Storage and** \\ **Luminescent Coating** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Tunnel Sidewalls Decorated with Gray Cloth** \\ **Tunnel Sidewalls Decorated with Gray Cloth** \\ **Luminescent Coating** \\ \end{tabular} } \\ \cline{2-5}  & \multicolumn{2}{c}{\begin{tabular}{c} **Single Lamp** \\ **Power of 10 W** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Single Lamp** \\ **Power of 18 W** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **Single Lamp Power of** \\ **10 W** \\ \end{tabular} } & \multicolumn{2}{c}{
\begin{tabular}{c} **Single Lamp Power of** \\ **18 W** \\ \end{tabular} } \\ \hline
2500 & 0.811 & 0.808 & 0.844 & 0.836 \\
3500 & 1.539 & 1.526 & 1.555 & 1.535 \\
4500 & 1.572 & 1.566 & 1.584 & 1.582 \\
5500 & 1.757 & 1.774 & 1.798 & 1.801 \\
6500 & 2.034 & 2.036 & 2.070 & 2.058 \\ \hline \hline \end{tabular}
\end{table}
Table 3: S/P value.  
Figure 7: Spectral distribution of LED light sources (S1 and S2 represent sidewalls with energy-storage and luminescent coating and gray cloth, respectively). (**a**) 2500 K, (**b**) 3500 K, (**c**) 4500 K, (**d**) 5500 K, (**e**) 6500 K.  
intermediate color (between 3300 K and 5300 K) and cold color (greater than 5300 K, and majority of the light is blue light) [31; 32; 34]. In general, the tunnel lighting environment was obviously dim when the color temperature was 2500 K, the light was relatively soft when the color temperature was 3500-5500 K, and the lighting environment gave the observers a high cold feeling when the color temperature was 6500 K.  
Figure 9 shows the visible distance for a small object with different color temperatures of LED light sources and pavement luminance (the data in the figure is the average value of three observers' visible distances). As mentioned in Section 3.3, it is beneficial to driving safety if the S/P value of the LED light source increases in the mesopic vision which is between about 0.001 and 3 cd/m\({}^{2}\); however, the data in Figure 9 illustrates that the increase in color temperature and S/P value of LED light had no obvious influence on the visible distance of human eyes. Compared with the color temperature and S/P value, the visibility of a small object was more sensitive to the brightness. The visible distance increased to a certain extent after the luminance of the sidewall and road surface in the tunnel improved by the energy-storage and luminescent coating. For the conditions of single lamp power of 10 W and 18 W, the visible distance of the small object increased by 6-10% and 3-10%, respectively.