In Figure 1A and Table S1 in the SI, the spectral parameters possessing the highest Lmes are listed for all four road lighting standards together with the scotopic and photopic vision regimes. Photometric and radiometric properties of the corresponding spectra are summarized in Table S2 in the SI. A careful investigation of Figure 1A and Table S1 reveals that at the very low luminance levels the maximum mesopic luminance is attained when all the color components have similar amplitudes. However, at higher luminance values the content of the red component is very dominant. The decrease in the relative amplitude of the blue emission is also another noteworthy feature at higher photopic luminances. Moreover, the rise in the yellow intensity is another remarkable fact as the luminance level increases. As a result of all these changes, the correlated color temperature (CCT), which is related to the shade of the white light, takes lower values that correspond to warmer white.
Figure 1 (A) Spectra possessing the highest mesopic luminance (continuous lines) and eye sensitivity function (dashed lines) for the scotopic vision regime, the mesopic road lighting standards 1–4, and photopic vision regime. (B) Enhancement in the mesopic luminance of the QD-WLED designs, which possess the highest mesopic luminance, compared to conventional light sources cool white fluorescent lamp (CWFL), high pressure sodium lamp (HPS), metal halide lamp (MH) as a function of radiance. These values were found by calculating the ratio of the difference in mesopic luminances of the QD-WLEDs and conventional sources to the mesopic luminance of the conventional sources at a given radiance.
Another interesting feature of these spectra that possess the highest mesopic luminances is the position of the peak emission wavelengths of the color components. We observe that the blue emission is very close to 460 nm in all of the cases. Additionally, the red component is located at ∼610 nm except for the scotopic case. A dedicated discussion on the criticality of wavelength selection is included in the next section. However, at this point this information shows us that the resulting spectra yield a good overlap with the eye sensitivity curves (Figure 1A).
In addition to the discussion of spectral parameters, the photometric properties of the QD-WLED emission deserve attention. Although we did not place any restriction on CCT, we found that the warm white emission (CCT≤4500 K) can be obtained by preserving very high mesopic luminance, CRI, and CQS at the same time. Furthermore, a short discussion on the photopic luminance levels and road lighting intervals is also necessary at this point. The QD-WLED spectra were generated such that their radiances are equal to that of CWFL for the scotopic regime and the mesopic road lighting standards, Mesopic 1 and 2. For the mesopic road lighting standards Mesopic 3 and 4, and the photopic regime, the radiances of the QD-WLEDs were selected equal to that of HPS. Therefore, it could be possible that the generated white LEDs do not remain within the standards mentioned in previous sections. However, the results show that the designed QD-WLEDs are within the standards that we consider in this work; therefore, they are still valid.
Achieving high photometric performance is crucial for high visual quality of the road lighting. This undoubtedly enhances the road safety and contributes to the life quality. However, the overall power conversion efficiency, which can be defined as the ratio of total radiant flux from the device to the supplied electrical power, is another important factor that has to be considered while designing a light source. For this purpose, we calculated the required radiance of the CWFL, HPS, and MH such that the obtained mesopic luminance is equal to the mesopic luminances of QD-WLEDs given in Figure 1A (and Table S1). The results are summarized in Table S3 in the SI. The minimum power conversion efficiency of a QD-WLED (ηQD-WLED), which is needed for consuming lower electrical energy than the conventional sources CWFL, HPS, and MH while generating the same mesopic luminance, can be calculated using Equation (1):
(1)
where ηconv is the power conversion efficiency of the conventional light source, Pconv and PQD-WLED are the radiances of the conventional source and QD-WLED, respectively. Taking the efficiencies of CWFL as 28%, HPS as 31%, and MH as 24% [34], the minimum required ηQD-WLED turns out to be between 21% and 24% when CWFL is employed (Table S4 in the SI). In the case that HPS is preferred for comparison, QD-WLEDs consume less electrical power than HPS while realizing the same mesopic luminance if their power conversion efficiencies become larger than the values presented in Table S4. Finally, the QD-WLEDs are required to possess power conversion efficiencies between 16% and 18% for them to consume less electrical energy than MH lamps while generating the mesopic luminance given in Table S3. Considering that 70% film quantum efficiencies of QDs can be obtained [9], achieving these required power conversion efficiencies is not a big challenge [34].
Another point that is worth mentioning is the efficiency of the luminaires. A luminaire is the system that delivers light to the target illumination volume and may include reflective surfaces as well as lenses and diffusers for mixing and managing the light. The light generated by the sources within the luminary is not delivered to the target volume with 100% efficiency. The typical efficiency remains around 60% for conventional sources [35], and conventional LEDs [36], the rest of the energy is lost as heat. However, the LEDs have the potential to possess higher luminaire efficiencies even up to 90% when more creative luminaire designs are employed [37–41].
