- Jan 7, 2002
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In the last 20 years, green lasers have shrunk from table-size lab equipment to pocket-portable presentation tools (not to mention cat toys). But making laser pointers a household item may have come at a cost. A new study from the National Institute of Standards and Technology reports that some cheap laser pointers can emit more than 10 times as much invisible infrared light as bright green light, making them more likely to blind kids and pets.
Its a serious problem, said NIST physicist Charles Clark, a coauthor of the study. If green goes into your eye, youll probably blink because you can see the green. But with infrared, you wont blink. The first indication that you have that infrared is coming in is that youd start to lose your vision.
Luckily, theres a science fair-worthy way to test your laser pointer for safety. All you need is a digital camera, a webcam, a CD and a few paper cups.
When green laser pointers first hit the market in the 1990s, they would set you back about $400. These days, they go for as low as $7.75 on Amazon. The average pointer makes its bright beam of light in three steps, each of which was a highlight in laser development when it first came out. Its like a little lesson on quantum physics all in itself, Clark said.
The trick is to convert two photons of long-wavelength, low-energy infrared light into one photon of short-wavelength, high-energy green light in a process called frequency doubling. First, two AAA batteries fuel a diode laser similar to a standard red laser pointer which emits infrared light at a wavelength of 808 nanometers. That light gets funneled into a crystal of a material called neodymium-doped yttrium orthovanadate, which is common to lab lasers. The crystals electrons respond by getting excited and emitting infrared light at 1064 nanometers, which goes through a second crystal made of potassium titanyl phosphate. That crystal combines two infrared photons into one photon with half the wavelength and double the energy, the familiar 532-nanometer green light.
The standard green laser pointer also includes a shield to keep any of the infrared light from escaping. But in the pointer that Clark and his colleagues examined, the shield was entirely missing. There wasnt even a holder where a shield should be.
That was a design choice, said NIST physicist Edward Hagley, a coauthor of the study. What we think happened is, if one of the suppliers decides to get rid of the filter and save 50 cents, they can reduce the price a little bit and drive everybody out of business. Then everybody else has to do the same thing.
Hagley noticed the problem when he bought three $15 laser pointers last December as Christmas presents for his in-laws. Each pointer claimed to emit 10 milliwatts of power, but one of them glowed with a much dimmer green beam. Not only was the dim pointer missing its infrared shield, it also turned out to emit 20 milliwatts of invisible infrared light during normal use. The extra infrared is probably due to a misalignment between the diode laser and the crystals, making the conversion from infrared to green light less efficient.
The total power isnt that much, about a thousandth of the output of a typical flashlight, Hagley noted. The danger is that laser light is a focused beam of a single wavelength of light, meaning 20 milliwats is enough to burn a hole in your retina before you blink.
It is a very big safety hazard, Hagley said. People who have these laser pointers shouldnt think theyre safe just because theyre not outputting much green. I know my kids would stick them right in their eyes. And that would be bad.
So before you let your cat chase a laser pointer beam across the floor, the authors suggest a do-it-yourself test to see how much infrared light your laser puts out. Most digital cameras or camera phones are sensitive only to visible light, but webcams can take images of light well into the infrared portion of the spectrum (or can be easily modified to do so). The authors suggest cutting a few notches in two paper cups, one to stabilize the laser and the other to hold a CD vertically. The CD acts as a diffraction grating, which spreads the laser light out across all its wavelengths.
Place a piece of paper with a hole in it between the laser and the CD, and aim the laser through the hole. The light reflects off the CD and onto the paper, where it can be photographed by either the digital camera or the webcam. Comparing the images reveals how much invisible light your laser produces.
The authors emphasize that you should always take standard safety precautions when doing experiments with lasers: Dont look into a direct, reflected or diffracted laser source; keep your eyes well above the laser level; wear safety glasses. The precautions are spelled out in detail in the NIST paper.
Its a simple setup, but its impressive even to other physicists. Their experiment design is very clever and illustrates the problem brilliantly, commented laser physicist Thomas Baer of Stanford, who was not involved in the study.
This isnt the only possible test, Clark added. We wanted to crowdsource a solution to the problem, he said. There are other methods people may think up. Having a method out there might stimulate community activity, quantify it further, and perhaps put pressure on the manufacturers to use safer designs.
Image: 1) Flickr/sara sotin 2) NIST 3) NIST. The top image shows the visible diffraction pattern; the bottom shows extra light in the infrared.
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