## 4 Results and Discussion
When the \(T_{a}\) increased from 30 \({}^{\circ}\)C to 60 \({}^{\circ}\)C, the illuminance of the low beam and high beam at illumination angles of 0\({}^{\circ}\), 90\({}^{\circ}\), and 180\({}^{\circ}\) decreased by 15.7%, 14.9%, and 19.2%, and 24.1%, 20.1%, and 21.0%, respectively. When the \(T_{a}\) increased from 30 \({}^{\circ}\)C to 60 \({}^{\circ}\)C, the power  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Deg. ** (\({}^{\circ}\))} & \multirow{2}{*}{\(T_{a}\)} & \multicolumn{4}{c}{**Low Beam**} & \multicolumn{4}{c}{**High Beam**} \\ \cline{3-10}  & & & **CCT** & & **CIE 1931 xy** & **CCT** & & **CIE 1931 xy** & \\ \cline{3-10}  & & **K** & **Duv** & **CIE x** & **CIE y** & **K** & **Duv** & **CIE x** & **CIE y** \\ \hline \multirow{4}{*}{0} & 30 & 5949 & 0.0319 & 0.3191 & 0.3981 & 5835 & 0.0304 & 0.3225 & 0.3978 \\  & 40 & 5963 & 0.0321 & 0.3187 & 0.3981 & 5854 & 0.0309 & 0.3219 & 0.3983 \\  & 50 & 5981 & 0.0324 & 0.3181 & 0.3981 & 5860 & 0.0309 & 0.3217 & 0.3983 \\  & 60 & 6005 & 0.0325 & 0.3174 & 0.3978 & 5875 & 0.0312 & 0.3213 & 0.3985 \\ \hline \multirow{4}{*}{90} & 30 & 5284 & 0.0389 & 0.3415 & 0.4379 & 5211 & 0.0398 & 0.3447 & 0.4433 \\  & 40 & 5315 & 0.0392 & 0.3403 & 0.4375 & 5203 & 0.0397 & 0.3450 & 0.4433 \\  & 50 & 5306 & 0.0398 & 0.3407 & 0.4376 & 5219 & 0.039 consumption of the low beam and high beam with illumination angles of 0\({}^{\circ}\), 90\({}^{\circ}\), and 180\({}^{\circ}\) decreased by 21.7%, 21.2%, and 21.3%, and 22.5%, 22.5%, and 21.6%, respectively. The illuminance is proportional to the luminous flux output, so the luminous flux output decreased with \(T_{a}\). This study combined the illuminance and power consumption of the LED lamp to further define the lighting efficiency (\(LEF\), \(\mathrm{lx}/\mathrm{W}\)) as the illuminance generated per unit of power consumption. The experimental results showed that increasing \(T_{a}\) caused the \(LEF\) of the LED lamp to fluctuate a little, which is not the same as the situation where high temperature reduces the optical efficiency of the LED chips. When the LED lamp was powered by a constant voltage, the rise of ambient temperature increased the resistance of the lamp and thus reduced the current injected into the LED chips, leading to low luminous output. Although higher temperature decreases the optical efficiency of LED chips, the reduced injection current to LED chips benefits the LEF [34]. Because these two-factor effects substantially canceled each other out, the LEF only varied from \(-6.2\%\) to \(8\%\) with the increase in the ambient temperature. The drop in the luminous output is mainly attributed to the increase in resistance of peripheral electronic devices of the LED lamp with the ambient temperature, instead of the generally expected factor where the optical efficiency of the LED decreases with the junction temperature. Generally, an LED lamp is used in a closed lampshade for the automotive headlamp. Qualified automotive headlamps must meet the specifications regarding light distribution and intensities of the sampling points in the bright and dark areas [29]. However, if the luminous flux output of the LED lamps decreases at high \(T_{a}\), the originally qualified automotive LED headlamps would probably become unqualified, especially in the hot-weather area or in vehicles with the internal combustion engine. Although raising the rated power consumption of the LED lamps to guarantee sufficient luminous flux output in the hot environment is intuitive, it would lead to energy waste and probably cause the intensities of some areas to exceed the maximal limit of the specifications, making the design of LED headlamps more difficult. Automotive LED headlights must have a total solution.