# Eye injury question.



## btcomm (Aug 23, 2006)

I've read that article that states even prolonged exposure to a 5 mw laser won't do permanent damage. I remember somewhere maybe on here someone was saying that the army was using a 25 mw laser that supposedly doesn't cause permanent eye injury. What about 50 mw lasers? If they get pointed into your eye and you blink to disrupt it would it cause permanent eye injury for that tiny time it got in your eye? What would be about the mw that would cause permanent eye injury if it went into your eye and you blinked right away?


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## The_LED_Museum (Aug 23, 2006)

I think 50mW into an eye would cause irreversible damage faster than the blink or aversion reflex could kick in and protect it.

Remember though, I'm not an opthalmologist, and don't play one on TV or on the internet, so please take this answer with plenty of crystalline sodium chloride (grains of salt).


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## btcomm (Aug 23, 2006)

I understand, it's your opinion thanks for replying though. Anyone else know any more info on this topic?


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## PhotonWrangler (Aug 23, 2006)

It depends partly upon how much of that energy makes it to your retina. A 50mw laser pointer seen from a few miles away (WITHOUT binoculars or a telescope) won't do much harm because the energy is dispersed over a wide area. On the other hand, if you're exposed to a 5mw laser really close-up, there's gonna be some damage.


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## CLHC (Aug 23, 2006)

I don't know, but did accidentally look into a standard over the counter laser on display once. Just wanted to see it work but it did not turn on the first couple of times, then when I looked at it is when the laser turned on and "hit" me in the eye! ! !  That did not feel good! Don't know how much damage if ever, but I still have two seeing eyes, albeit severely myopic.


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## Pila_Power (Aug 23, 2006)

I think the eye-safe ones are I/R


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## btcomm (Aug 23, 2006)

Photowrangler.

According to this study it seems that there really isn't going to be damage from a 5mw. I doubt they did these tests with someone miles away.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11115266&dopt=Abstract

So if you can have prolonged exposure to 5mw one up close with no permanent damage then I wonder if a 50 mw will damage you permanently if you just see it for 1/4 of a second. It might and I wouldn't really want to test it on myself but I'm just currious.

Pila power, you mean the laser that gets shot is in IR or laser pointers with IR filters are the safe ones?


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## allthatwhichis (Aug 23, 2006)

Pila_Power said:


> I think the eye-safe ones are I/R


 
:huh2: 
IR lasers or any laser over a certain mW will damage your eye... Just because yau can't "see" the beam doesn't mean the energy fromt he laser is not entering your eye. 

I think the FDA set the <5mW power rating because at less than 5mW your eye's


The_LED_Museum said:


> blink or aversion reflex could kick in and protect it


... That's why


CHC said:


> standard over the counter laser


... poitners even exist... 



CHC said:


> Just wanted to see it work but it did not turn on the first couple of times, then when I looked at it is when the laser turned on and "hit" me in the eye! ! !


... :candle:


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## CLHC (Aug 23, 2006)

Yes and that was really stupid if me looking at the "barrel" end of it. . .


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## btcomm (Aug 23, 2006)

My belief is that the reason they don't make class 3a lasers higher then 5 mw is because they are being way too over protective. If you can have prolonged exposure to a 5mw laser and not suffer permanent eye damage then the mw rating that would cause permanent damage at less then a second must be much more then 5mw. Whether it's 30 mw or 40 mw or 50 mw or 45 mw is what I want to know. Also I've heard that even 25 mw lasers won't damage your eye permanently if you let your blink reflex take over. So the I would really like to know what mw rating does the true danger start.


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## allthatwhichis (Aug 23, 2006)

btcomm said:


> My belief is that the reason they don't make class 3a lasers higher then 5 mw is because they are being way too over protective. If you can have prolonged exposure to a 5mw laser and not suffer permanent eye damage then the mw rating that would cause permanent damage at less then a second must be much more then 5mw. Whether it's 30 mw or 40 mw or 50 mw or 45 mw is what I want to know. Also I've heard that even 25 mw lasers won't damage your eye permanently if you let your blink reflex take over. So the I would really like to know what mw rating does the true danger start.


 
Just like parents... or teachers... and the govt are and will be over protective... Most of these tend to be over protective for our own good. OR at least they tell themselves that which from their perspective it the same. Drive 55 and stay alive, don't run down the hall, with scissors or at all...(That was so stupid it rhymed) Don't smoke crack was one my mom liked to say... I think the FDA just wants to keep pointers that can and WILL be used as toys where they belong, out of the hands of people who will use them as toys. You know, the ones that use them for popping ballons, lighting matches and all.  I know, I know, there can't be THAT many of them out there, but the FDA want them to have to go to great lengths and pay great prices for laser MODULES that can do those things...  They're all freaks if you ask me...  

