What's new here?

For the hobby of astronomy, the biggest obstacle of all time is light pollution. Hands down. The more lights in yiour area, the less you will see.

We try to fight it when we can. Petition for lights that reduce upward glare, maintain some dark sky locations, and advocate for limiting new lighted areas. But on an individual level, there is only so much we can do to fight light pollution.

One of those things is to employ a light pollution filter.


Light pollution filters work by blocking certain spectra of light while letting the imporant ones through. The light from stars, nebulea, and galaxies is pretty well measured and so has the light from our street lamps.  So by filtering out the spectra of the latter, you can provide the viewer with a nicer image of deep sky objects. These filters are not a substitute for dark skies, but they can help a lot when there are no other options.

However, light pollution filters depend on one thing: being able to block out those unwanted spectra.  They got a good handle on street lights, and the large lights that many retail centers might use, but it seems the street lights are changing.

I didn't ask for this!

I didn’t ask for this!

Up until recently, street lamps illuminate using High Pressure Sodium Bulbs. These provide a good modicum of light for less electricity than an incandescent bulb would, and have a longer life as well. They give consistant light over their life-span (unlike fluorescent bulbs which can fade over time). The only disadvantage to using HPS bulbs is that they do take some time to turn on and become fully lit. But given the amount of time it takes for the sun to go down this has not been much of a disadvantage.

However, times are changing. As LEDs have become more and more economical many cities have been switching from HPS to LED bulbs. There are good reasons for this, as studies have shown that the broad spectrum light from LEDs allowed pedestrians to better see possible tripping hazards and car braking reaction times are much faster. LEDs also may have longer life spans than the HPS bulbs.

But light pollution filters were made to mostly to combat HPS street lights. Let me demonstrate the difference. Here is an image of a HPS street lamp in the Manayunk neighborhood of Philadelphia:


Note the heavy orange-ish tone of the light. This has been the standard for decades. Now look at an LED street lamp in Center City Philadelphia:


Yeah, you might notice a bit of a difference there.

Now LED’s do have other advantages, like a possible energy saving ability to adjust to varying levels of moonlight. But it looks like they will be adopted more and more in the future, which may cause problems for astronomers who employs light pollution filters. They still will filter out much of the light that is produced by retail locations (which can be more problematic in some regions than street lights.


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