How quick can 5mW laser damage an eye?

[ultimatekaiser]

dunno. this is what sam came up with. I dont think you did your math right? but also keep in mind alot of assumptions have to be made when making these calculations from a non-scenario. but it has the potential to be that bright.

Intensity of a 1 mW Laser versus the Sun
Here is a comparison between the maximum intensity on the retina of the Sun and the beam from a 1 mW HeNe laser. (Adapted from one of Simon Waldman's optics lectures.)

Standard Sun:

Maximum intensity of sunlight at ground level (directly overhead, no smog, etc.) = 1 kW/m2 or 1 mW/mm2.

Assuming pupil diameter is 2 mm (i.e., radius of 1 mm), the area is approximately 3 mm2. So, the power of the sunlight through the pupil = 3 mW.

Focal length of eye's lens = approximately 22 mm. Angular size of Sun from Earth = 0.5 degree = 9 mR. Thus, diameter of image formed = 22 mm x 9 mR = 0.2 mm and the area of image = 0.03 mm2.

The intensity of the Sun on the retina (Power/Area) = 3 mW/0.03 mm2 = 100 mW/mm2.

Typical 1 mW HeNe laser (or laser pointer):

Power (P) = 1 mW, wavelength (l) = 633 nm, radius of beam (w) = 1 mm, focal length of eye (f) = 22 mm. So, the diameter of spot = (2 x f x l)/(w x pi) = 9 x 10-3 mm and the area of spot = 6 x 10-5 mm2.

The power density of the HeNe laser on the retina is 1 mW/(6 x 10-5 mm2) = 16,667 mW/mm2 = 16.667 watts/mm2.

So the 1 mW laser has the potential to produce an intensity on the retina 167 times that of direct sunlight! But there are many more factors to consider in determining the real risk of damage. In addition to those noted below, the actual focal point when looking at a laser at close range will not be at the retina so the spot size will most likely be much larger than the diffraction limit of the calculation. Even if the spot from the laser beam is smaller, natural eye movements or movement of the source (e.g., some moron waving a laser pointer) will result in it hitting any given point for a shorter time than the larger spot from the Sun (which usually doesn't move very quickly).

But, at least, perhaps you'll now have a bit more respect for that little HeNe laser or laser pointer!

(From: Jim Webb (jim@glservices.org).)

The real problem behind this is that it is assumed that the power density is the significant factor in the thermal damage mechanism. The ability of the retina to dissipate heat is not dependent on the area covered, but the periphery (circumference) of the exposed area! The blood vessels are in the retina and not the sclera (the surface under the retina) - it is the blood flow that dissipates the heat and so can only act on the *edge* not the middle of the exposed area. In circumference terms, the ratio drops to 7 times. Furthermore because the larger spot is less efficient at dissipating heat, the effective power delivered by the laser beam is only about 2 times greater than that of the spot formed by the sun.

 

[Atomicrox]

I reviewed my math and couldn't find anything obviously wrong. Can you point out the mistake?

The main problem I have with his explanation is that it uses two different formulas to compute the spot size on the retina: the diffraction limit is used for the laser and that assumes a perfect beam, which is never the case. The laser divergence is never considered and it won't be better than 0.5mrad for a real life pointer (which also won't have a 1mm beam if it's a diode laser).

My calculation is based on the power density hitting the eye, which seems to me a much better way to estimate damage.

 

[ultimatekaiser]

This is why I really don't like this, no matter how you look at it it's purely circumstantial when you're doing damage calculations. I've personally never come across a medical record saying that there was permanent damage from the laser that was a class IIIR. I think the worst one Ive ever come across was a 1 month of temporary vision accuracy loss or something. I believe there was a doctor that came up with doing a study on a voulenteer that already had a bad eye, where they compared red and green 5mW lasers and found that the green did more damage, but either way I'd never advocate aiming a laser into ones eye. I've had some personal experience in that area I can say it's easily more bright than anything else you could possibly put in your face that I can think of.

Edit:
And some diodes do have pretty circular beams. I have a few that are pretty darn close. I'll check the math later and see what I come up with I'm at work ATM.

