COSMIC MATTERS

By Andrew Cooper, W.M. Keck Observatory

Change is inevitable. LEDs are the most power-efficient commercially available light source. Their longer lifetimes, more light-per-watt and lower installation costs make it only a matter of time before most light sources in the world are LED. The fixtures and bulbs are still notably more expensive, but that’s offset by lower energy usage and longer lifetimes.

No light is exempt from the LED changeover, not even the ubiquitous streetlamp.

Streetlights consume an enormous amount of the power generated across the globe. Here in the islands, where power is more expensive than much of the nation, the power savings make LED streetlights very attractive. As Hawaii County taxpayers, this is something that we should all be concerned with. We all pay the power bill for the thousands of streetlights across the island.

The best competing technology is low pressure sodium (LPS). It is this streetlight technology that is currently used throughout Hawaii. LPS emits all of its light at a single wavelength, 589 nanometers, which gives them a golden yellow. Many people dislike the yellow color, and it can be confused with the yellow of a changing intersection signal.

Another advantage of the LEDs is reduced “light trespass.” That means directing the light properly onto the roadway and less light shining onto areas where it is not needed—into the windows of adjacent homes, upwards to light the sky, or outwards to confuse birds or sea turtles. The large LPS bulbs are difficult to design fixtures around, resulting in significant light trespass. Such fixtures are said to be poorly shielded. LED lamps are much easier to fully shield, because of their point-like light source.

LPS lights do have advantages, of course. They are quite power efficient, emitting all the light near the peak sensitivity of the human eye. Astronomers prefer the light of LPS lamps, as the single wavelength can be easily filtered or simply ignored when seen in their data. Between the power savings and the astronomy-friendly aspects, LPS had been the outdoor lighting of choice across the state. The result is the familiar golden glow above any urban area in the islands.

But change is coming, with LEDs already beginning to replace LPS streetlights. Hawaii County has planned for some time to convert to LED technology and evaluation of the new lamps has been going on at several test locations. With the success of those tests more widespread deployment is just beginning. This last month, the new LED streetlights were installed around the town of Waimea. For many, this is the first chance to see the new lights in place.

The recent conversion allowed an opportunity to compare the new with the old. Taken midway through the change out, the photo shows the new lights down the right side of the street, while the older low pressure sodium lights have yet to be replaced on the left side of the street.

The new lights are dramatically better than the old, poorly shielded LPS lights. Because of their vastly reduced glare, you simply do not see the lights themselves from any distance. Even nearby, the lights do not produce the distracting glare of the older designs. Note, also, that all of the older LPS lights create yellow halos of glare in the photo, all the way down the street. While the new LED lights become hard to see once you are out from underneath the light. In the photo, the third LED light from the camera is just seen and the fourth is hard to pick out.

The improvement is immediately noticeable while driving the main street of Waimea. The LED lights are difficult to see directly, while the light provided on the roadway is just as good if not better than the older LPS fixtures. This is a street with substantial pedestrian traffic and vehicles pulling in and out businesses, so good visibility is critical for safety.

Additionally, the new LED lamps installed on the island have specially designed blue cutoff filters. This is critical for the observatories. White LED lights use a blue or violet LED to excite a phosphor that converts the blue light to a white light. After the conversion, a large amount of this blue light leaks through the phosphor. It is this blue light that is most troubling to astronomers because it readily scatters in air, an effect called Rayleigh scattering. This is the same effect that makes the sky blue during the day. The night-time sky is naturally darker in the blue region of the spectrum, thus any light pollution at this wavelength creates a larger impact on astronomy. The special filters, therefore, keep the LED lamps from creating more light pollution.

So far, Hawaii County’s conversion to LED lights is just beginning. One thousand lamps were ordered, and only a handful have been installed, mostly near intersections. While we hope that the effects improve safety, reduce light pollution and reduce power bills, it will be some time before we see the real impact of the new lights and have a chance to measure the changes.

Andrew Cooper is an electrical engineer at W. M. Keck Observatory and a member of the West Hawaii Astronomy Club. His blog can be found at darkerview.com