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09-06-2009, 04:11 PM #1
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Trevor
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Laser Dot Danger

I remember reading here that you shouldn't look at a dot from a laser more powerful than 30mW. I was interested in seeing why this is, so I did some conservative math, and it didn't seem to work out. Here's my thought process:

Say we shine a laser pointer of 200mW at a perfect 90° angle at some sort of (flat) miracle matte surface that disperses the light evenly in all directions. The area of reflection would be a hemisphere whose radius is the observer's distance from the dot. The formula would be (4πr²)/2, or just 2πr².

Say we are observing at night from a distance of 2m. The area of the surface of the hemisphere whose surface touches the observer's eye(s) would be 25.132m². To make this number more manageable, let's turn it into cm². This yields 251,327cm². The intensity of the dispersed laser light would be 200/251327 mW/cm², or 0.000795mW/cm².

Even if the retina is as large as 1cm², the amount of light reaching the retina seems like it would be much less. It would take an error of nearly 1,000,000% before the power goes above the generally accepted "safe"(ish) 5mW.

My question to you is - what have I missed? Is a whole lot more light reflected back to the person holding the pointer, since no surface is perfect like the one I described? Have I missed an important physics concept in my crude "back-of-the-envelope" calculations? Is my error approaching 1,000,000%?

I figured I'd ask, since an entire forum of laser enthusiasts is very rarely wrong...

Trevor

Last edited by Trevor; 09-06-2009 at 04:15 PM.

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09-06-2009, 04:24 PM #2
 Class 1M Laser Join Date: Jul 2009 Location: Pacoima, California US Posts: 112 Rep Power: 9
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Re: Laser Dot Danger

5mW is not safe to look directly into the beam. It is only safe because you blink before it does any damage. If you purposely look into 5mW you will get eye damage.
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09-06-2009, 04:29 PM #3
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Re: Laser Dot Danger

Quote:
 Originally Posted by Invader Zim 5mW is not safe to look directly into the beam. It is only safe because you blink before it does any damage. If you purposely look into 5mW you will get eye damage.
Sorry, I wasn't clear. I do realize that 5mW is not "safe" (hence the -ish). I only used that example to illustrate the huge percent error needed to reach that threshold.

Trevor

09-06-2009, 04:43 PM #4
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Re: Laser Dot Danger

What you need to remember is that a laser beam does not diverge very much...
If you are looking at let's say a 100mW laser beam's dot on a Black Flat surface...
there will be a very low powered reflected beam... and would be much safer than
looking at the same dot on a White or even highly reflective surface..

The amount of reflection would only be measurable knowing the exact reflectivity
of the surface in question...
If the surface is a Front Surface Mirror.... the amount of the Laser's reflected beam
would come near 100% ...

That would be considered dangerous...

To avoid making a mistake or inadvertently shinning a Laser's beam onto a reflective
surface... Laser Saftey Goggles are a must....

BTW.... to the Forum...

Jerry
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Last edited by lasersbee; 09-07-2009 at 12:18 PM.

09-06-2009, 04:45 PM #5
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Re: Laser Dot Danger

Then, I really don't know either.
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09-06-2009, 04:56 PM #6
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Trevor
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Re: Laser Dot Danger

Quote:
 Originally Posted by lasersbee What you need to remember is that a laser beam does not diverge very much... If you are looking at let's say a 100mW laser beam's dot on a Black Flat surface... there will be a very low powered reflected beam... and would be much safer than looking at the same dot on a White or even highly reflective surface.. The amount of reflection would only be measurable knowing the exact reflectivity of the surface in question... If the surface is a Front Surface Mirror.... the amont of the Laser's reflected beam would come near 100% ... That would be considered dangerous... To avoid making a mistake or inadvertently shinning a Laser's beam onto a reflective surface... Laser Safey Goggles are a must.... BTW.... to the Forum... Jerry
Ahh, I see. So it's not the actual dispersed light that poses the danger, it's the possibility of hitting a slightly reflective or, worse, a very reflective surface and a low(er)-power beam intersecting with a retina. Makes more sense than my numbers.

