dangerous laser pointers

August 11, 2010 at 9:36 am | | lab safety, science and the public, science@home, stupid technology

NIST is warning us that some cheapo green laser pointers might be unfiltered and dangerous. Some manufacturers skip installing the IR filter, thus making a laser pointer that has a high-power invisible beam along with the green light.

The “green” of green laser pointers is 532 nm, doubled frequency of the 1064 nm emission from a neodymium (e.g. Nd:YAG, Nd:YLF, or Nd:YVO4) laser. A diode (e.g. 800 nm) pumps the neodymium laser, which emits 1064 nm light; a doubling crystal produces the green 532 nm light. But the doubling crystal is not 100% efficient, so an IR filter is necessary to block the remaining 1064 nm light that isn’t doubled (as well as block the 800 nm pumping light). The plot above shows how much 1064 nm light escapes if the filter is removed: it’s much more than the green light—if the 532 nm is 20 mW, the IR might be as high as 100 mW, certainly potentially damaging to the eye!

IR is especially dangerous laser light. First, it is invisible, so it is more difficult to identify and avoid stray beams. In this case, that’s less of a worry, because the green beam coaligned is visible. However, the second reason IR is dangerous is that, because it is invisible, you can’t tell how bright it is (see below). The final reason IR is dangerous is the biology of the eye, which is transparent to IR light, and focuses it to the retina (the nerves). IR can easily burn the retina permanently (causing blindness), or burn other parts of the eye or skin.

The simple method NIST suggests we can use to test our laser pointers is described in the announcement. Basically, they use a CD as a diffraction grating and a cheap webcam. The sensor of a digital camera is sensitive to IR light, but usually has a filter to see only visible; it is simple to remove the IR filter of a cheap webcam to make an IR sensitive detector. (Unfortunately, the sensitivity cuts out before 1064 nm, so the camera can only see the 800 nm pump light). The picture above shows the diffraction of the visible light with a normal digital camera; the bottom image is using the IR webcam. You can see the extra diffraction spots from the 800 nm light. Note also how much brighter the IR light is from the laser: even though you can’t see it with your eyes, it is very bright and dangerous.

By the way, my favorite line in the NIST report is the following: “The infrared light spreads out beyond the green, which could be injurious, for example, to a cat closely chasing a spot of green light.” Actually, that’s kinda sad: I hope the NIST folks didn’t discover this problem after they blinded their pet.

5 Comments »

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  1. I’m not actually sure that the camera can’t see 1064. It’s definitely much less sensitive for it, but I have seen 1064 nm light with a moderately priced CCD (not “scientific grade” but not a cheap webcam). The problem is that the 1064 nm light will be hidden by the 532 nm light, because the 2nd order 532 nm hits the paper at the same spot as the 1st order 1064 nm, and the camera is much more sensitive to 532 nm. They might see some 1064 nm light if they blocked the 532 nm light.

    Comment by MRW — August 11, 2010 #

  2. sure, but if the 1064 really is that much higher power, it should be very bright and overcome the 532 light. i suspect that the webcam can detect 1064, but with very low sensitivity, so it doesn’t appear bright. also, i don’t know if they used a CCD or CMOS detector, or which is more sensitive to 1064.

    Comment by sam — August 11, 2010 #

  3. i scooped wired:
    http://www.wired.com/wiredscience/2010/08/laser-pointer-hazard/

    Comment by sam — September 2, 2010 #

  4. […] Blue: You’re a bad-ass. You don’t care that blue lasers are more expensive and slightly harder to see, you want the audience to know that you’re a real laser jock. (Or maybe you’re worried about leaking 1064 nm from green laser pointers.) […]

    Pingback by Everyday Scientist » what your laser pointer says about you — March 10, 2011 #

  5. […] here. If I get a chance to try this out I’ll post a build log on it. This uses a DVD like similar low cost spectrographs have used in the past. I’m quite interested to try something like this to look at micro brew vs. name brand beers. […]

    Pingback by $35 Spectromoeter Project on Kickstarter | Austin's Imaging Blog — August 30, 2012 #

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