## 5 Conclusions  
This study investigates luminous and melanopic performance with tunable spectral characteristics, which sheds new light on convenient property evaluation and spectral composition for PC LEDs in practice. The peak wavelength and FWHM of phosphor and the intensity ratio of blue LED to phosphor exert different impacts on the luminous and melanopic performance, and the trade-off in low circadian effect lighting design. The investigations are summarized as follows:  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**CCT (K)** & 3000 & 4000 & 5000 & 6000 & 7000 \\ \(\lambda_{2}\) (nm) & 588 & 572 & 550 & 548 & 550 \\ \(\Delta\lambda_{2}\) (nm) & 100 & 109 & 105 & 100 & 102 \\
**B:Y ratio** & 0.3:1 & 0.32:1 & 1:0.98 & 1:0.68 & 1:0.5 \\
**MER (blm/W)** & 140.91 & 214.41 & 243.06 & 269.32 & 278.10 \\
**LER (m/W)** & 414.23 & 430.75 & 386.73 & 363.99 & 332.24 \\
**MER/LER** & 0.34 & 0.50 & 0.63 & 0.74 & 0.84 \\
**Ra** & 60 & 60 & 60 & 61 & 66 \\ \hline \end{tabular}
\end{table}
Table 2: Spectral characteristics with minimum MER/LER from 3000 to 7000 K.  
Figure 6: (a) Maximum and minimum MER/LER as functions of CCT; (b) spectra with minimum MER/LER from 3000 to 7000 K, of which LER is restrained at over 330 lm/W and Ra at over 60.  
Both MER and LER are approximately linear with the FWHM of phosphor within the peak wavelength range of phosphor in practical use. LER linearly decreases with the FWHM of phosphor, whereas MER linearly decreases only at shorter peak wavelengths. Adjusting the intensity ratio of the blue LED has less impact on LER and MER than the other two characteristics do, especially at the shorter peak wavelength of phosphor. Therefore, a single characteristic can play a different role in different spectral combinations.
2. LER reaches the maximum at a peak wavelength of phosphor of 545 nm for B:Y ratio \(>\) 1 and 560 nm for B:Y ratio \(>\) 1, respectively. Nevertheless, when maximizing LER at each CCT, the variation of CCT relies more on the change in the peak wavelength of phosphor. Lowering the FWHM of phosphor and the intensity distribution of the blue LED mainly helps to obtain higher LER at each CCT. However, the excessive reduction of blue light intensity is accompanied by a decline in color rendering.
3. The values of MER under LER-maximizing and MER-maximizing strategies are quite close and both increase with CCT, which suggests that a decrease in MER necessarily leads to a decrease in LER, especially at higher CCTs. Hence, neither simply maximizing LER nor minimizing MER in spectrum tuning can efficiently reduce the circadian effect.
4. Different optimization strategies correspond to various combinations of spectral characteristics, even at the same CCT. The spectral composition of maximizing LER with lower blue LED contribution is viable only at lower CCTs. As CCT increases, it is confronted with the trade-off between luminous efficiency, circadian effect, and color rendering. In this case, improving the blue light intensity is more appropriate for less loss of color rendering. Meanwhile, tuning the peak wavelength of phosphor close to the peak of LER and decreasing the FWHM of phosphor may still be a solution to help maintain luminous efficiency and lower the circadian effect, though the loss of LER is inevitable in the trade-off at higher CCTs.  
Conceptualization, Q.Y., J.J., and P.W.; methodology, Q.Y. and P.W.; software, P.W.; validation, Q.Y. and P.W.; formal analysis, Q.Y. and P.W.; investigation, P.W.; resources, Q.Y. and J.J.; data curation, P.W.; writing--original draft preparation, P.W.; writing--review and editing, Q.Y. and J.J.; visualization, P.W.; supervision, Q.Y.; project administration, Q.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Shanghai Gaofeng & Gaoyuan Project for University Academic Program Development.  
The authors wish to thank the anonymous reviewers for their valuable suggestions.  
The authors declare no conflict of interest.