MnAstro Forums

Does anyone know anything about these new LED streetlights that are really blue? There's some in Bloomington on 24th Ave between American Blvd and the MOA. Also, last month I went on a road trip to Des Moines, Iowa, and Lincoln, Nebraska. It seemed like every light on I-80 from the Iowa line into Lincoln was like this, as well as a significant portion of the city streets. But I have no idea why they are like this.

I assume that they don't do much to help light pollution, or wildlife, given everything I know about artificial light colors at night.

I'd bet they were selected for maximum visibility and energy efficiency. Unfortunately, you are correct in that high color temperature lighting does not help in the battle against light pollution.

A couple of years ago the YouTube channel "Technology Connections" did a couple of videos about street lighting. One of the things he pointed out was how human vision responds better to this color light.

It's worth watching the entire video, but for the gist of it, see 00:50 and 1:50 and 2:55 in this video: https://www.youtube.com/watch?v=wIC-iGDTU40 for the description of human eye response to various wavelengths under different lighting conditions. He presents a side-by-side picture of visibility of blue-white vs. warmer light at around 7:55. He does counter the visibility argument with the upset to circadian rhythms of the bluish-white light around 5:30.

He compares high color temperature LED to sodium vapor lamps throughout because of when the video was produced (2018), and we hadn't been installing a lot of "warm" 2700K to 3000K LED street lighting yet, so for the most part just substitute 2700K LEDs whenever he says "sodium".

With all that, I'm not saying I like it. 6000K lighting is harsh, and it's around the same color as starlight and moonlight and therefore cannot be filtered out using light pollution filters.

There is no perfect answer. Use 2700-300K LED lighting that is more pleasing to the eye, doesn't upset circadian rhythms and wildlife as much, is easier to filter out but consumes more power for a given level of apparent illumination due to our eye's spectral response. Or use 5500-6000K LED lighting that makes it easier to see obstacles on nighttime highways and uses less power for a given level of apparent illumination, but has biological impacts and can't be filtered out with LPR filters. And people still use way more illumination than they need!

This is really blue. I like 5500K lights indoors and think that looks white. This looks downright blue, not even blue-white. Definitely a different color than moonlight, but not as blue as the blue component in Alberio. It would definitely interfere with observing reflection nebulae that tend to be blue, like the nebulosity in M45.

Is there such a thing as a light temperature meter?

Deane Clark wrote: ↑Wed Jul 06, 2022 8:42 pm Is there such a thing as a light temperature meter?

I'd like one of those, if they exist!

FF2Rydia wrote: ↑Wed Jul 06, 2022 9:01 pm

Deane Clark wrote: ↑Wed Jul 06, 2022 8:42 pm Is there such a thing as a light temperature meter?

I'd like one of those, if they exist!

I googled it, but just found a lot of normal light meters.

Not as simple as a meter dedicated to the function, but I expect a spectrascope could be used to determine color temperature.

SEmert wrote: ↑Thu Jul 07, 2022 5:45 am Not as simple as a meter dedicated to the function, but I expect a spectrascope could be used to determine color temperature.

Yes, it’s a much more complicated task when I start to think about it. If the point is to identify the type of LED light, then what happens if you point your meter at a mercury or sodium vapor light? The meter would have to calculate an average light curve and select a representative point to call the “color temperature”, which would frequently not coincide with the strongest emission line.

It is not quantitative, but you can use a CD disc as a low quality diffraction grating to visually see the spectrum produced by a light source. LED’s usually produce monochromatic light (i.e. a single color). White LED’s either employ multiple smaller LED’s to get the components of white or else a single LED shining onto a coating containing several different fluorescent materials that absorb the light and then re-emit at other colors. Since fluorescent materials can only emit longer wavelengths than they absorb, I suspect that they would use a blue LED (i.e. short wavelength) as the primary LED and get the longer yellow, green and red wavelengths by fluorescence.

It is also interesting to look at other types of light sources. A fluorescent light bulb consists of a low pressure mercury lamp coated with several fluorescent materials. This will produce a few discrete colors, not a continuous spectrum. An incandescent bulb will produce a continuous spectrum that emits especially strongly in the red region.