Thanks for the clarification and the welcome.

Trevor

09-07-2009, 05:27 PM #7
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Re: Laser Dot Danger

You didn't miss much i think.. didnt check your calculation, but the outcome seems reasonable in value. There are some considerations:

- viewing distance 2 meters. If you view it from 20 centimeters distance instead, exposure rises 100-fold

- perfect matte re-distributing the light to a half-sphere evenly. There are few if any surfaces that do that. If you pointed the laser at it at a normal angle, even on a very matte surface the reflection would be more of a gaussian curve with the peak being straight back to the laser. This could easily be a 10-fold increase over your outcome as well.

But even with both considerations applied to the already worst-case calculation, its still under the generally considered safe levels. Looking at the dot on a -matte- surface is only dangerous from very close distances, or using very powerful lasers.

09-07-2009, 06:28 PM #8
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Trevor
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Re: Laser Dot Danger

Hmm, gaussian curve. Makes sense.

I have a paper to write for tomorrow, but my 9:30 class got canceled - so tonight I may see just how big the power difference is at different viewing angles. Bright white printer paper + wall + camera set to constant aperture and exposure + tripod + dot measurement = crude experiment. You piqued my curiosity. If I get any decent quantitative results, I'll post them.

Trevor

09-07-2009, 06:45 PM #9
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Re: Laser Dot Danger

You have to remember we are talking about the eye here, NOT just some "aperture."

Another consideration with laser safety is that the light (even when the amount is limited) is focused directly onto the retina by the cornea. Not only is the measured amout of light getting in, but it is very effectively focused where it will do the most damage to your vision.

5mW may not seem like much, but it is if it is focused on your retina.

Peace,
dave

Last edited by daguin; 09-07-2009 at 06:47 PM.

09-07-2009, 07:17 PM #10
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Trevor
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Re: Laser Dot Danger

Quote:
 Originally Posted by daguin You have to remember we are talking about the eye here, NOT just some "aperture." Another consideration with laser safety is that the light (even when the amount is limited) is focused directly onto the retina by the cornea. Not only is the measured amout of light getting in, but it is very effectively focused where it will do the most damage to your vision. 5mW may not seem like much, but it is if it is focused on your retina. Peace, dave
I do understand fundamentally why the dot itself is dangerous, thanks to lasersbee.

I'm interested in the different perceived strength at different viewing angles solely for the sake of academia.

Trevor

09-07-2009, 09:16 PM #11
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Re: Laser Dot Danger

One moment.

Then, looking at a 400mW green laser dot on a white wall isn't dangerous itself?

We must be missing something, since I've always been told looking at the sun is harmful (eclipses for instance) and I'm sure if you calculate the amount of light into the eye is so low.

I think it must have something to do with the fact that the light get's focused inside the eye and somehow the dot get's projected inside your eye, and so here's the danger I guess?

Yours,
Albert
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09-07-2009, 09:44 PM #12
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Re: Laser Dot Danger

Quote:
 Originally Posted by Hallucynogenyc One moment. Then, looking at a 400mW green laser dot on a white wall isn't dangerous itself? We must be missing something, since I've always been told looking at the sun is harmful (eclipses for instance) and I'm sure if you calculate the amount of light into the eye is so low. I think it must have something to do with the fact that the light get's focused inside the eye and somehow the dot get's projected inside your eye, and so here's the danger I guess? Yours, Albert
Well, looking into the dot from a 400mW laser would not be dangerous if it was projected onto this miracle matte surface that I made up for the sake of this post.

What I've gathered from the posts is that surfaces reflect most of the light back toward the source of the beam, and thus the person pointing it. There's also the possibility of a weak(er, but still dangerous) beam ending up in your eye.