No offence, but why does it matter? The only way to truly find out what mWattage is the bottom threshold that will damage your eye before your blink reflex can protect it is to try it out... and... good luck with that. All I know is when you hear a small pop, you've crossed it.


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## btcomm (Aug 23, 2006)

LOL well perhaps you're right. I thought maybe there was some scientific way for them to tell at what mw does the laser actually damage the eye without doing it to someone. It seems no one in this thread knows so perhaps there is no way. If anyone does know the answer I'd like to know. What does it matter? Well I would really like to have a laser that is powerful enough to see the beam clearly knowing that if something were to happen there would be no real danger of permanent eye injury. I already figure 25 mw would be pretty safe I just want to know how many more mw could be added onto that.


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## PhotonWrangler (Aug 23, 2006)

Laser eye damage thresholds for different wavelengths and powers (pdf).
Not the easiest chart to interpret but it shows the relationship between wavelength and power in terms of MPE (Maximum Permissible Exposure).


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## btcomm (Aug 23, 2006)

PhotonWrangler said:


> Laser eye damage thresholds for different wavelengths and powers (pdf).
> Not the easiest chart to interpret but it shows the relationship between wavelength and power in terms of MPE (Maximum Permissible Exposure).


 
Hmm, well yeah it's definetly not the easiest chart.  So lets say, if we were going to try and find out with this chart how long it would take for permanent eye damage with a green laser at 50 mw? Would it be possible to determine that with this chart? Second if it is, do you know how to figure it out? If you can figure it out what is the answer?


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## nero_design (Aug 24, 2006)

When you look at a lightbulb, it's image is reversed on your retina. But laserlight is not like the lightbulb. It get's focused by the eye's lens into a hot spot on the retina. And it get's magnified in strength. A green laser is focussed into a pinpoint of heat on the retina that is 100,000 times the strength of the laser. The area of your retina that processes images for your brain is actually really tiny... which is why you can only view a few letters on this message at a time. One hit by a 100mW laser and you will lose the use of that eye forever. The US army describes a 100mW laser as being capable of severe damage at a distance at only 10% of it's strength.

Sigh! Here we go again... ALL Class 3B lasers produce potentially dangerous specular reflections. 5mW green lasers were found to cause tissue destruction in the eye with JUST 60 seconds of exposure - red lasers (of the same wavelength) did not have the same results. Now these are available over the counter in most places!

If there are scientists talking about massive damage from a measly 15 mW and if 50mW is enough to cut plastic and thin rubber, what exactly makes it so hard to understand? Especially when people are constantly warned that these lasers are NOT toys?

http://www.pbase.com/image/52123862/original.jpg
http://www.pbase.com/image/52135235/original.jpg

Most correctly labelled lasers in the Class 3B range have a sticker that clearly states "DANGER". Not "caution". Would you stare at the sun with a magnifying glass in front of your eye? No, you certainly wouldn't! Because you know you'll lose all sight in that eye. A 50mW laser is close enough to sunlight & a magnifying glass if held at arm's length or closer. 

Stop being stupid, people. You all know better than this. The evidence is there, the experts are there to warn you. Victims have written about what happenned to them. And the DANGER labels are on the devices when they ship.

I simply can't wait to hear of some CLOWN destroying the use of one of his eyes because he deliberately shone the laser into his eye to prove a point. I mean that. What I DON'T want to read about is how some INNOCENT person lost their eyesight because of the ignorance and massive stupidity of someone else.


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## allthatwhichis (Aug 24, 2006)

:bow: :bow: :bow: :bow: :bow:


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## abeland1 (Aug 24, 2006)

nero_design said:


> When you look at a lightbulb, it's image is reversed on your retina. But laserlight is not like the lightbulb. It get's focused by the eye's lens into a hot spot on the retina. And it get's magnified in strength. A green laser is focussed into a pinpoint of heat on the retina that is 100,000 times the strength of the laser. The area of your retina that processes images for your brain is actually really tiny... which is why you can only view a few letters on this message at a time. One hit by a 100mW laser and you will lose the use of that eye forever. The US army describes a 100mW laser as being capable of severe damage at a distance at only 10% of it's strength.
> 
> Sigh! Here we go again... ALL Class 3B lasers produce potentially dangerous specular reflections. 5mW green lasers were found to cause tissue destruction in the eye with JUST 60 seconds of exposure - red lasers (of the same wavelength) did not have the same results. Now these are available over the counter in most places!
> 
> ...


