# Which colour light travels furthest?



## CTR (Aug 3, 2004)

Hi All ,

I'm wondering if anyone can tell me which colour light travels furthest and which doesn't travel as far. The colours I mainly want to know about are blue, yellow and white light.


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## PhotonBoy (Aug 3, 2004)

A lot of people swear by cyan, probably for two reasons:

1) monochrome LEDs (as opposed to white) are very efficient
2) the human eye is quite sensitive to this color


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## brickbat (Aug 3, 2004)

I put a white flashlight in my car a few years and its traveled over 25,000 miles...


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## Clint (Aug 4, 2004)

[ QUOTE ]
*brickbat said:*
I put a white flashlight in my car a few years and its traveled over 25,000 miles... 

[/ QUOTE ] /ubbthreads/images/graemlins/crackup.gif /ubbthreads/images/graemlins/crackup.gif /ubbthreads/images/graemlins/crackup.gif


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## brickbat (Aug 4, 2004)

CTR - Sorry about the smart-*** response. I see this was your first post. Welcome to CPF. You probably need to hone your question a bit - as you probably know, light travels forever, regardless of its color, unless it gets absorbed by something. So, maybe you're wondering which color is the best to see around in a dark environment? 

That sort of query has been discussed numerous times with a variety of opinions being expressed. Generally, white is the most practical color, but there are probably times when a colored light is advantageous... A lot of it has to do with what things you're trying to illuminate - If you know you're going to be using it in a wooded area during the summer (lots of green), green or cyan might work pretty well. Red would be a poor choice in this case. 

Some believe that nigh vision is best preserved by a particular color - again, opinions differ. 

White light has one huge advantage that just can't be overlooked though - it allows you to see colors accurately.


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## MacGyverS2000 (Aug 9, 2004)

Our eyes tend to be more sensitive to green during the day, and blue during the night.


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## PhotonWrangler (Aug 9, 2004)

[ QUOTE ]
*brickbat said:*
I put a white flashlight in my car a few years and its traveled over 25,000 miles... 

[/ QUOTE ]

Was it turned on at the time?


If you're travelling in a vehicle at the speed of light,
and you turn on your lights,
would they do anything?


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## eebowler (Aug 9, 2004)

Uh, the light will travel at the speed of light away from the car? That would make the light energy either stationary,(if it was travelling towards a non-moving observer) or traveling at twice the speed of light away from the non-moving observer??

Anyway, WELCOME to the CPF CTR. it is alwayse good to have another future flashaholic added to our list... Enjoy yourself here.

With respect to your question, as brickbat said, it really depends on what you are trying to see at a distance. Generally, white is the best choice since it will show all colours equally. 
Out of the colours you mentioned, I would think that yellow would be the worst colour for distance viewing.(no experience to back up my hypothesis)


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## brickbat (Aug 9, 2004)

The theory of special relativity says, among other things, that the speed of light is constant. Thus, the light emitted frm a moving headlight does not travel at the speed of light plus the vehicle's speed - it still travels at the good ol' regular speed of light - 299,792,458 m/s. Would the headlights do anything? Probably do something, but I don't think they'd illuminate objects in front of the vehicle.


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## PhotonWrangler (Aug 9, 2004)

[ QUOTE ]
*eebowler said:*
Uh, the light will travel at the speed of light away from the car? That would make the light energy either stationary,(if it was travelling towards a non-moving observer) or traveling at twice the speed of light away from the non-moving observer??


[/ QUOTE ]

Sorry, feeling a little punchy tonight and that drove my posting. But seriously, because the speed is relative to the observer, the lights would work as viewed from the perspective of the driver/passengers.

As far as visible throw at night, I'd vote for cyan. It won't preserve night vision very well, but in terms of a monochromatic source, it would probably appearto be the brightest given the same output power as other colors.


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## snakebite (Aug 11, 2004)

whichever is least attenuated/diffused by the dirt/water vapor in the air
[ QUOTE ]
*CTR said:*
Hi All ,

I'm wondering if anyone can tell me which colour light travels furthest and which doesn't travel as far. The colours I mainly want to know about are blue, yellow and white light. 

[/ QUOTE ]


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## HarryN (Aug 11, 2004)

OK, I will chime in with a thought on this as well.

There are some wavelengths which happen to cross high light sensitivity to the eye, as well as good efficiency in making the LED in the first place (somewhat related as well.)

In the red / orange area, near the 615 - 620 range, you will find LEDs to have very high Lumen output for a given power level (lets use 1 watt as an example) (see Lumiled's Luxeon data sheets as an example).

There is another similar point near 500nm (often called cyan). Interestingly, you need a 3 watt cyan to have approximate equivalent Lumen output to a 1 watt orange / red. This is very close to the traffic light "green" (505nm), so these are widely available.

If you happen to be beaming your light through ocean water, you will find 480 - 490 nm has the best penetration, and red is rapidly attenuated. This is a combination of effects that includes attenuation and light scattering.

There are some Lumen efficiency losses due to the use of phosphors to make the white, so it may not reach the same Lumens / watt as the others - however - I did some very simple visual testing on how long it took me to recognize various objects with red / orange, cyan, and white. As you might guess, I was able to recognize objects illuminated with white light substantially faster with similar lumen levels. (others have done this much more scientifically in the literature)

Interestingly, if you mix the red / orange light approx. 50 / 50 with the 480 - 490 nm light, you can create a false color white which lies in the "white light area" of the color chart. It is an unusual white color not normally seen - almost 10,000 K equivalent Plankian. I tried this, and it works, but it is hard to obtain LEDs binned in this area.

So in summary, - use white for your vision, and red / orange to be seen. (my humble opinion)


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## ViReN (Aug 28, 2004)

Speed of Light Does not Change (as some say)... & has been proved...

If You are traveling at a speed of Light(in your car with Flashlight ON) (if at all possible practically).. the light will still travel in a forward direction... with same SPEED !... 

So... they say.. Time is the thing.. that is actually changing... (scairy stuff /ubbthreads/images/graemlins/tongue.gif ) and If its time that is changing... you are Traveling Through Time !!!...

Its Not a Joke, Its a Fact & Proven (to some extent)

Thanks & Regards,
ViReN


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