 

[Atomicrox]

Agreed. I'd really like to see something more scientific, but I guess it's hard to experiment for obvious reasons.

Even the ones with circular beams have ~3mm diameter and 0.5mrad at best. Which is way worse than a good gas laser.

 

[ultimatekaiser]

Agreed. I'd really like to see something more scientific, but I guess it's hard to experiment for obvious reasons.

Even the ones with circular beams have ~3mm diameter and 0.5mrad at best. Which is way worse than a good gas laser.

He's using a hene in the example. My old laser pointer used to have a 1:1.45mm beam. But it also wasn't a cheap Chinese new wish pointer or anything either it was a good $35 pointer.

though it's beyond the scope of this argument, I do have some diode lasers that are A millimeter or less and circular.... But they're also about 2000 times more expensive than a typical laser pen

 

[hakzaw1]

only a broken laser is 100% harmless

look at the sun

then look again w/ a mag glass-Ouch-!
Im blinded==
except you may feel no pain- and your 'good' eye will help the damaged one (sorta)
so its not the power of the pointer but where you point it....


-the lens in your eye can make any laser too much..

treat like a gun--

 

[Faradayzed]

He's using a hene in the example. My old laser pointer used to have a 1:1.45mm beam. But it also wasn't a cheap Chinese new wish pointer or anything either it was a good $35 pointer.

though it's beyond the scope of this argument, I do have some diode lasers that are A millimeter or less and circular.... But they're also about 2000 times more expensive than a typical laser pen

wow, you're saying a laser pen can cost 2 grand? what makes it worth the price compared to cheap laser pointers?

 

[hakzaw1]

I doubt if you can really get a regulated 1mW laser as these low power laser are usually overspec .

Luis

AixiZ Laser stocks many <5 mW red modules-one (world's smallest) is ~1mW and is about as big as a grain of cooked rice.


BTW some member wrote that he took a one second eye hit--
IMpossible
longest you can keep from blinking/blocking is .25 seconds or less.

so when figuring the MPE we always use an exposure time of .25 seconds.

BUT a accumulated effect is possible with even low power if you get exposed enough.

 

[Alien Laser]

why not remove the batteries from the laser and your problem is solved ,

 

[diachi]

wow, you're saying a laser pen can cost 2 grand? what makes it worth the price compared to cheap laser pointers?

He's not talking about a pen, Ultimatekaiser has a fairly large collection of scientific/lab quality lasers.

 

[ultimatekaiser]

AixiZ Laser stocks many <5 mW red modules-one (world's smallest) is ~1mW and is about as big as a grain of cooked rice.


BTW some member wrote that he took a one second eye hit--
IMpossible
longest you can keep from blinking/blocking is .25 seconds or less.

so when figuring the MPE we always use an exposure time of .25 seconds.

BUT a accumulated effect is possible with even low power if you get exposed enough.

This is about the time that I use too. average blink time is about 100ms-300ms, so that's a good figure to work with. but is indeed a cumulative effect, especially with certain wavelengths.

He's not talking about a pen, Ultimatekaiser has a fairly large collection of scientific/lab quality lasers.

yes. which is why I said it was beyond the scope of this argument. ....and $2000? try more like $8000-10,000 or more! Hell I have a portable ruby that was $45K in 70s money! Probably still worth that too in theory.

 

[micheal rosen]

Well, honestly (as others have said) it depends on the person. However it also depends on the divergence of the laser. Gas lasers have been known to cause instant eye damage because of their near perfect beams, even though they have low output powers. This is partly due to the energy density, giving more energy going into the eye, and (potentially more importantly) the fact that they have nearly perfect beam charicteristics, and this means that when the lenses in the eye focus the light onto the retina (like they normally do) the laser light is converged to an absolutely tiny dot on the retina, giving massive power density on the retina, which results in serious retinal burns. Standard diode lasers often times don't have beam specs nearly as good as gas or DPSS lasers, however this does not mean they're safer, only that they will take longer, or require higher output powers to blind someone or cause permanent damage. I wouldn't be worried about friends and family, tell them that your lasers are dangerous and not to touch them without your permission, or keep them locked up or hidden away. If you want low power lasers, get one of the 1-5mw lasers from laserbtb.