Nero,
THANK YOU FOR THE EXCELLENT POST.


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## CLHC (Aug 24, 2006)

:thumbsup: *nero_design*

Well Put! ! !


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## livetofall (Aug 24, 2006)

with all these questions about which mw at what exposure time will cause damage to the eye... I think we might have finally found some use for pedophile prison inmates!


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## monkeyboy (Aug 24, 2006)

Pila_Power said:


> I think the eye-safe ones are I/R


 
IR lasers are the most dangerous of all lasers. This is due to the fact that the eye has no blink response to IR yet they are just as damaging as visible lasers.


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## Mihlrad (Aug 24, 2006)

Gotta love the world we live in. You cant have a laser that has a CHANCE to damage someones eye if misused or accidentaly, but you can buy at the age of 18, a .50 caliber that if misfired or whatever, would go directly through the target, and if the target is something living, odds are the target is dead. Not damaged, dead.
Its amazing isnt it. And in america, you cant drink till 21, but you can shoot people at 18...


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## N8YWF (Aug 24, 2006)




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## allthatwhichis (Aug 24, 2006)

livetofall said:


> with all these questions about which mw at what exposure time will cause damage to the eye... I think we might have finally found some use for pedophile prison inmates!


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## Pila_Power (Aug 24, 2006)

Sorry it took so long to reply - living on the other side of the earth and all that 

http://www.globalsecurity.org/military/systems/ground/an-pvs-6.htm

This was the laser I was referring to...


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## allthatwhichis (Aug 24, 2006)

Pila_Power said:


> Sorry it took so long to reply - living on the other side of the earth and all that
> 
> http://www.globalsecurity.org/military/systems/ground/an-pvs-6.htm
> 
> This was the laser I was referring to...


 


> The Rangefinders are being procured on a sole source basis from the manufacturer, Litton Laser Systems of Apopka, FL.


 
That's interesting, I drive through Apopka, FL on my way to and from work... It doesn't say, but I have a feeling that these rangefinder lasers are not that powerful. I doubt it would need to be.


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## jkaiser3000 (Aug 25, 2006)

> Sorry it took so long to reply - living on the other side of the earth and all that
> 
> http://www.globalsecurity.org/milit...nd/an-pvs-6.htm
> 
> This was the laser I was referring to... Yesterday 07:15 PM



That laser is "eye safe" only because it works in the mid to far IR part of the spectrum. By safe, they mean it won't get to focus on your retina, as the cornea itself is opaque to that wavelength. The thing is, with enough power, it's not your retina that will get damaged, but your cornea. Granted, you'll need a bit more power to damage it as the light is not being focused to a tight spot as in your retina, but damage will occur.

In this case, the most probable simptom of damage will be cloudy vision, and perhaps blurry too.

Incidentally, the same thing can happen with UV lasers.

As you can see, even if you can't see the laser, it's still harmful to your eyes, and even your skin in some cases.

Wear eye protection and don't play with lasers, even class II ones. They're not toys, remember?


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## abeepak1 (Aug 26, 2006)

OK this is to make it nice and simple for u.

1.No one is stupid enough to do that
2.Yes you will get permanent eye damage for sure
3.no one would operate a laser module of 15mw up without goggles
4.About 45mw would do the job of making u blind in an instance.
5.Check out this link(has nothing to do with the eye injury issues)
http://world4.monstersgame.co.uk/?ac=vid&vid=47010408
:laughing:


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## allthatwhichis (Aug 26, 2006)

abeepak1 said:


> OK this is to make it nice and simple for u.
> 
> 1.No one is stupid enough to do that
> 2.Yes you will get permanent eye damage for sure
> ...


 
:bow: :bow: :bow: :bow: :bow:


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## abeepak1 (Aug 26, 2006)

allthatwhichis said:


> :bow: :bow: :bow: :bow: :bow:


 
.......     (forgot to mention)

*No one will be stupid enough to point a laser at their own eyes but accident can happen!! so Get goggles(dont use goggles for red lasers on green lasers), dont try something stupid ,use common sense , end of story.
Also Go into this link and Register if u want to be part of it.

http://world4.monstersgame.co.uk/?ac=vid&vid=47010408


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## illumiGeek (Aug 29, 2006)

One common misconception I keep seeing here is the concept that exposure to an over-bright laser will "blind" you. This is not true. At least not for the kind of lasers we are talking about here. Small "pointer" or handheld type lasers focus to an extremely small spot on the retina, so the only way you could "blind" yourself with one would be to wave a high power laser around your visual field until you caused enough damage to a large enough area.

