Question about the new LED street lighting

[davelew.livejournal.com]

This is a long shot, but there's enough random information on this community that I thought somebody might be able to help.

The city of Somerville is switching from old sodium vapor street lights to new, more energy efficient LED street lights. That's great and I support it, it's good for the environment and for the city's finances. The problem is with my telescope. I have a filter for my telescope that cuts out light pollution from old-style lights, by blocking light at precisely the wavelengths of the emissions spectrum of sodium atoms. Unfortunately, that filter does nothing for the new LED lights. It's nice to have a filter for urban astronomy if you're looking at anything dimmer than the moon, and I don't know what filter(s) to get.

So, here's my random question: Does anybody know the emissions spectrum of Somerville's new LED lights? Or even a single particular wavelength that they don't emit? I'd even be happy with a model number so that I could contact the manufacturer.

Comments

[starphire.livejournal.com]

some notes in regard to your observations:
-both CFL and LED retrofit bulbs are in suboptimal packaging for a large percentage of all existing light fixtures in the world. Because an incandescent bulb is expected to generate mostly heat, fixtures were designed for appearance, nice light dispersion, and longevity using heat-resistant materials. Keeping the bulbs cool was not an important consideration for a century! But that's EXACTLY what you need for either a CFL or an LED bulb (because the ballast components are heat-sensitive, and failure rates for electronics generally follow the Arrhenius equation). With a CFL, the heat from the light-emitting part has a relatively large surface area, so it needs no metal heat sink, which adds cost and weight. But the CFL power supply (which also generates heat) has to be crammed into the base, as small and as cheap as possible to compete in stores with <$1 lightbulbs - because consumers tended to think of buying bulbs in terms of price at the register rather than in lifetime usage cost. That means billions of no-name bulbs dominating CFL sales, with rock-bottom component and construction quality, most installed in fixtures where heat is largely trapped around the bulb, where they fail long before their rated lifetime...or melt (as you note), or even catch on fire. Most CFLs have fine print saying "do not use in inverted (base up) position", precisely because the rising heat gets concentrated in the worst place. More costly CFL bulbs from brand-name manfacturers, in suitable fixtures, actually do live up to their claims - usually. Recessed ceiling fixtures are the worst...

-LED bulbs (the good ones you refer to) have those metal bodies to get heat away from the LED chips - because they produce heat in concentrated spots with little surface area and MUST have large heat sinks to keep those chips from frying. But they don't generate more heat than a CFL of equal wattage - it just feels that way to you, because aluminum conducts heat to your fingers so much better than glass or plastic! Their power consumption per lumen is FAR less than an incandescent, now better than CFLs too - and still getting better every year. There is no myth or marketing BS there, as you suggest - they consume what they say they do. US DOE expects the average LED bulb will be 120 lumens/Watt within 2 years, because LEDs coming out of the lab are already better than this.

-There are actually sound technical reasons why LED streetlights have better efficacy than sodium lights at getting light where it's wanted and not elsewhere, but no optical system is perfect in the real world. And as others noted, more light energy is matched to peak eye sensitivity. Interestingly, LED streetlights in colder climates have an advantage over ones in warmer cities, because the cooler the air temperature, the more efficient and long-lived the LEDs and drive electronics are! Also, electronic power supplies can have better Power Factor Correction than sodium ballasts, so better and more efficient for the electrical grid. Lots of small advantages that add up, so long as designs are robust and well-done and don't make silly compromises like trying to fit into a package that was designed for a different kind of technology. In that respect, LED streetlights are ALSO better off than LED retrofit light bulbs.

-Certain geek-magnet flashlights are designed with the aim of getting the most light out of the smallest package, which means those nice aluminum heat-conducting bodies get hot quickly and use up tiny batteries fast. That means nothing in terms of efficiency, it just means designers are making tradeoffs with certain performance priorities.