## 4 Discussion  
People have different needs for circadian rhythm during the day and night, leading to different spectral composition strategies in circadian lighting. In the daytime, sufficient circadian stimulus in lighting is expected, whereas lower melanopic suppression is desirable at night so as not to interfere with normal sleep. The International Well Building Institute [21] recommends Equivalent Melanopic Lux (EML) for circadian lighting evaluation by setting a threshold in various scenes, which requires no more than 50 EML in nocturnal lighting. In this standard, only the melanopic curve is taken into consideration to quantify the circadian effect of light, whereas there are five kinds of equivalent \(\alpha\)-optic illuminance for five photoreceptors, and all of them are proven to have impacts on rhythm [12; 17]. Nevertheless, EML is still instructive in circadian lighting for its relationship between melanopic sensitivity and light. The equation of EML is as follows, where \(\mathrm{E_{e}}(\lambda)\) refers to the SPD of a light source, and \(\mathrm{N_{\alpha}}(\lambda)\) refers to the spectral sensitivity curve of melanopsin. \[\mathrm{EML}\ =\ 72983.25\int\mathrm{E_{e}}(\lambda)\mathrm{N_{\alpha}}( \lambda)\mathrm{d}\lambda\propto\frac{\mathrm{MER}}{\mathrm{LER}}\phi_{\mathrm{ e}} \tag{6}\]  
On the right side of Equation (6), EML is proportional to the product of eye-level photopic illuminance \(\mathrm{\Phi_{e}}\) and MER/LER. Considering the results in Section 3.3, for light sources with fixed photopic flux, neither simply maximizing LER nor minimizing MER can efficiently reduce EML, since MER under the former strategy is almost maximum simultaneously, and LER under the latter strategy is particularly low. For the lighting scenes requiring sufficient EML, maximizing MER can obtain both high MER and relatively low LER at lower CCT, but improving EML at higher CCT tends to rely more on the decrease in LER, because the values of MER do not change much when LER continues to decrease. Hence, we adopted MER/LER as values of interest and calculated the maximum and minimum, respectively, as shown in Figure 6a. For light sources with fixed photopic illuminance, the change in MER/LER can represent the variation of EML. The figure reveals that both the minimum and maximum MER/LER increase along with CCT, but the maximum MER/LER at each CCT has much lower LER, which suggests that high LER plays an important role in low circadian effect. Furthermore, the results over 5000 K show that minimizing MER/LER accompanies the reduction of LER. The key to spectral composition is the trade-off between luminous efficiency and low circadian effect under the limit of color rendering. Figure 5: MER (y-axis on the left) and LER (y-axis on the right) as functions of correlated color temperature (CCT) under the LER-maximizing strategy and the MER-maximizing strategy with (**a**) B:Y ratio \(>\) 1 and (**b**) B:Y ratio \(<\) 1, where the red and blue solid lines with solid square markers represent MER and LER under the LER-maximizing strategy; the red and blue solid lines with hollow square markers represent MER and LER under the MER-maximizing strategy. We restrained LER at over 330 lm/W and color rendering index Ra at over 60. Figure 6b shows the spectra of MER/LER we eventually chose following the principle of minimum, and the corresponding spectral characteristics are listed in Table 2.  
Compared with the spectral characteristics under the LER-maximizing strategy listed in Table 1, all the values of \(\Delta\lambda_{2}\) at various CCTs still approach the minimum we set (100 nm). The spectral characteristics at CCT from 3000 to 4000 K are similar to those with maximum LER, which suggests that for light sources within this range of CCT, it might be a viable option to reduce circadian effect. When CCT is over 5000 K, due to the limitation of color rendering, the distribution of blue intensity should be improved.