In reality most people would probably not notice laser damage immediately. The odds of an accidental exposure causing enough damage to actually form a "blind spot" in your vision is very small, unless you are extremely stupid. Damage from the laser sources typically discussed here would most likely manifest itself as a very slight loss in visual acuity. This kind of damage may not be immediately apparent, but it is permanent, and cumulative.

So if you have accidentally zapped yourself with a class IIIb laser and thought you got "lucky" - you probably did't get as lucky as you thought.

Oh, and the person who said they read an article that said that prolonged exposure to a 5mw laser won't do damage was not reading about green lasers. Research I have read has shown that 5mw green lasers can do damage with exposure times of <60 seconds. Red, however was supposedly fine at exposure times of up to 15 minutes (I personally wouldn't recommend it, though).


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## jtice (Aug 29, 2006)

First off, I am no expert, I am only speaking from experience with my lasers.

I started out with a 15/52 mW dual stage pointer.
I spent many hours a day using it with a partly homemade scanner, that would project scanning patterns, and simple shapes.
Most of the time I was using the 15mW setting, but even then, after a few hours, 
I could tell it was taking it toll on my eyes.
Not blinding, and probably not perminate, but just hard on them, sorta like reading in the dark for a while.

Now, I have upgraded to a 100mW and 131mW laser. 
They are both hard on the eyes,
just looking at the dot they cast on the wall makes you squint.

Now, I have been blasted in the eye quite a few times with the 100mW one,
it was not direct though, as it had bounced off the scanner mirror.
So it was a bit more defused, but still a tight beam.
While it didnt hurt, it sure does kinda knock you simple for just a second, and you see spots for a while.

I dont think a quick blast to the eye will do much in terms of perminate damage.
Unless you do that evey day, then of corse thats going to add up.

There are professional lasers at rock shows that are WATTS of power.
Though these usually have a much larger beam.

I think long term exposure is what would be bad,
or staring at the dot on the wall every day.
Even staring at a 15mW dot on the wall every day is gonna add up, 
since you are using the same area of your eye each time.

Overal, dont be terrified if you get zapped real quick, 
but you definately dont want it happening often.
And if you are going to use something 25mW or more, for long durations, 
you might want some googles.

I cant even begin to align my 100mW unit with the scanner mirrors,
unless I wear my welding googles.
I cant even look at the scanner mirrors with the naked eye.
So if you are messing with lasers this powerful, DO use protection.

~John


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## Kiessling (Aug 29, 2006)

nero ... I quoted your excellent post in one of our stickies here, thank you.
bernhard


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## Kiessling (Aug 29, 2006)

Another thing ... monstergames are a topic for the Café, please


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## Kiessling (Aug 29, 2006)

And while I am on it ... accusations of one member being a troll is pretty serious, and while probably justified in some cases, it is often best to point this out to a moderator or admin in private unless the matter is terribly obvious.

Carry on now ...


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## livetofall (Aug 29, 2006)

thanks for the info IG, while this is a fresh thread, is IR also reflectable as the 532 or partial,etc? I am actually more worried about reflective light, as I dont know whether to get standard 532 range such as wickedlasers, or YAG Diode, until i know if the laser I am getting is emitting any IR, the YAG are NOT CHEAP


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## illumiGeek (Aug 29, 2006)

Just took a quick look at that chart PhotonWrangler linked to. There is something wrong with that chart. Look at the figures for 530nm. The "safe" 600 second exposure is much higher power than the "safe" 0.25 second exposure. That just doesn't make sense.

Safe power for 0.25 second exposure = 0.0000167 W
Safe power for 600 second exposure = 0.0025 W

Something is very wrong here...

(The 568nm numbers seem to be off as well.)


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## stockwiz (Sep 2, 2006)

Mihlrad said:


> Gotta love the world we live in. You cant have a laser that has a CHANCE to damage someones eye if misused or accidentaly, but you can buy at the age of 18, a .50 caliber that if misfired or whatever, would go directly through the target, and if the target is something living, odds are the target is dead. Not damaged, dead.
> Its amazing isnt it. And in america, you cant drink till 21, but you can shoot people at 18...




