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In our Physics lab we have a 1 milliwatt (0.001W) helium neon laser. Despite the low power, we were cautioned not to even look at reflections of the beam as it could cause permanent eye damage - why is such a low power level hazardous? I have a 3W LED which I can look directly into which is more than 3,000x more powerful.

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This is no joke, ok? You should really be careful. As they usually say "Don't look into the laser with your one remaining eye!" –  Rafael Jan 26 '11 at 17:49
    
@Rafael I will never look at the beam! I am only curious why it is so dangerous at a low power level. –  Thomas O Jan 26 '11 at 18:28
    
For those thinking of comparing to a laser pointer, note that a bench-top HeNe device is usually very well collimated, while the Laser Emitting Diode (and the distinction between Laser LED and light LED is going to be problem going forward) used for pen-sized pointers is only pretty well collimated. –  dmckee Jan 26 '11 at 18:36
    
The biggest danger from a 1 mW HeNe is from the high voltage power supply, not the laser light. –  nibot Jan 27 '11 at 0:39
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A 3W LED!? My goodness. Are you sure you're not talking about an LED lamp (i.e. a bundle of bright LEDs). That, or clearly LED technology has come a long way since I last used them three years ago! –  Noldorin Jan 27 '11 at 2:45
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Dear Thomas, the diameter of the beams of these HeNe lasers is between 0.5 and 1 millimeter, so the power 1 mW is coming to $10^{-6}$ squared meters or so. The ratio of power and area is $10^{-3}/10^{-6} = 10^{+3}$ Watts per squared meter.

On the other hand, when a 3W LED is watched from the distance 0.1 meters, the power of 3 W is divided to $4\pi R^2 = 0.13$ squared meters, so the power per units area is 3/0.13 = 23 Watts per squared meter. In both cases, the eye will refocus the beam so the power per unit area of the retina will actually be much higher in both cases.

In this estimate, the laser has about 40 times higher power per unit area than the LED light bulb. Of course, it's small, so it will only burn one small point in your retina, but that's a bad enough problem. ;-)

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It also matters that the laser light is coherent, while the LED is not. –  ptomato Jan 27 '11 at 0:11
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@ptomato : the coherence of the laser has only an indirect role, in the sense that this coherence is what allows to keep the the beam small. Coherent light is not dangerous in itself –  Frédéric Grosshans Jan 27 '11 at 9:00
    
There is a typo above: $10^{-3}/10^{-6} = 10^3$ not $10^{-3}$. –  ja72 Jan 27 '11 at 21:01
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@Frédéric Grosshans, well yes, but it matters in how the eye refocuses the beam, for example. I just thought this was the best answer except that it didn't mention coherence or collimation, so I wanted to add that. –  ptomato Jan 28 '11 at 8:46
    
@ptomato: OK, and maybe also the speckle related thing found in the other answer. Sorry for not seeing this. –  Frédéric Grosshans Jan 28 '11 at 18:10
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As far as I know you have to worry about how much energy being deposited per surface area. And the area of the "hotspot" of the laser can be very small. So the deposited energy it enough to kill cells on your retina.

Actually there is a whole article on Wikipedia about it.

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+1 for wikipedia link –  Frédéric Grosshans Jan 27 '11 at 9:01
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Because the light from the laser is collimated. This means that you have all this energy focused on a small spot. The larger power from a non collimated light source is spatially spread and the effective amount of energy that reaches your retina per time is much smaller.

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The laser is most likely not dangerous at all when used normally, because it is very low power and within the visible portion of the spectrum. The idea is that it is sufficiently low power that your blink reflex will protect your eye before any damage occurs.

The danger of a given laser source depends on its power, power density, and wavelength, as these determine what part of the eye will be damaged, how quickly, and whether you will blink. The Wikipedia article on Laser Safety gives a fairly comprehensive summary of the matter.

As others have written, the main reason why lasers are dangerous in general is because they can concentrate a large amount of power into a small area. The school most likely wants to instill a sense of respect for lasers and inspire good operating habits (both admirable goals) even though in this case they are probably over-stating the danger.

