Sodium Light

The golden yellow glow that has dominated the night for generations is disappearing.

Tenakee Docks at Night
Fishing boats under sodium lamps haunt the Tenakee docks
Low pressure sodium has been the standard technology for outdoor lighting for generations. the soft yellow glow is familiar to anyone who has lived in any urban area, coloring lives and countless photographs of the night.

The yellow glow of sodium light has been both celebrated and reviled. While the glow can be attractive in night scenery it also creates inhuman tones in faces and photos of people. Movies have been shot under sodium lights, songs reference the golden glow. Astronomers both professional and amateur prefer the lights as the light can be easily filtered from view.

The spectrum of low pressure sodium light
The spectrum of low pressure sodium light
LPS has been so dominant for several reasons, it was the most power efficient of the available lighting technologies. This allowed more light at lower cost than any other competing technology, a powerful motivator when buying decisions were made by cities, counties and highway departments.

This efficiency comes from the nature of sodium light. In LPS almost all of the light emitted in a wavelength usable by the human eye. That light is emitted at 589nm, a deep yellow. This is far different that most other lighting technologies that emit light at a wide variety of wavelengths. 589nm is relatively near the peak sensitivity of the human eye, making this light preferable for roadway lighting installed for driving safety.

Dual Lasers on the Galactic Center
Both Keck lasers aimed at the center of the Milky Way galaxy
Sodium yellow is also the color of many adaptive optics lasers. Stabbing forth from the observatory these yellow beams excite a layer of sodium atoms found high in the atmosphere to create reference beacons used to analyse atmospheric distortions.

Aside from AO lasers astronomers love low pressure sodium lights because of the monochromatic nature of the light these lamps produce. With the light centered at a very sharp wavelength it is easy to simply remove any signal from your data while the remainder of the spectrum is unpolluted. See a peak at 589nm in your spectrograph data? Ignore it, it is likely to be the light of streetlights from a nearby city or town.

Alternately you could install a filter designed to block the wavelengths of known light pollution sources while allowing other wavelengths through. Such filters are commonly available in optics catalogs or equipment websites frequented by amateur astronomers. You can simply buy a filter that can be installed on your camera that will block these wavelengths while allowing a clear view of the night sky.

LED versus Low Pressure Sodium
Downtown Waimea with new LED streetlights and old LPS
The reign of LPS is coming to en end, as more efficient LED lighting is installed to replace the older technology. With LPS cities are losing their golden glow, becoming brighter and more energy efficient. The change is visible from space, photos taken by space station crews note the white cores of many cities as the central urban areas are changed first.

The new technology on the street, LED based lighting brings many issues. The white or blue-white light of LED is more harmful to human health and wildlife. Many studies note the harmful effects blue light has on our own health, in disrupting our circadian rhythm, to supressing metatonin production, and a wide range of other possible effects.

The New LED Lights
The electrical contractors show off the new LED lights for our neighborhood
The issues with LED can be mitigated to some extent. Filters or special phosphor blends can change the spectral content of the lights, removing the more troublesome blue end of the spectrum. This creates a warmer or greener result. while not as harmful, it does at least minimize the issues. The new LED streetlights in Hawaii county are filtered like this. The filters result in a light that looks a bit more like the old LPS, a soft greenish glow in place of the yellow.

Kawaihae Small Boat Harbor
Fishing boats under sodium lights in the Kawaihae small boat harbor

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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