I agree with you on the drinking thing, but I can understand why the government regulates laser pointers. They are bright, and people misuse them far more then a .50 caliber weapon. They are available to everyone, at any time, without a licence, background check, etc.

I myself don't trust others with laser pointers especially in bars or clubs because they seem to love pointing them at eyes. At sporting events, people love to point lasers at the eyes of announcers and the like. I certainly wouldn't want the masses running around with 20 mw laser pointers they got off ebay.


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## K-T (Sep 3, 2006)

Let's keep this thing ontopic regarding "eye injury", everything else is OT and can be discussed elsewhere.

Keep in mind that injuries to the retina done by a laser does not hurt and the defects created might be small and compensated by the brain at first. Multiple and repeating damage will reduce the sight for sure. To find out whether your retina and eye sight has already been damaged you better stop by at your local ophtalmologist (sp?) and let them have a look at your eyes. "I got hit but blinked with my eye and it doesn't hurt" does not mean that damage has already been done.

k.


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## ajohnson (Sep 3, 2006)

illumiGeek said:


> Just took a quick look at that chart PhotonWrangler linked to. There is something wrong with that chart. Look at the figures for 530nm. The "safe" 600 second exposure is much higher power than the "safe" 0.25 second exposure. That just doesn't make sense.
> 
> Safe power for 0.25 second exposure = 0.0000167 W
> Safe power for 600 second exposure = 0.0025 W
> ...


 
Try appendix B of this one then:
http://w3.ouhsc.edu/ehso/policies/Laser%20Safety%20Program%202004.pdf

It looks like they got swapped. What's also scary is that's direct from OSHA
http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_6.html


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## jrmcferren (Sep 17, 2006)

I got hit with a class III (forget a or b though, but if it was B it may have been running in a as it was on batteries) once. It was around 01 or 02 and I don't think it affected me too bad or at all. I remember the experience slightly and I will try to post it here. I was playing with a laser DJ light (like a DJ Widow) and for some reason I had it pointing straight up. It hit my eye briefly, it did not hurt too much, but I knew it was the laser though.


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## YAG (Sep 18, 2006)

You guys really need to be careful with these larger laser "pointers".

They all have a very small beam diameter and good quality beam. If you get tagged directly in the eye, all of the beam is likely to enter your pupil because of the beam size, and like was stated above, you may not notice the damage right away. The best measure of protection is not being careless with any laser... you only get one set of eyes, and any retinal damage you do is permanent. Treat these things like a loaded gun with bullets that ricochet.


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## The_LED_Museum (Sep 18, 2006)

The warning I place on some of my web pages for ultraviolet LED products is appropriate here too:

*WARNING!!!*​*You only come with one set of eyeballs, and if you ruin them, you can't just go to the local 7-11 for some "Eyeballs-In-A-Can" and pop some new ones in. In another 1,000 years maybe, but not now.*


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## YAG (Sep 20, 2006)

Agreed... ultraviolet ocular toxicity is a big deal with a lot of these newer deep blue and ultraviolet LEDs. Be careful people.


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## Canuke (Sep 27, 2006)

YAG said:


> Agreed... ultraviolet ocular toxicity is a big deal with a lot of these newer deep blue and ultraviolet LEDs. Be careful people.



Not to mention the organic photoablation factor, once you get into the blue and UV range. It has already been noted in this thread that green becomes a problem faster than red at equivalent wavelengths... with blue and UV this effect will be *much* more pronounced.

I would not be surprised to see the power levels for each class lowered for wavelengths <500 nm.


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## soapy (Sep 29, 2006)

The reason that green has more of an effect is because your eye is far more sensitive to it. That is why it is so much brighter at the same power level. The reason it seems brighter is because we grew up in the jungles and plains, where much was green, so green was most important. Blue is less important, as is red. IR is impractical due to our warm blooded natures, nIR would be good, we just don't have the receptors (yet) and as for UV, well, our transparent corneas are simply not opaque to it. If they were, we would see UV just fine (my grandad can, in one eye) as the blue detectors pick it up fine. It is, of course, very harmful to the rest of the eye, though, as well as causing cataracts in the cornea.

Looking at the mechanism for how the eye detects a photon, the energy from the photon causes a chemical reaction in the sensors in the back of the eye. What is effectively a dye is split apart, giving off an electron, and the resulting electrical signal is sent to the brain. This reaction is reversible, but it detects far faster than it reverses, and so when you look at the usual ever-changing scenes, you barely notice the slightly negative picture that presents itself when you more your eye, or the picture changes. However, these effects are easily noticed when you look at an optical illusion that uses persistance of vision. The one where you stare at a green dot for 30 seconds, then see vermillion? That's the type I mean. 