Up to this point we compared the performance of the QD-WLEDs proposed in Figure 1A and the conventional sources by fixing the mesopic luminance that the sources are capable of achieving and calculating the required radiances. An alternative approach is to investigate the enhancement of the mesopic luminance when the QD-WLEDs and conventional sources possess the same radiance. When all of these sources have radiances equal to the ones given in Table S2, the QD-WLEDs in Figure 1A are found to exhibit 15–29% higher luminances than CWFL as presented in Figure 1B and summarized in Table S4. This enhancement becomes 8–172% in the case of comparing to HPS and 33–42% in the case of comparing to MH. The results reveal that the use of QD-WLEDs can promise higher mesopic luminances than the conventional sources while consuming less electrical energy.
From an experimental point of view, obtaining a QD-WLED having a specific spectrum, which exactly satisfies the theoretically determined spectral conditions stated in the previous section, is a very challenging task due to uncontrollable deviations from the ideal design during implementation. Therefore, elucidating the strict and flexible parameters is of significant importance in practice. In our study we use the standard deviation to evaluate the criticality of the spectral parameters. Before investigating this, however, it is beneficial to know what would be the standard deviations if we had uniform distribution of these parameters. We may use this information when concluding the importance of various spectral parameters. Our calculations revealed that if we had a uniform distribution of peak emission wavelengths, we would have a standard deviation of 17.1 nm for the blue component and 14.2 nm for the remaining three color components. The full-width at half-maximum would have a standard deviation of 5.6 nm for all four color components in the case of uniform distribution. Finally, the standard deviation of relative amplitudes would be 229/1000.
The average and standard deviations of the peak emission wavelengths of the spectra passing the thresholds are given in Table S5 in the SI and illustrated in Figure 2A. The results indicate that the blue and red peak emission wavelengths should be very carefully adjusted whereas the green and yellow components are not as critical. We found out that obtaining high mesopic luminance requires a blue emission peak located around 460 nm and a red peak around 610 nm. As very low standard deviations show, only small deviations from these average wavelengths are tolerated. However, the choice of green and yellow peak emission wavelengths turned out to be not very critical as their very large standard deviations indicate. Therefore, it is safe to choose QDs emitting relatively far from the average values given in Table S5. Another remarkable point is that the choice of the blue wavelength preserves its importance under all luminance conditions whereas the red choice is partially relaxed in the scotopic and photopic vision regimes. This can be due to the fact that the red component gets closer to 620 nm for high efficiency as the luminance level increases towards photopic levels [7].
Figure 2 Average and standard deviation (error bars) of (A) peak emission wavelength, (B) relative amplitude, and (C) full-width at half maximum belonging to the spectra passing the thresholds at the simulated luminance levels. The colors indicate the data points belonging to the corresponding color component.
The average and standard deviations of the relative amplitudes of the spectra passing the thresholds are given in Table S6 in the SI and illustrated in Figure 2B. From these results, we can draw several important conclusions. First of all, for obtaining high luminance in the mesopic regime one has to keep the blue content weak, between 145/1000 and 175/1000. The standard deviations around 25–30/1000 do not allow for a stronger blue content without sacrificing good color rendition and high mesopic luminance. Another interesting feature is that the blue amplitude and its standard deviation decrease as the radiance increases. In addition to the blue content, the red exhibits interesting properties as well. First, the red should be the dominant color component in the spectrum at all the luminance levels we tested. The average amplitude values are located around 330/1000 for the scotopic vision regime, and between 450/1000 and 475/1000 for the four road lighting standards and photopic regime. For the scotopic regime, the low standard deviation around 27/1000 shows that any large deviation will cause a decrease in the photometric performance. Actually, for the scotopic region all the standard deviations of relative amplitudes are very low. Therefore, designs should be strictly based on the values presented in Figure 2B and listed in Table S6. However, at higher luminance levels the standard deviations of the red are in the range of 45/1000–60/1000, which gives some flexibility during the spectral designs. When it comes to the green and yellow components, we observe that the green takes intermediate values whose average intensity is almost the same at all the radiances. On the other hand, the yellow content remains very low, even lower than the blue component in some cases. However, both of these color components have very large standard deviations, which increase the freedom of designers in adjusting their weights.
The average and standard deviations of the full-width at half-maximum belonging to the spectra passing the thresholds are given in Table S7 in the SI and the results are illustrated in Figure 2C. Here we observe that the blue, green, and yellow color components have similar average values and similar standard deviations regardless of the luminance level. Moreover, the resulting standard deviations (around 5–6 nm) are very close to the case of uniform assumption (5.62 nm). Therefore, we conclude that the designer has a high flexibility for selecting the full-width at half-maximum values of these emitters. The only critical color component turns out to be the red one. In that case the full-width at half-maximum values are the lowest ones except the scotopic case. More importantly, for the luminance standards of road lighting the standard deviations are narrower than the others. Therefore, the full-width at half-maximum selection only for the red component should be carefully selected. It should be narrow (34–35 nm) and large deviations are not tolerated at the appropriate luminance levels of road lighting.
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