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How quickly do laser beams lose their power as they travel?

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When looking into safety issues and safety goggles, I read that a laser beam that hits you directly in the eye from close range might damage your eye, depending on its power, but that the same laser at a longer distance might not do damage even if it hits you square in the eye.

Thats pretty obvious, as everything loses power as it travels farther away from it's source, but how quickly or at what rate do lasers actually lose their energy?

I would imagine it would depend on how clear the room or air is, and maybe the light color frequency plays a role(?) Do different colored lasers lose energy at different rates? I'd imagine so........

If you have a 532nm green laser of 200mW, and you point it across a field 1/2 mile, on a clear night(or day), with no fog, smoke, particulates, is there some formula or equation that determines the loss of energy per meter, assuming the beam stays relatively concentrated, and doesnt spread out too much? How would that laser do compared to 200mW lasers of 405, 450, 635, 650 as far as maintaining its energy over distance?

If you accidentally hit a person in the eye, who isnt wearing safety goggles with a quality 400mW 650nm laser at 300 feet or 100 meters, would it still likely do damage if the person doesnt blink or move for a few seconds?
 
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John Capelo

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I would think it depends more on the divergence of the beam, since divergence is a measure of how much the beam spreads out over a certain distance. Larger divergence means larger spread, which means less power focused in one spot on the person's retinas.
 
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Wolfman29

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John is right - lasers intrinsically lose very little power on the way to a target. The only power lost is that which you can see in the beam (VERY minute). However, the power is spread over a larger area due to the laser's divergence, making it less dangerous after a certain point.
 
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I would think it depends more on the divergence of the beam, since divergence is a measure of how much the beam spreads out over a certain distance. Larger divergence means larger spread, which means less power focused in one spot on the person's retinas.


I forgot to factor in divergence, as I'm new at this.

So I altered the OP a bit, so it now allows for a laser with a beam that stays together well, without too much spread over distance.
 
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Wolfman29

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In the vacuum of space, if you had a laser with 0 divergence (impossible, but for this thought experiment's sake), it would travel infinitely without losing any power (until it struck something of course). There is no "friction" to light. Either it hits something or it doesn't. If there is nothing in the way (air molecules count as stuff), then it will just keep on going.
 
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In the vacuum of space, if you had a laser with 0 divergence (impossible, but for this thought experiment's sake), it would travel infinitely without losing any power (until it struck something of course). There is no "friction" to light. Either it hits something or it doesn't. If there is nothing in the way (air molecules count as stuff), then it will just keep on going.

Yeah, I realize that a laser beam would travel forever in space, but the OP was dealing with Earthbound lasers, as I asked the question with clean, smoke or fog free air to be factored in.
 

Wolfman29

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Point is, the amount of energy that a laser loses is nearly negligible - otherwise you would always see the beam ;)
 

hakzaw1

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Fog-make-haze does not lessen the lasers power. It only effects how easy it is to see them beam.
Due to the fact that we are never completely still for very long--and most time the laser is also not always perfectly still exposure is very limited.
Once we see any bright light-or sun we close or eyes or look away or block the light. A conservative guesstimate is less than .25 seconds. So we know the exposure time-
The lens in our eyes makes any power laser--regardless of wavelength or beam size - much more dangerous.
A good rule is NEVER lase horizontally above the knees or less than 10 feet high.

doing that pretty much eliminates our eye's lens making things 1000 times worse.

hak

ps

If you had used the best search terms you prolly could have answered your own question..
For more info than you want see this thread.

 
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RedCowboy

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Fog eats up the beams power leaving less at a given target distance, I have seen this 1st hand over just a few hundred feet during a foggy night, walking to the target during heavy fog there was only a dim spot, but walking to the same target on a clear night the same laser shown from the same position made a bright spot at the same target.

Some wavelengths of electromagnetic spectrum travel through our air better than others, see chart >

See the source image





There are also charts for eye safety based on a lasers power and divergence over distance, just do a search but always use extra caution when lasing and never shine a laser at anyone or any vehicle/aircraft.

See the source image
 
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hakzaw1

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yeah-- I did not get that completely right.. distance controls divergence. Not fog etc is what I meant.
Thanks RC..
 
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RedCowboy

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Some wavelengths traverse fog better than others such as IR but visible lasers I have noticed get consumed as they make a much brighter beam when shown through fog and it figures as the light we see as the beam is energy and has to come from somewhere, I understand Rayleigh scattering but in heavy fog the beam can light up the immediate area brightly and what's left at the target is a dim spot when the fog is heavy and the beam is bright, I have seen this 1st hand many times, even in a heavy man made fog inside a room the 1st few 10's of feet will be very bright and casts light upon all surfaces in the room around the beam through heavy fog and then the beam diminishes quickly.

However as far as crowd scanning you can not count on fog making a beam safe, that's not what I am trying to say at all, just that fog will bleed off the strength of a beam on it's way to a target.
 
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T_Warne

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You guys do realize the last post in this thread was from April of 2012, right?
 

RedCowboy

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No, I did not notice that, strange...........

Someone may have responded then deleted their post because I thought I saw it at the top of the recent list I'm pretty sure before hak responded.
 

hakzaw1

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OMG
MY BAD..
i MISSED THAT... having all these old titles showing is inviting necros.

so that is another minus for the new forum..the list grows!!

sorry

____ OR maybe there was another post--add another neg for this new forum-- no way to know a post has been deleted... either way I should know to read back SEVERAL posts.. NOT just the last one.
 
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