The reason it works is because the continued sensing of one bright colour depletes the reserves of that chemical in your eye. When you remove the stimulous, you see an after effect. Shining a laser into your eye depletes the reserves in fractions of a second, same as a glance at the sun. Hence you get after-spots, and they heal after some time, which depends on the total power exposure.

However, the far higher powers of the sun and lasers (compared to your monitor) means that you can also pump too much energy into your eye, which has fairly poor cooling. As you can imagine, a laser pointer that will set a match on fire or pop a weather balloon isn't going to be nice to your eye!

Personally, I know that my 45mW 532nm diode laser will pop said latex weather ballons, as I did it once. I also know that the tests for eye laser damage are a lot more in-depth than "I can still see green!" as I went through them all at university, where I did a laser course.

Back then, the official line (this is 10 years ago) was that anything above 1mW was dangerous to the eye. There were cases every week of people claiming they were blind due to a 0.9mW pointer being flashed into their eye. I rebutted dozens of people by shining my 4.5mW 670nm into my own eye. No problem, except for some after effects. I spent on the order of hours staring into ~2mW of red HeNe, with occasional flashes of the 12.8mW beam, with very little issue.

Moving on to the green lasers, I imported from the US two such beasts. One is about 4.5mW, the other is 45mW. I marked the uprated one to comply with the law, and getting them confused can be painful to the eye! I would not want to get hit in the eye by either of these. As I said above, I can pop a 3mm thick latex balloon instantly with the 'big' one. The other week I was playing with it in a club, shining it off the mirror ball to great effect. But I always keep the beam moving fast, and ensure that it is at least far enough to spread the beam a bit before it hits. I know I can light up targets miles away. The diffuse light physically hurts the dark adapted eye.

This brings me on to another important point. When you walk from the dark into the sunlight, you tend to stand around blinking until your eyes get used to the change. The iris changes size in about 2 seconds, changing the amount of light that can get in by about a factor of ten by making the hole far smaller. In bright sunlight, you will obviously block most beams at least partly, and the beam loiter time will the lower. At night, your eye will be getting ten times the power. And that is without the increased sensitivity at the back of the retina, that is just from the iris. Hence getting a 1mW beam in the dark will make you react a lot more than a 1mW beam during the day.

I am fairly sure I have a few tiny spots that are burned out. My night vision is nothing like as good as it used to be. My tolerance for bright sunlight is reduced. Of course, I'm ten years older now, but I'm only early 30's.

It will be interesting to see what happens as the local chavs can afford lasers that will invisibly burn your eyes out. The first time a club full of people go blind due to some prat with a UV laser getting it wrong will be the first law banning them fron the public, in the same way as the first prat to shoot up a school resulted in a handgun ban here. And it won't stop further problems, and, in fact, they will increase, exactly in line with the number of handgun deaths in the UK. (Up 300% in a few years)

In ending: Do not stare into the beam with remaining eye!


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## jkaiser3000 (Sep 29, 2006)

There are a few points to this post that make me wonder.



> The reason that green has more of an effect is because your eye is far more sensitive to it




I'm not so sure of this. Our eyes are more sensitive to green due to the increased number of "green receptors" in the retina. If you have a 1mw red and a 1mw green, the energy reaching your retina will be the same, regardless how "bright" it looks. In theory they should both be equally dangerous. It is when we consider the absorption of the retina that we can determine which is more dangerous. The retina is a redish color due to the blood vessels and the thinnes of the "skin" protecting them, and seeing as red absorbs green very well, you can see why a lower powered greenie can be more dangerous than a higher powered red. That's why red baloons are easier to pop than green and white ones. By the same token, red absorbs blue very well too, so a blue laser should be as dangerous as a green one, and maybe even more so, as our eyes aren't as sensitive to blue, you'll think it's dimmer, and hence not dangerous (same effect as with IR). This is mere speculation on my part, though.



