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What CCT Lights are Cities Installing?

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#26 GeorgeLiv


    Your Light Pollution Info

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Posted 16 April 2024 - 06:19 PM

So, if I am understanding this correctly, the lower, warmer temperatures are better for those of us in the astronomy community? If so, is the benefit for visual as well as photographic?

This depends on where you observe or image from.


If you're located within a few feet to a mile or two from the LP source, then it's true that in exceptionally dry & clear conditions, per lumen of visible as well as UV & IR light, bluer sources scatter more than redder. It's important to understand that the shorter the wavelengths, such as from UV & violet rich sources (like mercury & metal-halide lights) the greater is the scattering compared to any redder sources.


For anyone in a darker suburban or rural location, this is NOT necessarily true! Far from the city LP, meaning at least 30 miles or 50 kilometers away it's the opposite (assuming no other additional outdoor lights). That's because the shorter blue wavelengths get scattered out in the air immediately near the sources. Redder sources, like from HPS or LPS, will penetrate farther.


For a dark-sky astronomer, his/her biggest urban pollution in the recent past, was the redder light from HPS. I definitely DO NOT wish for LED to achieve the color temperature of HPS, nor do I believe any narrow-band amber LED will be better. For anyone far from a city, the red sources will penetrate deeper in a clear & dry atmosphere.


My opinion is that a fairly good balance for LED should be at around 3000K to 3500K, where the phosphor peak (at ~555 nanometers) is still slightly higher than that for the blue diode (at ~450 nm). This scenario keeps the scattering from the diode (~450 nm) definitely less than what was possible from mercury or metal-halide lighting. See here lower down the real jpgs of my clear sky-glow. Scattering from blue rich sources is nothing new. It's a new issue due to recent studies about the disease potential of blue light at inappropriate times.

Edited by GeorgeLiv, 16 April 2024 - 10:14 PM.

#27 earlyriser



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Posted 19 April 2024 - 04:38 AM

I’m actually happy to see that the city reconsidered its 4000K deployment. However, they are using this as an opportunity to increase the level of lighting in almost every situation. I’ve measured up to 130 Lux on South Street with the LEDs.

The good news is, the fixtures will be dimmable, so communities can request changes if they find the lights too bright.

I believe 40 lux is the standard for brightly lit streets. Swapping 4000k for 3000k with three times the illumination seems like a bad trade-off that will greatly increase the streetlights’ contribution to sky glow and wipe out any energy savings from switching to LED. 

Edited by earlyriser, 19 April 2024 - 04:49 AM.

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