Long-distance visibility of single-color LEDs.

finn

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If my math and understanding of rayleigh scattering is correct, royal blue @ 440nm should scatter 4x as quickly as red @ 620nm and hence be much less visible at long distances. Has anyone noticed this effect in long-distance visibility of light sources?

I can't find any data out there at all -- I may have to set up some emitters in the wilderness and try 1km visibility tests.
 

Bazar

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light houses are interesting to study. some intentionally use scattering, others are more focused and have a reddish frequency as you say. by the way, it is 10 mile tests that you should use. rayleigh scattering is almost unaffected at 1 single kilometer, unless you are at sea level and the moister is high.
 

finn

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I know it is possible to see both blue and red components of emergency vehicle lights from many km away. I figure there's a chance rayleigh losses are small enough to be minuscule vs inverse square losses at tens of km, which would practically invalidate my theory.
 

Curious_character

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If red light traveled farther than other colors (wavelengths), wouldn't white light appear redder and redder as you got farther and farther from the source?

c_c
 

finn

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If red light traveled farther than other colors (wavelengths), wouldn't white light appear redder and redder as you got farther and farther from the source?

c_c

That does happen. It happens with the sun over several hundred km of atmospheric transmission -- hence tint shift of sunrise/sunset. Whether it would make a difference at 10km with 10 lumens is what I am wondering, and it feels like it may be a stretch.
 

Ken_McE

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Another consideration. The human eye is not equally sensitive to all wavelengths. Green is what we see best.
 

JoakimFlorence

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If my math and understanding of rayleigh scattering is correct, royal blue @ 440nm should scatter 4x as quickly as red @ 620nm and hence be much less visible at long distances. Has anyone noticed this effect in long-distance visibility of light sources?

I can't find any data out there at all -- I may have to set up some emitters in the wilderness and try 1km visibility tests.
Not completely sure about this but I think I've seen figures that estimated losses from atmospheric scattering to be roughly about 5 per cent per kilometer, in clear skies. The scattering is not completely proportional to the inverse fourth power because part of the equation is aerosol scattering, in addition to Rayleigh scattering. (particles larger than a few micrometers, like fog or cloud, have no wavelength dependence)
 

vadimax

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May sound a bit off topic, but why are orange/yellow filters or lights being used to penetrate fog/smoke? May this work from the other end as well?
 
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Genes

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Not a scientific test, but I can spot blue lights at night much further and brighter than red. While driving on straight stretches of Interstate roads at night, I can spot emergency "blue" lights long before I can ever pick out the red lights which may adjacent to the blue lights on the same vehicle. Also, there are a lots of cars driving around with blue lighting showing to the rear of the vehicle. While technically, blue lights showing to the rear are against the law in most states, it is very common to see cars with blue illuminated license plates. I can see vehicles with these blue license plate lights many miles ahead. My eyes seem to strongly favor the blue spectrum at night.
 
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May sound a bit off topic, but why are orange/yellow filters or lights being used to penetrate fog/smoke? May this work from the other end as well?

They don't penetrate any better, your eyes are just not as glare sensitive to orange/yellow, and there is less internal scattering in the eye at these colors or with a reduced bandwidth with your pupils fully dilated.
 
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Not a scientific test, but I can spot blue lights at night much further and brighter than red. While driving on straight stretches of Interstate roads at night, I can spot emergency "blue" lights long before I can ever pick out the red lights which may adjacent to the blue lights on the same vehicle. Also, there are a lots of cars driving around with blue lighting showing to the rear of the vehicle. While technically, blue lights showing to the rear are against the law in most states, it is very common to see cars with blue illuminated license plates. I can see vehicles with these blue license plate lights many miles ahead. My eyes seem to strongly favor the blue spectrum at night.


Your night vision (scotopic) is much more sensitive to blue light than red light, the opposite of your day vision (photopic). Blue also creates a stronger glare response which increases awareness. It is a rarer color "in the wild" as well, so we appear to detect it quicker as it is anomalous.
 
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