> Looking at the mechanism for how the eye detects a photon, the energy from the photon causes a chemical reaction in the sensors in the back of the eye. What is effectively a dye is split apart, giving off an electron, and the resulting electrical signal is sent to the brain. This reaction is reversible, but it detects far faster than it reverses, and so when you look at the usual ever-changing scenes, you barely notice the slightly negative picture that presents itself when you more your eye, or the picture changes. However, these effects are easily noticed when you look at an optical illusion that uses persistance of vision. The one where you stare at a green dot for 30 seconds, then see vermillion? That's the type I mean



If this process is reversible, wouldn't the reverse give out light?, that is, if a photon gives an electron off from the dye, wouldn't an electron hitting the dye give off light?. Somehow I can't see people shooting light out their eyes :laughing:. Furthermore, where would that stray electron be coming from?. I agree with the photon giving off electrons, but this theory does not explain the after images experienced by everyone, basically because those after images will need to be electrons being transmitted to the brain, otherwise it wouldn't register as an image in the first place. As far as I know, the after images effect is not well understood yet. Some people think these are psicological rather than physical.



> as for UV, well, our transparent corneas are simply not opaque to it. If they were, we would see UV just fine (my grandad can, in one eye) as the blue detectors pick it up fine. It is, of course, very harmful to the rest of the eye, though, as well as causing cataracts in the cornea.



I think you meant to say the corneas are opaque to UV. In fact, it's due to the absorption of UV that the cornea developes cataracts with time, and it's the reason why laser eye surgery can be performed. Although, the wavelength could be a factor too. Maybe the corneas are transparent to "near UV" (UVA I think). Of this I am not sure . Anyway, we are not sensitive to UV because we lack the necessary receptors. The pigments you speak of are sensitive to certain wavelengths (photon energies) and so can't percieve outside their range. Blue receptors can see into the UV slightly, just as we can see into the IR slightly, and every person has different sensitivities. 400nm is generally considered the lower limit for people to see in the blue-UV, but some people can see down to 350nm or so, just as 700nm is considered the top limit in the red and some people can see 800 and even 900nm. It's a matter of luck, I guess 



> Back then, the official line (this is 10 years ago) was that anything above 1mW was dangerous to the eye.



And then you stated this:



> I rebutted dozens of people by shining my 4.5mW 670nm into my own eye. No problem, except for some after effects. I spent on the order of hours staring into ~2mW of red HeNe, with occasional flashes of the 12.8mW beam, with very little issue.



and finaly:



> I am fairly sure I have a few tiny spots that are burned out. My night vision is nothing like as good as it used to be. My tolerance for bright sunlight is reduced. Of course, I'm ten years older now, but I'm only early 30's.



I have nothing further to say to that. I rest my case


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## allthatwhichis (Sep 29, 2006)




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## Athoul (Oct 1, 2006)

Wow, interesting couple posts there. Anyway, jkaiser3000 is right in that eye sensitivity to a colour is not what determines the potential to do damage. For potential to do damage to the eyes, consider same outputs in the entire visible sepctrum to be equivalent, 100mW of green will be as dangerous as 100mW red. Technically the shorter wavelengths may do more damage because the will be focused to a smaller point, but this is not going to mean much to your retina. The invisible wavelengths will instead damage the cornea or lens depending on the wavelength involved.


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## soapy (Oct 3, 2006)

> If this process is reversible, wouldn't the reverse give out light?, that is, if a photon gives an electron off from the dye, wouldn't an electron hitting the dye give off light?. Somehow I can't see people shooting light out their eyes . Furthermore, where would that stray electron be coming from?. I agree with the photon giving off electrons, but this theory does not explain the after images experienced by everyone, basically because those after images will need to be electrons being transmitted to the brain, otherwise it wouldn't register as an image in the first place. As far as I know, the after images effect is not well understood yet. Some people think these are psicological rather than physical.


The electrons come from all over the place, including from other dye molecules that are being split by other incoming photons. This neatly explains the negative image, since your eye is then not detecting that colour any more, and is in fact abosbing that colour alone, where the aftereffect is. This explains why a bright red beam would cause your depleted red detectors to register red everywhere else, but nothing in your after effect affected areas, and then to register as green when a white light is looked at - it isn't letting the signal through, it is absorbing it instead.

As for the sensitivity curves for the eye, they are easily found online. Whether the higher sensitivity is due to better dectection in each cell or more cells I don't know. I believe they are roughly equal in number, but it was ten years ago.

Re: "not opaque" - yes, wrong way around. UV won't go through the cornea, hence eventually causes cateracts. This, of course, protects the eye for short periods.

100mW of red light is not the same as 100mW of green. The energy per photon is higher at higher frequencies/lower wavelengths. (That's why IR warms you and UV burns you (No, not really))

Right, the speed of light, c, is constant. The variables are linked by c=fy (where f is frequency and y will stand in for lambda, for wavelength)

Energy in a photon is, however, E=hf (where h is Planck's Constant) so photons with a high frequency (more blue) have higher energies than lower frequencies (more red). It is the absorbtion of this photon that causes an electron to be ejected from the dye in the back of the eye.

