405 nm LASER SAFETY

[The_Doctor]

405 nm has long been in the standard visible range of 400 to 700 nm, and is not generally considered a hazard as sources well beyond 400 nm are. If this needs serious revision, don't blame me for accepting it, I didn't establish this accepted status. As Bill said, until such time as this is viably disproved as good status, we should go with it.

Sure, inverse square law might be the main thing we rely on to protect us, but has it ever been otherwise? That helps even with new sources. If we know a visible-range source IS 100 mW and treat it with the same cautious avoidance of direct exposure or staring at bright diffuse sources as with obviously bright ones, then we do most of what needs to be done.

What reading? You need to be more specific. I was always told that moderate long-wave UV exposure either had little or no effect on the body, or was localised to surface effects, in the skin or corneas. No mention of internal organ failure. You need to cite the sources to be explored for that one. People have lived with equatorial sunlight for millenia, and it's known to cause damage to skin, even just accelerated aging of that skin. And cataracts. I never heard anyone say it caused internal organ failure, even after a lifetime.

EDIT: I know that a melanoma can go agly and migrate cells to damage other organs, but this is an extreme case, and plenty of obvious things go wrong before that happens. Such damage is well beyond 'creeping up' on its victim.. Probably results from traumatic exposure rather than incremental, too.

[p1t8ull]

How would the UV radiation from a 405nm laser compare to that of 400W UV cannons (mercury vapour lamp) used in nightclubs? I presume the cannon would be nearer to 390nm, but people are often exposed to these for 8 hours at a time.

And also ar lasers, isn't 351nm one of the strongest lines and a fair bit of power produced <407nm?

[The_Doctor]

And also ar lasers, isn't 351nm one of the strongest lines and a fair bit of power produced <407nm?

If exposed directly to the arc tube glow at close range that might be troublesome but I think the energy in that wavelength is reduced in the beam by selective use of mirror coatings. Very little of anything but the designed-for wavelength will get into the beam, as one strong resonance robs energy from other potential ones. For more on that I will defer to Steve, this is definitely his territory.

[tocket]

I have had some thoughts regarding the safety of these lasers as well. While I initially regarded them as equally "safe" as other visible wavelength lasers I'm not so sure anymore.

While it is not by definition UV light, it is rather energetic and still sufficient to cause photochemically induced damage in tissue (and DNA) via free radical mechanisms. Consider that the irradiance of a 100 mW laser focused to 1 mm² (100 kW/m²) is over 2000 times higher than what you receive from the sun in the form of UV-A radiation (45 W/m²). At such a high irradiance the natural defense against free radicals might be severely overpowered, causing far more damage than the sun would.

As for eye safety, which is probably our primary concern, it is my belief that there's no reason to treat it any differently than a laser on the other extreme of the visible spectrum. It is easy to underestimate the radiant power of these lasers because the eye responds poorly to them. If you're considering audience scanning with them (not recommended) you really need to look at the radiant power, not perceived brightness. If you don't understand these terms you shouldn't be doing any audience scanning whatsoever.

We are exposed to a lot of UV light from the sun on a daily basis. The solar spectrum is surprisingly flat in the 400-700 nm region. The irradiance at say 395-415 nm is only 30% lower than at 520-540 nm. Sure, high exposure to UV light from the sun does cause eye problems, but the sun is a lot stronger than a laser or blacklight (they are very inefficient).

If you're looking for safety goggles for 405 nm, think sunglasses. They're supposed to have 0% transmittance at wavelengths < 400 nm. This means very little transmission at 405 nm, because the UV-absorbers have rather broad peaks. Just make sure to test them first as they might not be as good as they should be.

[The_Doctor]

tocket, I agree with all points there, but this one needs a bit of exploration:

While it is not by definition UV light, it is rather energetic and still sufficient to cause photochemically induced damage in tissue (and DNA) via free radical mechanisms. Consider that the irradiance of a 100 mW laser focused to 1 mm² (100 kW/m²) is over 2000 times higher than what you receive from the sun in the form of UV-A radiation (45 W/m²). At such a high irradiance the natural defense against free radicals might be severely overpowered, causing far more damage than the sun would.

While that's also true, I can assure you that more than a very few tens of seconds of exposure to 1mm square of 100 mW hurts. Avoidance is immediately demanded strongly by several body mechanisms after periods well under 2000 times shorter than we can tolerate hard sunshine. Also, it's very localised. I suspect that if such exposure was nonlocalised enough for free radicals to overwhelm that region of the body, then traumatic damage of greater significance would result. Like a serious burn.

I think that the most likely result of a small point exposure is an irreversible generation of melanin, a pigmented spot. Several causes for these exist, most of no consequence. Just watching for odd activity of such, as with any mole, might be enough to prevent cancer or other nastiness developing unchecked.