Source: I have to take a laser safety course every year for my job. I work with lasers every day.

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P.S. I wouldn't want to stare directly into a 3W LED, myself. –  nibot Jan 27 '11 at 0:31
    
It causes temporary blindness and is safe to look at (what's the term, when you can only see a bright spot for about 30 seconds?) –  Thomas O Jan 27 '11 at 16:06
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A laser is coherent, collimate and intense

It may be a small amount of power but all that comes down a precise beam, in the same phase, at the same time. Your eye is a lens and focuses it into an even smaller spot, burning the retina.

Try staring at a 200w bulb for 1 minute, note the temporary black spots when you look away. That 200w is radiating in all directions, is incoherent, and wastes a huge amount as heat. The 0.1mw laser delivers more in a shorter time period to your retina than the bulb will

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1mW is not enough to do damage, its more of just an annoying distraction. Going to middle school then high school, 1997-1998 they ended up being banned because of people shining lasers into other peoples faces. Once you go beyond 5mW the blink reflex no longer offers protection.

I perminently damaged the CCD chip of a $200 digital camera with a 30mW green laser. I own a 1,100mW blue laser so powerfull, the dot on a wall is bright as the sun to a digital camera, and without eye protection, blue light is espescially dangerous at high levels because of photo chemical reactions in the eye, even just looking at the dot. 1,100mW will burn skin instantly, and even start fires, in the eye, tissue will get hot enough to boil the fluid likely as 1,100mW will boil coffee at point of beam contact. And I shined the beam on a drop of water on wood, and the drop sizzled away.

Lasers can be dangerous, but the 1mW red diode lasers you can get for $2 are harmless.

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The danger is that the 1mW laser you buy for a $1 are not exactly carefully calibrated 1mW, they may be 3 or 5mW. They might also be emitting dangerous amounts of near IR which you can't see and your blink reflex doesn't protect you from –  Martin Beckett Jan 22 '12 at 1:37
    
Just DPSS Lasers do have IR radiation. For cheap green pointers, it is a serious problem. But not for HeNe, red laser diodes etc. –  Alex1167623 Mar 12 '12 at 20:23
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If you have ever looked at a laser beam expanded and projected on a screen you may have noticed it has a speckled appearance. A red HeNe beam spot after it passes through a divergent lens is not a uniform illumination. Those spots are little regions with much higher irradiance. If that beam goes directly into the eye, where the lens focuses the light, some retinal cells can be zapped. If you get even low power laser beam directly into the eye with considerable repetition you can accumulate retinal damage.

Of course now this is compounded by the green lasers of considerably higher power that knuckleheads use to play as Starwars light sabers or worse to flash at aircraft. These are only to point at the sky, where it reflects off the sodium layer and lets one point out stars etc

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This is actually incorrect. The phenomenon of "speckle" does not represent intensity variations in the beam. Speckle is actually an interesting effect which results from coherence. When a coherent beam scatters from a rough surface, an interference pattern is produced, which is the speckle pattern you see. The intensity can actually be quite uniform, but the phase is a randomly distributed complex value. –  Colin K Jan 26 '11 at 19:10
    
Then the little laser safety course I took some years ago had it wrong. –  Lawrence B. Crowell Jan 26 '11 at 19:25
    
Edited xxxxxxxxxxxxxxxxxxxxxxxx –  Georg Jan 26 '11 at 19:26
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To convince yourself that the speckle arises from coherence effects on the surface get a bright beam, point it at a surface far enough away that the spot has non-trivial size, pick a bright sparkle, and gently move the beam around such that the spot you picked stays inside the lit up area. Beam moves but the "hot" spots remain stationary with respect to the surface. –  dmckee Jan 26 '11 at 19:33
    
@dmckee, @Colin K : The origin of the speckle does not affect the eye security aspects of the answer, no ? –  Frédéric Grosshans Jan 27 '11 at 9:10
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