Like many of you, I never head off to a star party without my trusty green laser pointer close at hand. These great and increasingly inexpensive gadgets have really revolutionized how we point out sights in the night sky to newbies and veteran skywatchers alike.
Also, like many of you, I'll admit to having a little "beam envy" — I sometimes get into friendly little competitions to see who's got the brightest laser. It's well known that the output can vary dramatically from one laser pointer to another, no matter what the label (or the dealer) might say. In fact, I've sometimes wondered why the beam on a particular model looked so anemic.
Now I know, thanks to a recent article prepared by researchers at the U.S. National Institute of Standards and Technology. It turns out that these laser pointers don't create green light from the get-go, but instead generate a collimated beam of infrared energy that's converted to green light when it passed through a special crystal. (Try saying "neodymium-doped yttrium orthovanadate" three times — I dare you!)
In most units, a careful combination of transmissive and antireflective coatings, together with an infrared-blocking filter right at the end of the beam path, keeps all the infrared energy bottled up — all that comes out of that little hole is pure green light at a wavelength of 532 nm.
But the researchers found that really inexpensive green lasers not only have very poor conversion efficiency but can also lack an infrared filter altogether. In their study, the researchers tested a flawed unit that releases nine times more infrared energy than green light. Normally this wouldn't be a concern — anyone who accidentally gets an eyeful of laser light will blink and recoil instinctively. However, shine this defective laser at a modern energy-saving window, and the infrared beam gets reflected back at you (or someone else nearby) even though the green light passes through.
I've seen some lasers powerful enough to burn a hole through a black plastic trash bag, and one dealer's website even touts the ability of its products to pops balloons, light matches, and set paper afire. I just checked my green laser, and it had no effect during an impromptu "trash-bag" test. So is it safe, at least from an infrared standpoint? Not necessarily, says Alessandro Restelli, one of the paper's authors. The filterless laser tested by the NIST team pumped out 20 milliwatts of infrared energy, not enough to pop a balloon but plenty strong enough to cause retinal damage.
Nor does a weak beam mean you've got a stealth weapon in your hand. Restelli notes that many factors can influence green-light output strength, among them variations in the doped crystal, its operating temperature, optical alignment, and the quality of the surface coatings.
One way to ensure that your laser is free of spurious emission is to use the novel setup detailed in the NIST article and illustrated at right. By using a CD's surface as a diffraction grating, the green and infrared components can be separated. Your eye can't detect the diffracted peaks of infrared energy at 1064 nm shining on the paper screen, but a simple webcam that's had its infrared filter removed can. I encourage astronomy clubs to build one of these gizmos to test their members' laser pointers. Also, it'd be a good idea to give everyone a tutorial on how to use them safely.
The lesson here is "buyer beware." If you're in the market for a green laser pointer, buy it from a reputable dealer who'll both certify the output wattage (lest you be disappointed with its performance) and insure that the unit is infrared free.