[mixedgas]

melonoma migrates inside from skin via the lymph system faster then you can say "black death"

The 351 line of argon is strong, but its ARIII+ instead of ARII+, so tubes get driven 2-3 times harder to get the 351/365 nm lines. At 40 A, the laser I used to work on did 9+ watts all lines visible. At 55 amps, it did 350 mW all lines UV or about 100 mW single line , single frequency, but one little organic film on the brewsters, or any dust at all in the cavity and its not lasing. The UV would slowly cook the teflon in the brewster stem covers and deposit it on the windows. It broke down skin cells and turned them into window films as well. Had to wear a smock, mask, and cotton gloves to work on it.

Even a fraction of a mm of polycarbonate was all you needed for safety glasses for 351, but we would still test all the glasses to make sure.

Shortly after I left that lab a chemical engineering grad student who wouldn't listen decided that running the tube at J=1100 A/cm^2 vs 700 A/Cm^2 (J = current density from the gas laser equation)current density was gonna get her graduated faster by speeding her sample exposure times, She cooked the tube in a week. It still would do 5 watts of all lines vis, but a inside window film prevented UV. I got the call to check it out, and she was dum enough to write the current levels in the log. She'd go to class or lunch and leave it at 65 amps. She didn't care, its not her money. I hope she enjoyed delaying her graduation for a year. UV argon runs that energetic inside.

But that beam was scary, I had the doors interlocked, outside and inside warning lights, and the orange shielding plastic and phosphors around the beam paths, if it drifted off a pinhole, you got a glow, and a bright one. Failure to wear goggles in that lab ment two weeks of not doing your experiment and a day long laser retraining session with me. You only that option once.

BTW the lower laser level is a 170 nM excimer like photon radiated before going back to ground state.
It doesn't make it out the windows, thank God.

Steve

[tocket]

While that's also true, I can assure you that more than a very few tens of seconds of exposure to 1mm square of 100 mW hurts.

Oh, I know. It stops hurting once you get deep enough though, but that takes a few minutes.

No, seriously, 100 mW/mm² doesn't hurt instantaneously. It takes a few seconds. You need to focus it further to get instant pain and smoke.

Regarding laser vs sun, I suspect 1 s of laser exposure is worse than 2000 s of sun exposure (with UV-B sunblock). The reason being antioxidant depletion; their concentrations are fairly low and they're transported relatively slowly. Once the antioxidants have been depleted, radicals will be free to cause more damage before they are finally eliminated (radicals are not consumed in a reaction, so a single radical can potentially do damage to thousands of positions in your DNA). In the case of sun exposure the antioxidants can easily be transported quickly enough to maintain a sufficient concentration to efficiently scavenge radicals.

Of course this is just my hypothesis, I don't have any data to back it up. It might be completely wrong.

Oh, you don't get a sunburn from 405 nm by the way. You need UV-B to activate that mechanism. You will indeed darken existing melanin though, giving you something of a tan.

[mixedgas]

[QUOTE=The_Doctor;74041]If exposed directly to the arc tube glow at close range that might be troublesome but I think the energy in that wavelength is reduced in the beam by selective use of mirror coatings.

UV does leak out of the OC. Its not lasing, but there is one member of PL who will tell you that years of being around the UV from laser cathodes and yag pump lamps has earned him double cataract replacement surgery.

Steve

[mixedgas]

Look guys, my worry is that enforcement of basic technical safety principles is going to "Hell in a Handbasket" very fast. Somebody in a low production cost country we all know and love , does something like combine 20 units of 405s in a Arctos style array and we have a exposure problem, a second Moscow incident waiting to happen.

Since they cant seem to make 1 watt plus at the prices they make 100 mW for, we seem to have reached a temporary safety plateau. I have frequent nightmares about this.

We are having this discussion so that our peers on this board are more aware of a issue that can rapidly scale at 4 pieces of 200 mW 405 in a projector. That's why I have been goading the doctor..........


The jury is still out on long term or corneal exposure.

Steve
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[Laserman532]

I started working at Spectra-Physics when I was 22 years old. One of my jobs for years was final test alignment on argon & krypton lasers. I spent 8 hours a day (ore more) hunched over open cover cathodes running while aligning lasers. I left in 1984 so that was 6 years of "average" exposure of the intense uv from cathodes. 12 years later in 1996 about one week after my 40th birthday, I went blind in my right eye from a very quickly developing cataract. I had a lens implant surgery and total vision was restored in that eye perfectly. Two years later my left eye developed a cataract very quickly and I also had a lens implant in that eye restoring my vision perfectly. I am not saying that UV exposure was the cause, but I dont have enough data to say it wasnt.

Understand that "WE ARE" the lab rats for cumlitave laser exposure.

I am now 52 years old and waiting to see if any ill effects come my way because while also working there we were working with laser dyes and DMSO at the same time. We use to wear the dye on our hands as a badge of honor to show how hard we were working. The more different colored laser dyes you had on your white lab coat was also considered "experience" I also used UV lasers in the 3.5 watt watt range to pump laser dyes...double wammy!

Stay tuned for updates....