Evaluating potential spectral impacts of various artificial lights on melatonin suppression, photosynthesis, and star visibility

Abstract

Artificial light at night can be harmful to the environment, and interferes with fauna and flora, star visibility, and human health. To estimate the relative impact of a lighting device, its radiant power, angular photometry and detailed spectral power distribution have to be considered. In this paper we focus on the spectral power distribution. While specific spectral characteristics can be considered harmful during the night, they can be considered advantageous during the day. As an example, while blue-rich Metal Halide lamps can be problematic for human health, star visibility and vegetation photosynthesis during the night, they can be highly appropriate during the day for plant growth and light therapy. In this paper we propose three new indices to characterize lamp spectra. These indices have been designed to allow a quick estimation of the potential impact of a lamp spectrum on melatonin suppression, photosynthesis, and star visibility. We used these new indices to compare various lighting technologies objectively. We also considered the transformation of such indices according to the propagation of light into the atmosphere as a function of distance to the observer. Among other results, we found that low pressure sodium, phosphor-converted amber light emitting diodes (LED) and LED 2700 K lamps filtered with the new Ledtech's Equilib filter showed a lower or equivalent potential impact on melatonin suppression and star visibility in comparison to high pressure sodium lamps. Low pressure sodium, LED 5000 K-filtered and LED 2700 K-filtered lamps had a lower impact on photosynthesis than did high pressure sodium lamps. Finally, we propose these indices as new standards for the lighting industry to be used in characterizing their lighting technologies. We hope that their use will favor the design of new environmentally and health-friendly lighting technologies.

Figure 1. x and y chromaticity coordinates for each lamp listed in Table 2.

On that figure, the spectral locus, which is the line for monochromatic light, is shown by the thick black line. Thin black lines indicate color zones. Black squares show monochromatic values, while small black circles are lamps.

Figure 2. Various spectral sensitivities: human eye photopic and scotopic spectral sensitivity; MSAS: human melatonin suppression action spectrum; PAS: photosynthesis action spectrum.

Figure 3. Constant lumen spectral power distributions.

A subset of the spectrum used in this paper is shown here. Pane (a) shows HID lamps, pane (b) illustrates white LEDs, (c) shows low blue content broadband LEDs and (d) shows thermalized spectra such as for halogen and incandescent lamps and our reference D65 illuminant from the Commission Internationale de l’Éclairage (CIE). D65 illuminant corresponds to a midday Sun in Western/Northern Europe with CCT∼6500 K. LED filtering was performed using the Equilib optical filter commercialized by Ledtech International.

Figure 4. Spectral impact of the atmospheric transfer function as a function of the distance for the CIE D65 illuminant without cloud cover in (a) and with cloud cover in (b).

Direct SPD relates to the original SPD before any atmospheric transformation. Compared to direct SPD, scattered SPD is bluer at short distances and redder at long distances.

Figure 5. Atmospheric scattered Melatonin Suppression Index dependency, with distance between lamp and observer, (a) and (c) for usual street lamps and (b) and (d) for new technologies that reduce light pollution.

The two upper plots (a) and (b) are for clear sky conditions, while (c) and (d) are for cloudy sky conditions.

Figure 6. Same as Figure 5 but for Induced Photosynthesis Index.

Figure 7. Same as Figure 5 but for Star Light Index.