Trevor

09-07-2009, 10:59 PM #13
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Re: Laser Dot Danger

Another very hazardous surface to be avoided other than mirrors and other shiny surfaces, are "retroreflectors". Even at a steep angle these reflect a significant portion of the beam back towards the source of light.
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09-07-2009, 11:16 PM #14
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Re: Laser Dot Danger

Quote:
 Originally Posted by Hallucynogenyc One moment. Then, looking at a 400mW green laser dot on a white wall isn't dangerous itself? We must be missing something, since I've always been told looking at the sun is harmful (eclipses for instance) and I'm sure if you calculate the amount of light into the eye is so low. I think it must have something to do with the fact that the light get's focused inside the eye and somehow the dot get's projected inside your eye, and so here's the danger I guess? Yours, Albert
Yes... Albert... IT IS DANGEROUS... (again... depending what distance you are from
that white wall)... without Laser Safety Goggles...

That's what most of the above posts state....

Jerry
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09-07-2009, 11:36 PM #15
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Re: Laser Dot Danger

The 5 mW safety limit is there because you will blink before any damage to the retina occurs - its a matter of both power and exposure time.

Looking directly into the sun might be more dangerous than most people think: The sun blasts up to 1300W/m2 onto the earths surface. In more conventient units, this is 130 mW/cm2, which could actually be above the safe limit depending on the size of the pupils. Normally the pupils would be fully contracted under such conditions, to about 3 mm diameter or 7 mm2, catching about 10 mW of energy (still twice what is considered safe).

Mind you, looking into the sun when its blasting at full power (i.e. directly overhead in clear skies) is -very- painful, and advised against. Normally people just tend to not do so, but in case of, for example, observing a solar eclipse, protective eyewear is recommended.

The 5 mW concept stems from people blinking in time. If you force yourself not to blink or look away, and just stand the pain, exposure to these power levels is potentially harmful. So do not stare into your 5 mW pointer, nor into the sun!

10-07-2009, 03:57 PM #16
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Re: Laser Dot Danger

Quote:
 Originally Posted by twhite828 I remember reading here that you shouldn't look at a dot from a laser more powerful than 30mW. I was interested in seeing why this is, so I did some conservative math, and it didn't seem to work out. Here's my thought process: Say we shine a laser pointer of 200mW at a perfect 90° angle at some sort of (flat) miracle matte surface that disperses the light evenly in all directions. The area of reflection would be a hemisphere whose radius is the observer's distance from the dot. The formula would be (4πr²)/2, or just 2πr². Say we are observing at night from a distance of 2m. The area of the surface of the hemisphere whose surface touches the observer's eye(s) would be 25.132m². To make this number more manageable, let's turn it into cm². This yields 251,327cm². The intensity of the dispersed laser light would be 200/251327 mW/cm², or 0.000795mW/cm². Even if the retina is as large as 1cm², the amount of light reaching the retina seems like it would be much less. It would take an error of nearly 1,000,000% before the power goes above the generally accepted "safe"(ish) 5mW. My question to you is - what have I missed? Is a whole lot more light reflected back to the person holding the pointer, since no surface is perfect like the one I described? Have I missed an important physics concept in my crude "back-of-the-envelope" calculations? Is my error approaching 1,000,000%? I figured I'd ask, since an entire forum of laser enthusiasts is very rarely wrong... Trevor
This is one of those issues that confuses people no end. My wife for example won't let our children view the beam spot of any of my lasers because she cannot understand the difference between looking at a beam spot and looking into a beam. Sigh.

The maths of course does not lie. Unless the beam spot reflection has a significant specular component, it will be safe for any reasonable power and from any reasonable viewing distance. Still, at very close range (say less than 30cm) the glare from a higher powered unit is uncomfortable. I wouldn't want to do burning or power checking without goggles. Just too damn bright. And there is always the possibliity of accidents.

As for in-beam viewing, it can be done safely but only by defocusing. I have looked into my 270mw 658nm beast, but the beam spot was over 2 metres across so that it was the equivalent of 1mrad at 2 kilometres (and I only looked at the beam edge). Very bright but not dangerous. Still, this is not one for the kids to try at home. It also makes you think what one of those things would be like if the beam accidentally strayed into your eye at close range - i.e. what I saw multiplied by a factor of about 80,000. Unimaginable.

David
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