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Scott K. Johnson from Ars Technica reports on how Flagstaff, Arizona, switched to energy-efficient LEDs without giving astronomers a headache from the harsh blue-rich light they produce. An anonymous reader shares several excerpts from the report: Flagstaff became the first city to earn a designation from the International Dark Sky Association in 2001. That came as a result of its long history of hosting astronomy research at local Lowell Observatory, as well as facilities operated by the U.S. Navy. The city has an official ordinance governing the use of outdoor lighting — public and private. A few years ago, though, a problem arose. The type of dark-sky-friendly streetlight that the city had been using was going extinct, largely as a casualty of low demand. In fact, as of this summer, there are none left to buy. Meanwhile, the age of the LED streetlight has arrived with a catch: limited night-sky-friendly LED options. The problem with LEDs boils down to blue light. Older streetlights are high-pressure sodium bulbs, which produce a warm yellow glow around a color temperature of 2,000 K. The bulbs Flagstaff relied on for most of its streetlights were low-pressure sodium — a variant that only emits light at a single wavelength (589 nanometers) near that yellow color, producing something resembling candlelight. Many of the LED streetlights on the market have much cooler color temperatures of 3,000 or even 4,000 K.

[…] Narrow-band amber (NBA) LEDs […] actually use a type of LED that only emits warmer colors from the start. In this way, they actually compare pretty well to the low-pressure sodium streetlights that recently went extinct. The range of wavelengths emitted is a little broader, but the practical effect is about the same. Separately from all this wavelength wrangling, though, LEDs do have a strong natural advantage — they’re highly directional. That is, LED streetlights do a much better job of only lighting the street (rather than the adjacent homes). That means that fewer lumens coming out of the fixture can give the same result you had before. Flagstaff’s plan is generally to swap in NBA LEDs for all the low-pressure sodium lights, and PCA LEDs [lights known as phosphor-converted amber (PCA) shift all the light out of the blue and into the yellow part of the spectrum at the cost of some efficiency] for the high-pressure sodium lights that are used along the busier streets (as they’re a little brighter). The better directionality of LEDs — combined with resident requests for slightly dimmer lighting on residential streets — actually means that the total output of the city’s streetlights is going to drop from about 29 million lumens to about 19 million lumens. That’s not unusual. In closing, Johnson says Flagstaff’s hope is to produce the first dark-sky ordinance updated to deal with LEDs that could give other cities an example to follow.