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.