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Does a laser beam have to hit the eyes in order to damage them?

Or can a persons eyes get damaged by looking at a beam that goes past their eyes (e.g. looking at a laser beam moving inside an enclosure)?

If just looking at beam that goes past a person can damage the eyes, what factors would affect whether it is potentially dangerous? (like how strong would the beam have to be and how long would a person have to keep looking at it?)

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  • $\begingroup$ For class 4 lasers, your eyes can be damaged sometimes even if you look at diffuse reflections of the beam. That depends on the power output of the laser, of course $\endgroup$ – Jim Jan 13 '15 at 15:34
  • $\begingroup$ Like @Jim said, it depends of course on the power of the laser. A classification scheme exists with safety recommendations for each class. The wikipedia page is informative. I quote: "Even moderately powered lasers can cause injury to the eye. High power lasers can also burn the skin. Some lasers are so powerful that even the diffuse reflection from a surface can be hazardous to the eye." $\endgroup$ – andrepd Jan 13 '15 at 15:39
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The Wikipedia article on Laser Safety completely covers how lasers damage the eye, what types of lasers there are (in terms of damaging potential), and other related topics. So I'll give the brief summary of relevant info here.

There are 4 main classifications of lasers (aptly named class 1 through class 4).

Class 1 lasers are completely safe. This is usually because the laser beam itself is hidden, but for class 1 lasers with open beams, you can stare into them as long as you want without damaging your eye (unless you do something stupid like focus the beam with a telescope).

Class 2 lasers are safe because the time it would take to damage your eye if looking directly into it is greater than the time it takes for your brain to say "Ah, bright light! Blink, you dummy!" and then carry out the blink, thus protecting your eye. Again, don't be stupid and look at the beam through a magnifying glass or something.

Class 3 lasers should be handled carefully. They can damage the eye if you directly view the beam (although the low-power end of this classification carries a low risk of eye damage). It is perfectly safe to view the diffuse reflection of a class 3 laser (that is a reflection off a non-shiny surface. It is also perfectly safe to look at the beam indirectly (such as using mist or smoke to illuminate the beam path). Protective goggles are a recommendation when there's a chance of directly viewing the beam (that is, putting your eye right along the path of it).

Class 4 lasers are the most dangerous. Anything too powerful to be a class 3 is listed as a class 4. You can damage your eyes by looking at a class 4 laser beam in any way (directly, indirectly, even looking at the laser dot on a surface can damage your eyes). Protective goggles are always required. Class 4 lasers can even burn your skin or clothes and ignite combustible materials. There is little chance of most people encountering a class 4 laser, but if you do, make sure you are protected, don't walk in front of the beam, and most importantly, don't be stupid.

The main factor in determining the class of a laser is its output power. Class 2 lasers don't go higher than $1~\rm mW$. Class 3 lasers are separated into 2 subcategories; 3R for lasers below $5~\rm mW$, and 3B for lasers up to a continuous beam power of $500~\rm mW$. Above $500~\rm mW$ is class 4 lasers. I've worked with a few class 4 lasers myself, the lowest power one was $700~\rm mW$ and the beam was warm to the touch even then. So above $500~\rm mW$, no-touchy.

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  • $\begingroup$ I trust the safety guidelines, but I'm missing something because I don't understand how the diffuse reflection from even a 10 W laser could be more dangerous than a 60 W incandescent light bulb, which also emits ~10 W of visible light in all directions. $\endgroup$ – user1247 Jun 16 '16 at 15:37
  • $\begingroup$ @user1247 I usually stick to trusting the safety guidelines too. I have often wondered the same thing myself, but not being a doctor and not knowing as much about the eye, I don't myself know exactly what makes the diffuse reflection so dangerous. It is worth looking up though $\endgroup$ – Jim Jun 22 '16 at 12:54
  • $\begingroup$ After thinking about it since my comment, I think the reason is because 1) the beam spot can be diffusely reflected only a few cm from someone's eye if they are in the right position (same is technically true of a light bulb, but it would hurt to press your eye against a light bulb), and 2) practically most surfaces are not 100% diffusely reflecting, and even a few % specular reflection can be really bad. $\endgroup$ – user1247 Jun 22 '16 at 16:09
  • $\begingroup$ @user1247 you're on the right wavelength there (pun intended). I'd bet the guideline was implemented with the idea that most diffuse reflections are partially composed of many small specular reflections. But also it's worth mentioning that safety guidelines say you shouldn't look directly into a light bulb either. My guess is that looking at the diffuse reflection of a class 4 laser is like staring directly into a light bulb. It may not cause serious damage immediately, but you'll definitely be seeing spots at least. $\endgroup$ – Jim Jun 22 '16 at 17:28
  • $\begingroup$ Well, that doesn't make me feel safe and secure about the irony of my optometrist dilating my pupils and shining a ridiculously bright light in my eye that leaves worse spots than when I stare at a lightbulb! $\endgroup$ – user1247 Jun 22 '16 at 18:03
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Some of the light from the laser beam must strike your eye to do any damage, but if no light is reaching you you cannot see the laser at all. Laser light can be scattered from imperfections or dust on mirrors or other surfaces and this is generally what you see when you see a laser spot. If there is sufficient energy in this scattered light then it can still damage your eyes.

As Jim mentioned there are several classes of laser. Class 3 can damage your eye if viewed directly but not from scattered light. Class 4 lasers are more powerful and even diffuse reflections can be dangerous.

If you are using classs 3B or 4 lasers you should wear the correct lasers googles when using the laser and propably go on a laser safety course.

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  • $\begingroup$ Anybody who runs a class 4 laser would be foolish to let you in the room with it without very close chaperoning or extensive safety training. $\endgroup$ – The Photon Jan 13 '15 at 17:56
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Infrared lasers are used for industrial metal cutting. These lasers will badly harm any part of your body exposed to their light. kW infrared lasers are tested as weapon systems as well.

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Here is the authoritative, perhaps complex, answer: https://www.lia.org/store/ANSI+Z136+Standards

Lasers come in many wavelengths, EVEN THOSE our eyes cannot SEE. Nevertheless they can all hurt retinal neural tissue, the "video camera" built into our eyes.

An illustration: Let's say you have a smart phone, whatever kind. Usually you aim it so that the most important part of the shot is in the center. Now, imagine that, for some reason, the center of the light sensor has a defect and records only black. Would you keep that smart phone, or give it back for a new one? Of course.

But since it was designed into your head, you cannot give it back broken (to whom?). If you are lucky you have a second healthy eye.

So: Don't look into a laser with your remaining eye.

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