Eyes also emit light. That's why we can't see IR, as the heat of our own eyes would drown out the far weaker signal from outside. However, as regards re-emmission of light by the dye absorbing an electron, that would be a bad choice of dyes, and evolution would have sorted that out long before. As far as I know, the dyes are replenished over a fairly long time, hence the depletion issues with over exposure of light lasting for so long.

(http://dsc.discovery.com/news/briefs/20050905/handlight.html is interesting, too, though not really relevant.)

As regards the 5mW red thing, I shone it into my eyes many times, with no issues, as was confirmed by the tests carried out. A 5mW red leaves a far less lasting aftereffect than the same power level of green, because your eye is less sensitive to it. Once you get up to higher powers, however, energy flux and heating come into play. I know the spots in my vision are due to the far higher powered green lasers than the lower power and far (perceptually) less bright red lasers. 

Once you get into the UV, the actual energy in each photon starts to have an effect, and then it isn't heating that is the problem, it is the molecules getting blown to bits that is the issue. Unlike the dyes in your eye, your cells, DNA, etc. are not designed to have this happen, and that's why you end up with free radicals (due to that stray electron from each photon absorbed) which have been linked to cancer.

-
If you want to see a truly beautiful thing, get a pinhole, and a lense, and focus your laser down through the pinhole. It is a spatial filter, and will generate the most incredible true Gaussian curve for you. Shine onto a smooth white wall in a darkened room for full effect. Jaw dropping.


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## soapy (Oct 3, 2006)

> Anyway, we are not sensitive to UV because we lack the necessary receptors. The pigments you speak of are sensitive to certain wavelengths (photon energies) and so can't percieve outside their range. Blue receptors can see into the UV slightly, just as we can see into the IR slightly, and every person has different sensitivities. 400nm is generally considered the lower limit for people to see in the blue-UV, but some people can see down to 350nm or so, just as 700nm is considered the top limit in the red and some people can see 800 and even 900nm. It's a matter of luck, I guess


Sorry, forgot to address this. The response curves of the eye have been tested in vitro. These show that you can see very long wavelengths, but, the power levels required to "see" them, even with a dark adapted eye, mean that heat build up will destroy the eye. The last thing a lot of people blinded by IR lasers report seeing was a bright light, then darkness.

As regards UV, your blue pigments in your eye pick them up fine. Corneal absorption is the limit here. In fact:



http://education.guardian.co.uk/higher/medicalscience/story/0 said:


> What does ultraviolet look like? Prof Stark possesses UV vision because he is aphakic in one eye and, with Professor Karel Tan, has published research on the nearest visible equivalent. His conclusion is that it looks whitish blue or, for some wavelengths, a whitish violet.
> 
> This appears to be because the three types of colour receptor (red, green and blue) have similar sensitivity to ultraviolet, so it comes out as a mixture of all three - basically white, but slightly blue because the blue sensors are somewhat better at picking up UV.


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## jkaiser3000 (Oct 3, 2006)

I'm in a bit of a hurry now, so I'll address only one point. The rest require some time to think about :laughing:.



> 100mW of red light is not the same as 100mW of green. The energy per photon is higher at higher frequencies/lower wavelengths.




You're correct on that. Blue photons do have higher energies than red ones. But, as far as I know, mW is an energy meassure, so 100mw of red=100mw of green, at least as far as energies go. I might be wrong though.

As far as damaging effects, there are many things to consider, and they have to be considered as a whole, simultaneously, like power, beam size, color absorption, etc.

Later


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## allthatwhichis (Oct 3, 2006)

But... we can ALL agree, that 100mWs of any laser will blind you if you do not have goggles to reflect whatever frequency laser you are using. Yes?  POP goes the retina...


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## jkaiser3000 (Oct 4, 2006)

allthatwhichis said:


> But... we can ALL agree, that 100mWs of any laser will blind you if you do not have goggles to reflect whatever frequency laser you are using. Yes?  POP goes the retina...



Absolutely, we all agree on that. Which makes me think we were diverging a bit from the topic at hand .


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## Athoul (Oct 4, 2006)

Yep 100mW is 100mW is 100mW, it's a measure of energy. The only thing that is different is that the shorter wavelengths can be focused to a smaller point, and in this regard have greater burning ability. Though for most purposes that we use them for, this won't make any noticable difference.


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