Is wikipedia wrong?

Either my math, Wikipedia's laser class designation, or every label on every laser pointer is wrong.

Class IIIa

Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter or power density. Output power may not exceed 1–5 mW. Beam power density may not exceed 2.5 mW/square cm. Many laser sights for firearms and laser pointers are in this category.

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2.5mW/cm[sup]2[/sup]? If a 4mW red pointer has a diameter of 5mm:

Diameter = 5mm
D = .5cm

Area = (D/2)[sup]2[/sup]pi
A = (.5cm/2)[sup]2[/sup]pi
A = 0.0625pi cm[sup]2[/sup]
A ~ .2cm[sup]2[/sup]

Power Density = 4mW/.2cm[sup]2[/sup]
D[sub]p[/sub] = 20mW per cm[sup]2[/sup]

A 4mW green With a 1mm beam diameter yields a substantially higher result at 510mW per square cm - clearly above the 2.5mW limit set by the class. So what gives? What did I miss?

Wikipedia is not the best source of information, it is open to change from anyone. At Uni we are not allowed to use it. We have edited it before and put in false information ( a few choice swear words in our second language ;D) so don't believe everything you read

Google didn't really turn up any results including power density with the class, and Sam's Laser FAQ makes some reference to it but isn't sure of itself.

Another fact I found while looking: The sun has a power density of 100mW/cm[sup]2[/sup]. So you can tell your friends your green pointer is 5 times brighter than the sun... kinda.

Check 21 CFR 1040.10 and see if it mentions that. I bet it doesn't. Wikipedia is trying to paraphrase that 21 CFR 1040.10, but someone probably misinterpreted it, or just pulled that info out of their... hat.

it could mean that if the entire beam was expanded to 1cm(sq) that the power density would be less than 2.5mW

Just in regards to wikipedia relying upon the info that people provide - here is an awesome article that proves just that. It is about how Steven Colbert gets banned from wikipedia because of stuff that he did... it's pretty funny.

http://spring.newsvine.com/_news/20...ses-chaos-on-wikipedia-gets-blocked-from-site

Colbert praised Wikipedia for "wikiality," the reality that exists if you make something up and enough people agree with you - it becomes reality.

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Colbert goes on to declare that he doesn't believe George Washington had slaves.

"If I want to say he didn't that's my right, and now, thanks to Wikipedia *taps keyboard* it's also a fact."

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enjoy

A 4mW green With a 1mm beam diameter yields a substantially higher result at 510mW per square cm - clearly above the 2.5mW limit set by the class. So what gives? What did I miss?

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Your calculations are correct, but it's all about how dangerous something actually is to the eye.

Let's assume you are looking directly into a laser from a relatively large distances (10 meters or so). In that scenario, the laser aperture is a point source even if it is a few mm in diameter.

In such case, the actual diameter of the beam at the eye can be 0.5 or 3 mm, but that doesnt matter much in terms of safety. In both cases all the light wil pass throug the pupil and be focussed into a single area on the retina that will be (termally) damaged.

Only if the beam is larger than the pupil, a less collimated laser becomes less dangerous under these circumstances, simply because a portion of the light will not reach the retina.

Another fact I found while looking: The sun has a power density of 100mW/cm2.

Click to expand...

Which is a good reason not to stare directly into the sun... although the pupil will maximally contract to 1 mm2 or so - giving only 1 mW of real energy exposure.

This 1 mW or so is safe in terms of not termally damaging the retina right away, but it's not a good idea to prolonge the situation and look away or blink - exactly the reaction that makes 1 mW laser pointers fairly safe for others even if in the hands of an idiot.

MarioMaster said:

it could mean that if the entire beam was expanded to 1cm(sq) that the power density would be less than 2.5mW

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And then because of the wide beam diameter, it would fall into class IIM because less than 1mW would enter the eye because of the pupil diameter!

So, I'll take my (hypothetical) He-N with 2mW, fire it through a focusing lens, where the beam gets continuously smaller until it hits focal point at which point it continues to expand until it hits the edges of the universe.
Thus hypothetically, there is a class II beam, that, somewhere around the focal spot, turns into a class III beam, and same distance after focal, turns into a '"mild" class II again.

Am I right about this?!? Cool, then in some jurisdictions, a lens would make a laser illegal - but only partially!

Cyparagon said:

Either my math, Wikipedia's laser class designation, or every label on every laser pointer is wrong.

Class IIIa

Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter or power density. Output power may not exceed 1–5 mW. Beam power density may not exceed 2.5 mW/square cm. Many laser sights for firearms and laser pointers are in this category.

Click to expand...

2.5mW/cm[sup]2[/sup]? If a 4mW red pointer has a diameter of 5mm:

Diameter = 5mm
D = .5cm

Area = (D/2)[sup]2[/sup]pi
A = (.5cm/2)[sup]2[/sup]pi
A = 0.0625pi cm[sup]2[/sup]
A ~ .2cm[sup]2[/sup]

Power Density = 4mW/.2cm[sup]2[/sup]
D[sub]p[/sub] = 20mW per cm[sup]2[/sup]

A 4mW green With a 1mm beam diameter yields a substantially higher result at 510mW per square cm - clearly above the 2.5mW limit set by the class. So what gives? What did I miss?

Click to expand...

I don't know if the math is correct, but yes a class IIIa can be dangerous if optics are used to focus the beam. This free download program should be useful
http://www.laservisualsresearch.com/sglite.php

Cyparagon said:

Another fact I found while looking: The sun has a power density of 100mW/cm[sup]2[/sup]. So you can tell your friends your green pointer is 5 times brighter than the sun... kinda.

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You mean 5 times more powerfull kinda, not 5 times brighter kinda...

I don't know if the math is correct, but yes a class IIIa can be dangerous if optics are used to focus the beam. This free download program should be useful
http://www.laservisualsresearch.com/sglite.php

Click to expand...

One of the biggest problems with audience scanning (in regards to laser projectors - even though it is almost 100% illegal in the US) is you have to factor in Eyewear. Like glasses for example can help focus the light even better. I've gotten plenty of 5mw beams into my eyes.... I regret it. I've gotten 50mw direct, which was very discomforting.

Another issue is in regards to pupil dilation, (we just had this discussion on PL) some people might be under the influence of drugs or alcohol (of course we would hope our pilots aren't drunk or stoned) but that increases the reaction time it takes to blink which can increase the dangers associated and can make a class IIM dangerous to the eye. Even though under normal circumstances it is regarded as being safe.

That said you cannot take a class IIM laser and point it into your eye and claim to your friends that you will be fine because its "not dangerous" those are guidelines which take into account your natural blink reflex.

steve001 said:

don't know if the math is correct, but yes a class IIIa can be dangerous if optics are used to focus the beam.

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Now there's another point I don't understand. Like Benm said, the power density doesn't really matter in terms of safety because the beam is focused by the eye. Focusing of the beam on the retina occurs regardless of what optics you view the beam with.

The eye will focus it assuming you're focusing at the right distance from the start.

Steve, you often seem to assume two things I take issue with:
1. You can focus light down to an infinitesimal area.
2. Two sources have the same burning power, regardless of initial power, as long as they achieve the same irradiance.

By that logic, any non-zero quantity of light can cause damage to anything. I disagree with this :-/