Yellow light is night-friendly light: Low-pressure sodium (or ) is not just for turtles and astronomers.
|Effect of changing from Low-pressure sodium to4100K|
|Sky as seen from Sunset Crater National Monument, with artificialarising 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 , from 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 .
The bottom line is simple, if unexpected to many: Yellow or amber light sources (such as , or, the best, * or ) cause the least 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 ”) cause greater impact, usually much greater. Read more…
* Low Pressure Sodium Lighting.) no longer recommends for new installations due to notice from the current sole manufacturer of discontinuation in 2020. See
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 :
- 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, fluorescent and metal halide) has a dramatically greater impact – lumen-for-lumen – on than the currently most common high-pressure sodium ( ) and especially low-pressure sodium ( ) and narrow-band amber ( ).
Nearly all discussion about theimpacts of outdoor lighting, particularly lighting, has focused on the “correlated color temperature” ( ), or sometimes “percent blue” (percent or light emissions at wavelengths less than 500nm) as the way of gauging . 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
(relative to )
(relative to )
Low-pressure sodium – a nearly monochromatic yellow-orange light source used mostly in areas near astronomical observatories and sea turtle nesting beaches.
3 Narrow-band amber – a narrow-spectrum yellow-orange nearly equivalent to in impacts. 0.23-0.30 0.4
High-pressure sodium – A golden-yellow light source, widely used throughout the world.
4 Phosphor-converted amber – Similar to though products vary. 0.45-1.0 0.7-1.6
FLED5 Filtered warm-white light-emitting diode – a straw-yellow lamp with a filter that removes most emission with wavelength shorter than 500 nanometers.
Light-emitting diode with “correlated color temperature” () of 2200K – a “warm-white” . This type of has not seen wide use.
Light-emitting diode with “correlated color temperature” () of 3000K – a “warm-white” .
4100K Light-emitting diode with of 4100K – a “cool-white” . This is a common type in recent area lighting installations.
5100K Light-emitting diode with of 5100K – a “cool-white” . This also is a common type in recent area lighting installations.
Notes to Table:
1 Ratios vary with distance and position in the sky: values shown are for 1 km distance and overhead in the sky.
2 – based on sky-glow impact of the spectrum only.
3 AlInGaP with peak wavelength between 590nm and 595nm. The range is for different peak wavelengths, with the lower S/P for the longer peak wavelength.
4 Some phosphor-converted amber (PCALED) have impacts very similar or even lower than .
5 The filtered used on the island of Hawai’i is different than the FLED analyzed by Luginbuhl et al. The Hawai’ian version has an estimated impact 4.4x and 1.8x – very similar to the 2400K
6 S/P ratio for a given 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 whitesources cause much brighter . from the lowest-impact commonly used (4100K ) appears nearly seven times as bright as that from an equal amount of or , and ~2.5x times brighter than or good . This is a dramatic effect. Even without changing light amount or shielding, switching a lighting installation from to 4100K will increase as if the amount of light had been increased 170%, or nearly tripled; if changing from the brightness would increase 540%.
A focus on using lowerLEDs misses much of the problem, because the colors causing the greatest visual impact (blue-green and green) are still strong in low- LEDs and in filtered .
And brightermeans fewer stars are visible. In a moderately polluted sky with artificial caused by mostly 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 to 4100K , the artificial component of 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” 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 fromis somewhat less dramatic, with the visual brightness of the artificial component increasing 2.7x (170%); if the example above was switching to 4100K from , 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.