## 4 Results and Discussion  
Figure 4a-c shows the spectral measurement results of LED lamps at three illumination angles 0\({}^{\circ}\), 90\({}^{\circ}\), and 180\({}^{\circ}\), respectively. Each figure contains the measurement results of two types of LED chipsets (low beam (L) and high beam (H)); each type of LED chipset was measured under four ambient temperatures of 30, 40, 50, and 60 \({}^{\circ}\)C, respectively. Figure 4 indicates that the spectral distribution of the LED lamp under various conditions was very similar, and its spectral intensity decreased with the ambient temperature \(T_{a}\). The illumination angles of 0\({}^{\circ}\) and 180\({}^{\circ}\) mean that the LED chips on the right and left sides, respectively, faced the measuring hole (vent). Therefore, the received luminous flux for the illumination angles of 0\({}^{\circ}\) and 180\({}^{\circ}\) is higher than 90\({}^{\circ}\). The spectral intensity of the high beam is much higher than that of the low beam for the illumination angles of 0\({}^{\circ}\) and 180\({}^{\circ}\). However, the spectral intensity of the high beam is lower than that of the low beam for the illumination angle of 90\({}^{\circ}\). It is because part of the light of the low beam LED chipsets was reflected in the 90\({}^{\circ}\)direction by the reflector, while most of the light of the high beam LED chipsets was emitted toward the directions of 0\({}^{\circ}\) and 180\({}^{\circ}\). To evaluate the effect of the temperature on relative spectral distribution, the data on each curve in Figure 4 are divided by the maximum value on that curve and multiplied by 100 to form Figure 5. Figure 5 indicates all the normalized spectral distributions of different \(T_{a}\) almost coincided, which means that the temperature had little effect on the color rendering of the light. In general, the higher junction temperature causes the dominant wavelength to drift higher, e.g., \(\sim\)0.1 nm/\({}^{\circ}\)C for an InGaAsIP LED. In the experiment, the raw data of the measured spectrum indicate the drift of the peak wavelength of the blue light was about 1-1.5 nm. Such a small drift can hardly be observed in Figures 4 and 5. To further inspect the change in the color rendering, the spectral data of the LED lamps in Figure 4 can be further calculated into the CCT, Duv, and chromaticity coordinates (CIE x, y) shown in Table 2, which shows that the \(T_{a}\) had little effect on the CCT, Duv, and  
Figure 4: Spectral measurement results of the LED lamp: (**a**) 0\({}^{\circ}\), (**b**) 90\({}^{\circ}\), and (**c**) 180\({}^{\circ}\). chromaticity coordinates in the short-term test regardless of illumination angles, high beam, or low beam [32]. In addition, the CCT of the high beam and low beam of LED lamps at the illumination angles of 0\({}^{\circ}\) and 180\({}^{\circ}\) was very close to the original specifications of the LED lamps (CCT 6000 K). However, the CCT at the illumination angle of 90\({}^{\circ}\) was significantly lower than 6000 K, mainly because of the color unevenness of the light emitted from the LED chips in the angular space. In general, slanted light from the white-light LED at a large angle with a normal emitting area tended to be yellowish (lower CCT) [33]. The low beam of the LED lamp measured at an illumination angle of 90\({}^{\circ}\) included the slanted light and the reflected light that was originally toward the normal around by the reflector, and the high beam of the LED lamp measured at an illumination angle of 90\({}^{\circ}\) only included the slanted light. Therefore, the high beam of the LED lamp measured at an illumination angle of 90\({}^{\circ}\) displayed a lower CCT than the low beam of the LED lamp. Table 3 lists the illuminance, power consumption, and lighting efficiency of the LED lamp. The illuminance decreased with the \(T_{a}\), which has the same trend as the spectral intensity. In addition, the power consumption decreased with the increase in the \(T_{a}\).