ACTUAL eye damage threshholds?

[ossumguywill]

I was just reading a thread where some guy claimed to be a doctor, saying that an "energy density of .4 joules" causes blindness. Obviously joules is not a measurement of energy density. I am wondering how many (mw/cm^2)*time or joules/cm^2 actually causes blindness or eye damage. I haven't heard a straight answer yet.

 

[pullbangdead]

The "straight answer" is, by necessity, very nuanced. There is no single, magic number. Professionals use the standards available, such as ANSI Z136, to evaluate individual instances of laser use and exposure, since every situation is different.

Here is just about the shortest explanation I've seen that still conveys many of the essential elements necessary, the chart of MPE is really nice. Notice a hole or 2 in that nice chart though, because it doesn't include the wavelengths between 351nm and 514nm that are actually very common now. You can extrapolate somewhat though, and it gives you the general idea of where you need to look for further information.

http://www.safety.vanderbilt.edu/pdf/laser_exposure_limits.pdf

ETA: some of the numbers in that handy chart are also transposed: for the krypton row, the 2.5s and 600s values have been switched for 2 of the wavelengths, so there may be another typo or 2 on there. But it gives you an idea.

 

[ossumguywill]

OK, cool. I will take a look.

 

[Grix]

The rule of thumb is 5mW / 0,38 cm^2 (Area of pupil).

 

[millirad]

One thing that seemed to be clear from the chart and rightfully so..... Short wavelengths in the UV range are extremely hazardous.

 

[Benm]

Even if you can calculate the treshold at all, which one would you call the 'actual' damage?

Its similar to 'how poisonous is that substance?', and 'what is a lethal dose?'.

With poisons, the lethal dose is often stated as the LD50, which is the dose that killed 50% of test subjects in research. Ingesting this dose isn't a guarantee you will die from it though, its the point where the odds are 50-50. With chemicals other doses are sometimes determined as well, such as LD10 or LD90.

Getting back to the lasers: the '5 mW into the eye' gives a small, but accetable, chance of eye damage. The 'safe limit' where no damage occurs is much lower, in the order of 0.5 to 1 mW into the eye for visible wavelengths. I'm not sure about the exact definition of safe here, but its something like 'less than 1% chance of damage during 15 minute exposure'.

On the other side of things, i don't think that any power level has been defined that is guaranteed to cause eye damage - and i hope no one here will figure that number out the hard way!

 

[ossumguywill]

Even if you can calculate the treshold at all, which one would you call the 'actual' damage?

Its similar to 'how poisonous is that substance?', and 'what is a lethal dose?'.

With poisons, the lethal dose is often stated as the LD50, which is the dose that killed 50% of test subjects in research. Ingesting this dose isn't a guarantee you will die from it though, its the point where the odds are 50-50. With chemicals other doses are sometimes determined as well, such as LD10 or LD90.

Getting back to the lasers: the '5 mW into the eye' gives a small, but accetable, chance of eye damage. The 'safe limit' where no damage occurs is much lower, in the order of 0.5 to 1 mW into the eye for visible wavelengths. I'm not sure about the exact definition of safe here, but its something like 'less than 1% chance of damage during 15 minute exposure'.

On the other side of things, i don't think that any power level has been defined that is guaranteed to cause eye damage - and i hope no one here will figure that number out the hard way!

Yeah, according to that graph 2.5mW/cm^2 is appropriate at 514nm, so .5mW over the pupil seems like a good estimation for any wavelength.

 

[LSRFAQ]

I'd have to dig up a copy of Sliney's book, but IF I remember right:

Class I is intrinsically safe.
Class IIA is 1:1,000,000 odds of detectable damage
Class IIIA is 1:100,000 odds of detectable damage.
Anything above IIIA, all bets are off.


This work was done in the 1970s. Instrumentation was crude by today's standards and damage thresholds to animal, cadaver, and volunteer eyes was somewhat subjective.

The .4 joule number, for 100% blindness in one eye, I'm quite sure, is for hitting the Fovea in the Macula and thus killing/damaging the optic nerve.

You want:

Sliney D H and Wolbarsht M L 1980 Safety with Lasers and Other Optical Sources (New York: Plenum) c

Kind of oudated, but is a good, thick, detailed, book.

My local library has placed it in hard storage or I would look up the numbers for you.

 

[ossumguywill]

I can't think why the hell I would volunteer to have a laser shot at my eye, no matter how much you paid me.... Well, maybe depending how much you paid me. But that's good to know. I'm really interested in the distance at which powerful lasers become safe, I asked about NOHD a while back but the thread didn't get much attention. This is a start, certainly. Like a 1 watt blue would have to be around 12 centimeters across to be safe for .25 seconds. A 2.5mrad blue would have to be over 150 feet away to be that wide. That's what I'm after :thanks:

 

[Benm]

I think most forum members count on the blink-reflex timings to determine what is relatively safe, and accept the remainder of the risk as part of the hobby.

Personally i just go for cigar-box calculations and common sense. If the exposure level exceeds 10mW/cm2 or so, i make sure not to look into the beam and take precautions not to do so accidentily either (i.e. safety goggles and/or ensuring the beam cannot hit me).

The MPE limits are geared towards protecting audiences and other third parties, and are more conservative for that reason.

Different limits for operations and bystanders are not that uncommon either. Since i took a course in radiology, i can now safely expose myself to 200 mSv of ionizing radiation a year, instead of the 20 mSv considered safe for the general public. Obviously my physiology has not been changed in the course, but its a matter of awareness and likelyhood of taking prevention measures that allows higher limits for those who have a clue what they are doing.

With lasers its kinda similar: unsuspecting audience might do their best to stare into a laser show projector, even if it is very bright and even somewhat painful. Anyone used to working with lasers knows to look away from the source and not attempt to look at it again.

Most people don't have a clue that light can be dangerous and would willingly stare into the sun if requested to do so. Solar eclipses are good sources of eye injuries too - people peering into the sun just before and/or after the eclipse finishes, using binoculars or telescopes if stupid enough

 

[pullbangdead]

I can't think why the hell I would volunteer to have a laser shot at my eye, no matter how much you paid me....

I've seen some studies where they got volunteers who were going to have their eye(s) removed due to other health problems. Since the eye was going to be removed anyway, they did some science before removing it.

 

[millirad]

^ Almost as bad as electroshock therapy before a lobotomy. J/K

 

[LSRFAQ]

If you give me specific diameter, divergence and wavelength numbers, I will run them through Skyzan software for you. But the distances have to be longer then say 20 meters and the powers higher then say 50 mW. The software has some lower limits.

Keep in mind you have to consider the laser's mode as well. A TEM00 beam spread to 12 Cm might have a very small central bright spot. This is why it is always best to actually measure what is there.

One other caveat, never assume a laser stays at a constant power.

Steve

 

[ossumguywill]

I've seen some studies where they got volunteers who were going to have their eye(s) removed due to other health problems. Since the eye was going to be removed anyway, they did some science before removing it.

Ahh, I suppose that makes sense... It might just pay for a new glass one anyway!

@steve
thanks for the offer-I don't have a specific laser in mind, I just wanted to be able to do rough safety calculations.

will