Scientific Method / Science & Exploration

Achtung! Test safety of laser pointers with paper cups, webcam

Your mom was right: those little laser pointers can be dangerous! Some …

In recent years, handheld green lasers have become the hardware of choice for everything from delivering lectures to entertaining cats, in part because the human eye detects their light more efficiently than their red counterparts. But researchers at the National Institute of Standards and Technology (NIST) have found that shoddy manufacturing processes can result in these lasers emitting up to ten times as much infrared light as they produce in the visible spectrum, creating a potential hazard. 

The NIST document describing these results starts out as a primer in laser physics, then turns into an exercise in homebrew hacking as the researchers describe a test for IR emissions that involves CDs, paper cups, and a webcam.

The physics of green

We'll do as the authors did, and cover the physics first. Right now, it's possible to make red and infrared diode lasers that are power-efficient enough to run on a pair of AAA batteries. At the moment, however, we do not have any such green diode lasers (although that may be changing). So how did these tiny green lasers come to market?

The trick involves an optical effect called Second Harmonic Generation (SHG). Given an input of laser light of one wavelength, some materials that are ordered on similar scales can take two low-energy photons and combine them into a single, higher energy one. In the process, the wavelength of light is cut in half, and the intensity of the light drops considerably. 

So, to get green light at a wavelength of 532nm, all one needs is a more intense source of light at 1064nm (in the infrared range) and an appropriate crystal—a crystal of potassium titanyl phosphate is apparently the material of choice for the green lasers on the market. Bam! Green laser pointer.

But implicit in that description is the fact that green lasers will need a fairly powerful source of infrared radiation, which they get via a diode laser containing neodymium-doped yttrium orthovanadate crystals (say that five times fast). 

This actually produces light at too short a wavelength, so a second bit of material is used to step things down to 1064nm. Then the potassium titanyl phosphate cuts the wavelength in half, turning the light green.

Even though there are losses at all of these steps, the whole system is cheap and efficient enough to fit within a battery's power envelope. This mere fact seems to still amaze the guys at NIST, who note, "Twenty years ago, 10mW of coherent green light could be obtained only from research laboratory-grade lasers that cost over $100,000 and occupied much of an experimental laser table. Today's green laser pointer delivers much the same performance for less than $20, in a package the size of a slim cigar, powered by two AAA batteries."

Safety costs money

According to NIST, though, there's a problem: it's cheaper still for the manufacturers to skip an important safety component, an infrared filter that ensures no unconverted light makes it out of the device. 

This feature gets even more important when manufacturing is shoddy enough that the three key components—laser diode, frequency shifter, and SHG material—aren't aligned properly. Any problems of this sort let infrared light escape, even as it ensures that the amount of green light produced drops.

As the visible light intensity drops, the laser is less likely to set off a blink reflex. That could potentially leave an infrared laser (producing a much higher power beam than the device's rating) pointing right into someone's eye, with the victim unable to detect it. The authors note that there's also a lot of infrared-reflecting glass that could separate the visible and IR portions of the laser.

Although these don't seem like common risks, this might be a case where a product recall could get any defective devices off the market. Unfortunately, these lasers are now cheap and ubiquitous, and rarely have detailed manufacturer and model information on them. So, the authors suggest that it would be tough to get problem lasers off the market.

They therefore suggest an alternative that, while just as likely to be ineffective, is seriously cool: they describe how to make a homebrew safety test.

Those CDs are still good for something

All you need is a CD, a few paper cups, a sheet of paper, and an infrared-capable webcam. Cut a few notches in cup #1 and you can nicely stabilize the laser. Ditto for cup #2, which should hold the CD in a vertical position. The CD acts as a diffraction grating, separating out all of the wavelengths of the laser. The sheet of paper goes between the two, with a hole cut out for the laser to pass through. Reflected light from the CD ends up spread by wavelength across the opposite side of the paper.

Now, as noted above, you can't actually see infrared light, which is where the webcam comes in. If you don't have one already, the authors recommend buying a night vision capable version, which will detect infrared light. If you've already got one that's not night vision capable, have no fear. It turns out all webcams can pick up IR, and the authors have downloaded instructions off the Internet that tells you how to remove the webcam's IR filter and convert it to night vision. They include these instructions in the document "for completeness."

Point the webcam at the paper and presto—the setup should be able to show whether there are significant amounts of light that are not visible to the unaided eye. And, if so, consider not pointing the laser at your eye.

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