Lamp Spectrum and Light Pollution

Color Matters!

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.

OfficeMax Using Flagstaff Lighting Code
Fully shielded, low-pressure sodium lighting at 50,000 lumens per acre in this Flagstaff OfficeMax parking lot – exhibits all three aspects critical to minimize sky glow, glare, and ecological impact. Recent research shows that high-pressure and low-pressure sodium lighting cause 1/3 and 1/6 the sky glow (respectively) from the typical 4100K CCT white LED or metal halide.

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:

  1. Shielding of fixtures
  2. Spectrum of lamps
  3. 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.

Two recently published studies (Luginbuhl et al., 2014; Aubé et al., 2013) have evaluated the visible sky glow brightness caused by the following lamp types:

TypeDescriptionS/P RatioSky Glow1
(relative to LPS)
Sky Glow1
(relative to HPS)
LPSLow-pressure sodium – a nearly monochromatic yellow-orange light source used mostly in areas near astronomical observatories and sea turtle nesting beaches.
LPS spectrum 400-650nm
NBA LED2Narrow-band amber LED – a narrow-spectrum yellow-orange LED nearly equivalent to LPS in light pollution impacts. NBA-LED spectrum 400-650nm0.23-0.301.00.4
HPSHigh-pressure sodium – A golden-yellow light source, widely used throughout the world.
HPS spectrum 400-650nm
PCA LED3Phosphor-converted amber LED – Similar to HPS though products vary. PCA LED spectrum 400-650nm0.45-
FLED4Filtered warm-white light-emitting diode – a straw-yellow LED lamp with a filter that removes most emission with wavelength shorter than 500 nanometers.
Filtered LED spectrum 400-650nm
Light-emitting diode with “correlated color temperature” (CCT) of 2400K – a “warm-white” LED. This type of LED has not seen wide use.
LED 2400K CCT spectrum 400-650nm
LED 4100KLight-emitting diode with CCT of 4100K – a “cool-white” LED. This is a common LED type in recent LED area lighting installations.
LED 4100K CCT spectrum 400-650nm
LED 5100KLight-emitting diode with CCT of 5100K – a “cool-white” LED. This also is a common LED type in recent LED area lighting installations.
LED 5100K CCT spectrum 400-650nm

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.

Sky Glow Brightness for Lamps
Sky glow impacts, relative to low-pressure sodium lighting, of common outdoor lighting types.

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.

To celebrate, promote, and protect the glorious dark skies of Flagstaff and northern Arizona.