Yellow light is night-friendly light: Low-pressure sodium (or narrow-band amber LED) is not just for turtles and astronomers.
|Effect of changing from Low-pressure sodium to LED 4100K CCT|
|Sky as seen from Sunset Crater National Monument, with artificial sky glow arising from Flagstaff AZ. Hammer-Aitoff equal-area all-sky simulations by D. Duriscoe (U.S. National Park Service) and C. Luginbuhl (U.S. Naval Observatory Flagstaff Station)|
The spectrum of outdoor lighting influences many aspects of light pollution, from glare and human health to activities of animals (notably sea turtles) and insects and biological processes in many organisms – a good overview of these issues can be found here. The blue and green part of the spectrum especially has disproportionate impacts (see here). On this page we describe the influence on the darkness of the sky and the visibility of stars, specifically the results of recent research on the visual brightness of sky glow.
The bottom line is simple, if unexpected to many: Yellow or amber light sources (such as high-pressure sodium, PC amber LED or, the best, low-pressure sodium* or “narrowband” amber LED) cause the least sky glow by far, as little as one half or even one quarter that of the best (lowest color-temperature) white light sources. The other side of the coin is that all white light sources (even “low CCT”) cause greater impact, usually much greater. Read more…
* FDSC no longer recommends LPS for new installations due to notice from the current sole manufacturer of discontinuation in 2020. See Low Pressure Sodium Lighting.)
As described on the Outdoor Lighting Codes page, to effectively limit adverse impacts of outdoor lighting, lighting codes must address the three principal aspects of lighting that increase light pollution:
- Shielding of fixtures
- Spectrum of lamps
- Amount of light
Though the negative impacts of poorly shielded fixtures and overlighting are widely understood, the impact of lighting color is not widely known, and most lighting codes do not address lamp types. But recent research shows that white lighting (such as LED, fluorescent and metal halide) has a dramatically greater impact – lumen-for-lumen – on sky glow than the currently most common high-pressure sodium (HPS) and especially low-pressure sodium (LPS) and narrow-band amber LED (NBALED).
Nearly all discussion about the light pollution impacts of outdoor lighting, particularly LED lighting, has focused on the “correlated color temperature” (CCT), or sometimes “percent blue” (percent or light emissions at wavelengths less than 500nm) as the way of gauging sky glow. Light pollution research shows that these measures are not accurate for this purpose – that the most accurate measure by far is instead the “scotopic to photopic ratio,” or S/P.
Type Description S/P Ratio Sky Glow1
(relative to LPS)
(relative to HPS)
LPS Low-pressure sodium – a nearly monochromatic yellow-orange light source used mostly in areas near astronomical observatories and sea turtle nesting beaches.
0.23 1.0 0.4
NBA LED2 Narrow-band amber LED – a narrow-spectrum yellow-orange LED nearly equivalent to LPS in light pollution impacts. 0.23-0.30 1.0 0.4
HPS High-pressure sodium – A golden-yellow light source, widely used throughout the world.
0.64 2.4 1.0
PCA LED3 Phosphor-converted amber LED – Similar to HPS though products vary. 0.45-1.0 1.8-4.1 0.7-1.6
FLED4 Filtered warm-white light-emitting diode – a straw-yellow LED lamp with a filter that removes most emission with wavelength shorter than 500 nanometers.
0.9 3.6 1.5
Light-emitting diode with “correlated color temperature” (CCT) of 2400K – a “warm-white” LED. This type of LED has not seen wide use.
1.15 4.3 1.8
LED 4100K Light-emitting diode with CCT of 4100K – a “cool-white” LED. This is a common LED type in recent LED area lighting installations.
1.65 6.4 2.7
LED 5100K Light-emitting diode with CCT of 5100K – a “cool-white” LED. This also is a common LED type in recent LED area lighting installations.
2.05 7.9 3.3
1 Ratios vary with distance and position in the sky: values shown are for 1 km distance and overhead in the sky.
2 AlInGaP LED with peak wavelength between 590nm and 595nm. The range is for different peak wavelengths, with the lower S/P for the longer peak wavelength.
3 Some phosphor-converted amber LED (PCALED) have sky glow impacts very similar or even lower than HPS.
4 The filtered LED used on the island of Hawai’i is different than the FLED analyzed by Luginbuhl et al. The Hawai’ian version has an estimated sky glow impact 4.4x LPS and 1.8x HPS – very similar to the 2400K CCT LED
5 S/P ratio for a given CCT varies – value shown is approximate
Due primarily to the increased sensitivity of the human eye to blue and green light at the very low brightnesses seen in the clear night sky – even in light-polluted skies – all of the white LED sources cause much brighter sky glow. Sky glow from the lowest-impact commonly used LED (4100K CCT) appears nearly seven times as bright as that from an equal amount of LPS, and 2.7x times brighter than HPS. This is a dramatic effect. Even without changing light amount or shielding, switching a lighting installation from HPS to 4100K LED will increase sky glow as if the amount of HPS light had been increased 170%, or nearly tripled; if changing from LPS the sky glow brightness would increase 540%.
A focus on using lower CCT LEDs misses much of the problem, because the colors causing the greatest visual sky glow impact (blue-green and green) are still strong in low-CCT LEDs and in filtered LED.
And brighter sky glow means fewer stars are visible. In a moderately polluted sky with artificial sky glow caused by mostly LPS outdoor lighting (here we assume the sky is 50% brighter than a natural sky at the zenith), about 2,700 stars are visible. If outdoor lighting were changed from LPS to 4100K CCT LED, the artificial component of sky glow would increase 6.6x, and total (artificial + natural) would appear 3x brighter (now the sky would be 200% brighter than a natural sky at the zenith). Instead of 2,700 stars you would now see only 1,500 stars. Simply changing the lighting type to a purportedly “environmentally friendly” LED light – with no increase in the amount of light (in lumens, footcandles or lux) – would obscure almost half of the stars in the night sky.
Flagstaff Low-Pressure Sodium Area Lighting
The effect of switching from HPS is somewhat less dramatic, with the visual brightness of the artificial component increasing 2.7x (170%); if the example above was switching to 4100K CCT LED from HPS, the total sky brightness would increase 1.9x at zenith.
We often hear that we must use white lights (especially in recent years LEDs) because “everybody” wants or needs white light, or “nobody” likes yellow light, or that white light is better for visibility. Yet if the benefits and drawbacks of all lamp types are fairly described, many communities may choose the lower impact yellow light, as Flagstaff, Sedona, and Coconino County Arizona have.