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# How the heck does it work?

#### Mrnewbie

##### New member
We all know that Lasers are dangerous gadgets. So I have a hypothetical question on how it all works.

If we all go back to when we were kids with a magnifying glass, we know the light beem has to be focused pinpoint beam to burn that ant!

We know a 100mw laser when pinpoint focused is more than capable of burning black tape or to cause injure the human eye.

If one had a 100mw laser..but focused to a 120inch beam at 100ft, would it be dangerous to look at from 100ft?

My way of thinking is something like this. Not taking beam signal distance losses into account

100mw/120in=.833mw per inch

I'm thinking the human eyes ..about 1 in wide would only be exposed to .833mw...each eye

Not a bright thing to do, but I would think it would not be harmful

There has to got be a math formula for this..

So, How does it work?

#### Wolfman29

##### Well-known member
You've got that exactly right. But just because it's below 5mW does not mean it's not dangerous. It's just 5mW is the threshold for damage in under 0.25 seconds. I would bet that damage could occur even at <1mW.

#### Hap

##### Super Moderator
Staff member
You've got that exactly right. But just because it's below 5mW does not mean it's not dangerous. It's just 5mW is the threshold for damage in under 0.25 seconds. I would bet that damage could occur even at <1mW.
Of course damage could occur at <1mW, but even when focused, that is such low power that it would still take awhile for any eye damage!

Hint: Just never point any laser at you're eyes/anyones eyes, and you will be alright.

#### cantthinkofone

##### New member
Look at laser tag.... .95 mW of 532 in the hands of 7 year olds. And WE'RE the ones the FDA worries about...

#### Mrnewbie

##### New member
Ok, well we figure it not a good thing, but you won't be joining the land of the blind.
.83mw exposure...and all exposure to be avoided.

Next question, same senario 100mw,120" beam, 100ft.

This time its you..your pointing twards a wall 100ft away, throwing a pretty 120" RED or Blue laser at it.
So the laser is traveling 100ft to wall, then bounce back 100ft to your eyes. I would think the reflectivity of the wall would play a major part in this one..lets call it 50%.

Your not wearing any goggles.. your not looking directly at the laser, your looking at the bounce back from the wall.
How would this one work out? 100mn/120in=.83 .83 x50% = .415mw

I suspect no problems at all, but just kinda currious

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#### Hap

##### Super Moderator
Staff member
Ok, well we figure it not a good thing, but you won't be joining the land of the blind.
.83mw exposure...and all exposure to be avoided.

Next question, same senario 100mw,120" beam, 100ft.

This time its you..your pointing twards a wall 100ft away, throwing a pretty 120" RED or Blue laser at it.
So the laser is traveling 100ft to wall, then bounce back 100ft to your eyes.

Your not wearing any goggles.. your not looking directly at the laser, your looking at the bounce back from the wall.
How would this one work out?

I suspect no problems at all, but just kinda currious
Hey Mr,

I can say from experience a 120mW red will not cause any harm to you're eyes at all a wall 100ft away. Hec, my 500mW green is easy to look at 50ft away.

Also when light hits a wall it does not reflect, concrete being a non reflective surface will instead absorb the light and spread it out.

If this was GLASS instead of a wall at 100ft away you would be screwed :beer:

So if you are worried about 100ft away, then no. No damage possible.

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#### Mrnewbie

##### New member
I was more concerned about understanding the math end of things.
The theory and reasoning behind it.

#### Hap

##### Super Moderator
Staff member
I was more concerned about understanding the math end of things.
The theory and reasoning behind it.
Ahhh, then sorry man. Can't be of very much help

:shhh: Math isn't my best subject!

#### Mrnewbie

##### New member
(If one had a 100mw laser..but focused to a 120inch beam at 100ft, would it be dangerous to look at from 100ft?

100mw/120in=.833mw per inch

I'm thinking the human eyes ..about 1 in wide would only be exposed to .833mw...each eye)

You know guys, I think the above thinking is flawed.

Some of you fellas have tools to measure laser power..not sure how it works.
But here is what I was thinking.

When you measure your laser output..are you not measuring it in a focused beam. Lets just call it 1/16th of an inch beam.

So if you were to take a 100mw laser ..stretch it out to 1 inch..it would it not be measured as a 100mw laser it anymore. It would have 1/16 of the output power of 100mw. Or 6.25mw because the original beam is already enlarged by a factor of 16.
I think a more correct version of the posted hypeteticl formula should be.

(100mw* .0625(1/16th))/120in = output
100x.0625= 6.25mw 6.25mw/120in= .0520mw...

Just hinking aloud....

#### Wolfman29

##### Well-known member
Hey Mr,

I can say from experience a 120mW red will not cause any harm to you're eyes at all a wall 100ft away. Hec, my 500mW green is easy to look at 50ft away.

Also when light hits a wall it does not reflect, concrete being a non reflective surface will instead absorb the light and spread it out.

If this was GLASS instead of a wall at 100ft away you would be screwed :beer:

So if you are worried about 100ft away, then no. No damage possible.
He was talking about a direct hit at 100ft, if I understand correctly.

#### SKeeZ

##### New member
Ok, well we figure it not a good thing, but you won't be joining the land of the blind.
.83mw exposure...and all exposure to be avoided.

Next question, same senario 100mw,120" beam, 100ft.

This time its you..your pointing twards a wall 100ft away, throwing a pretty 120" RED or Blue laser at it.
So the laser is traveling 100ft to wall, then bounce back 100ft to your eyes. I would think the reflectivity of the wall would play a major part in this one..lets call it 50%.

Your not wearing any goggles.. your not looking directly at the laser, your looking at the bounce back from the wall.
How would this one work out? 100mn/120in=.83 .83 x50% = .415mw

I suspect no problems at all, but just kinda currious
it would be even less, .20525mW because the beam expands further while on its way back, to 240in. thus halving the power again.