Why are some streetlights red?

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Offline thedoc

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Why are some streetlights red?
« on: 16/10/2014 17:30:01 »
Muhammad Nouman asked the Naked Scientists:
Some streetlights are deliberately manufactured to provide illumination with a reddish color. Based on the EMR and spectral reflectance relationship, can you suggest why?

What do you think?
« Last Edit: 16/10/2014 17:30:01 by _system »


Offline evan_au

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Re: Why are some streetlights red?
« Reply #1 on: 19/10/2014 05:00:58 »
I have seen sodium vapor streetlamps, which have two closely-spaced orange-yellow spectral lines. When these lamps first turn on, they have a reddish glow, but it soon turns orange.

Because the energy is concentrated in the visible band, these lights are fairly efficient. They are used in the town of Coonabarabaran, near Australia's Siding Springs observatory, because it is easier for astronomers to ignore two narrow spectral lines (with zero red-shift) than it is to eliminate the effects of broadband light, such as from an incandescent or fluorescent lamp.

I am not familiar with any intentionally red street lighting. In fact this could be a bit dangerous, as it might make it slightly harder to spot red traffic lights or red brake lights.