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Today I got some medical laser treatment to treat some tendon irritation. The doctor used a red or IR laser with 7 W of power. He casually applied the laser onto the knee and was panning over the area. I then asked about the laser power and he said that he is going to apply 300 J there. It was done within a a minute or so. And I could feel that the spot got warm.

In my studies, I have performed a laser experiment (magneto optical trap) with a 12 mW laser locked to some IR rubidium frequency. There the people from the university have told us to be super careful, wear safety goggles and not look into the laser at any time. If we think that we have looked into the laser, we should get a retina scan.

The tutor who did the experiment with us told us that when you know what you are doing, you can fine-tune the mirrors without the safety goggles. Being a theoretician, I just wore the goggles :-).

Either way, I find it extremely fishy that a 12 mW laser is to be considered harmful and the 7 W medical laser seemed to have no precaution whatsoever. I think I never saw the end of the optical fiber directly. My eye lens does not seem to be opaque or anything like the stuff they told us at the safety briefing. They especially told us that even scattered light can be harmful if the laser is powerful enough.

Even if the beam is divergent, 7 W will be still quite a lot in at a distance of around one meter. So what is the reason that this powerful laser can be operated without anyone wearing goggles, without the doctor telling me anything (not even a “close your eyes”)?

In case it matters, this is in Germany.

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    $\begingroup$ When I did some analogous laser therapy, they indeed gave me goggles... $\endgroup$ – yuggib May 12 '16 at 14:05
  • $\begingroup$ 1 mW is a safe range for laser light, this is the approximate power of a laserpen, which is still not something you want to shine in your eye. So, it's no wonder you have to wear goggles with 12 mW, let alone 7 W. Granted, the chance that something aimed at your knee will hit you eye is extremely low, but not wearing goggles here just seems like negligence. $\endgroup$ – Feyre May 12 '16 at 14:10
  • $\begingroup$ I layed down, the doctor was at my knee. He let the laser shine downwards. The light would have to be reflected around 90 degrees in order to get in my eye. And I was looking at the doctor at first, so the optical fiber was in my field of view. My girlfriend uses like a 1 W green laser for her experiment. And they have the whole experiment mounted in a black box which they close every time they go above say 1 mW. $\endgroup$ – Martin Ueding May 12 '16 at 14:20
  • $\begingroup$ Seems really weird. Anything over ~0.5 W has the chance to cause permanent damage. These are class 4 lasers, and you should always wear goggles when near them. web.stanford.edu/dept/EHS/prod/researchlab/radlaser/laser/… $\endgroup$ – Feyre May 12 '16 at 14:31
  • $\begingroup$ I guess the difference is that the 7W laser is highly uncollimated. While a collimated beam retains its power density at huge distances, an uncollimated one loses the power density proportionally to square of distance to it or so (depending on the angle of divergence). $\endgroup$ – Ruslan May 12 '16 at 16:27
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Lasers used in medicine can have a range of different wavelengths depending on their application, and these wavelengths will be absorbed differently depending on which part of you they are being used on... Unfortunately this means that it isn't enough to just consider power when it comes to laser safety, but also the wavelength, duration of the light pulses at the laser output, and their energy. Telling us that the laser emitted "red or IR" radiation at about 7 W is unfortunately nowhere near specific enough. Laser classification is standardized and takes these details into account. You can find tabulated information on the subject here, but often precautions are country specific.

I mentioned that wavelength was important. For example, 1 $\mu$m light is considered infra red as we can't see it (although it is often treated as visible because there are no special experimental precautions which need to be taken, unlike when using ~10 $\mu$m wavelengths). As such, we have no blinking reflex to it, and yet it passes straight through our cornea, through the fluids in our eye, and to the retina where it can cause a lot of damage. However, using a laser which has a wavelength of 1.3 (or 1.5) $\mu$m but is otherwise equivalent might not cause any damage as it is absorbed by the various fluids in your eye where the small amount of heat quickly dissipates, so never reaches the retina. Such wavelengths are generally referred to as "eye safe", although this is a bit of a misnomer as high average and peak power lasers with eye safe wavelengths can still ablate the cornea.

The only answer that it is possible to give here is a guess: either the Dr. wasn't wearing goggles because s/he was hugely irresponsible (unlikely), or the laser operated around an eye-safe wavelength. However, as you point out, 7 W is still a lot of power*.

It was mentioned in the comments that the divergence of the output beam is also important, and this is definitely true. If the light was coming from a multimode fibre and wasn't collimated at the output, then we can assume that $\text{NA}\approx 0.2$. Assuming (for the sake of argument...) a top-hat beam profile from this fibre, this means that at a distance of around 1 m between your knee and your eye, the beam will have a diameter of about 40 cm. This gives an intensity of 55.7 W/m$^{2}$. Assuming that the radius of your pupil during the procedure was about 2 mm, your pupil would cover an area of $12.5\times 10^{-6}$ m$^{2}$. This means that when looking at the beam from the multimode fibre directly at a distance of 1 m only around 700 $\mu$W of power would enter your pupil. This isn't much even for visible wavelengths, let alone eye safe ones.

It's important to note that this high divergence doesn't mean that the laser is a lamp, as was suggested in a comment. Also, the fact that it is coherent definitely can hurt, especially if you are talking about spatial coherence, which the divergent laser beam will still have. A high spatial coherence will allow the beam to be focused more tightly than if the spatial coherence were low, and will therefore result in much higher power densities than spatially or temporally incoherent lamps would provide. If this happens in your eye with wavelengths around the visible or 1 $\mu$m region then that can be very bad news even for low power levels.

*Edit: It could also be that the laser manufacturer has labelled the box with the highest-power light source that it contains. It's possible that there is 7 W of light somewhere in there, but it might be a pump source, or it could be attenuated either by design or loss before being emitted by the fibre.

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    $\begingroup$ Another thing to keep in mind is that skin is not particularly specular. $\endgroup$ – WhatRoughBeast May 14 '16 at 21:06
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I have talked to the manufacturer of the laser. They told me that one must use safety goggles at all times. Of course, they have to say this by law. Even if there is some mechanism that makes the laser safe at usual operation, they have to be careful. They ship the device with safety goggles. On my last visit I checked them closer and they are class 5 to 7 in the range of the laser. So that is good I think.

Then the doctor talked to the sales representative. That person said that he has given many seminars with the laser and only wore goggles when there were shiny objects that could reflect the laser. So far he still has his vision. On some occasions he noted that he had some visual effects at the evenings. The description sounds like visual irritation by laser speckle patterns. The doctor has used the laser this way lots of times as well and has no eye problem.

Either way I had my eyes checked out, they are fine. So although there were 7 Watt of power involved without goggles once, nothing happened. That does not prove that it is safe, of course.

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  • $\begingroup$ Ask for goggles if you have a similar procedure again. As someone who works with these sorts of systems daily, I find the lazy "I still have my eyesight so it must be fine!" attitude absolutely terrifying... Better safe than sorry! $\endgroup$ – user113857 May 23 '16 at 23:16

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