Some Pointers on the Use of Laser Pointers
By Richard Tresch Fienberg
A recent addition to the backyard astronomer's toolkit has been flagged as a potential weapon in the terrorist's arsenal. The humble laser pointer, used by thousands of skygazers to show beginners the way to stars and constellations, is coming under fire from US federal and state authorities following several recent incidents in which laser beams have "painted" aircraft in flight.
In the most notorious case, on January 4, 2005, a New Jersey man was arrested after allegedly shining a laser at a small passenger jet on approach to a nearby airport. The suspect claims he had been showing his daughter around the night sky, using his laser pointer to direct her gaze at particular stars and planets. Now he faces a possible jail term and six-figure fine. The incident sparked a media frenzy, with many articles appearing alongside other news from the War on Terror.
Depending which newspapers you read or which television stations you watch, you may hear that handheld laser pointers — commonly available for less than $100 from a host of retailers and online dealers — are either perfectly harmless or capable of bringing down a jumbo jet. Naturally, the truth lies somewhere in between. Used properly, laser pointers are quite safe. But used improperly or maliciously, they can be dangerous indeed.
Here is some basic information about the laser pointers typically used by amateur astronomers, along with some tips on using them safely.
Laser-Pointer Basics
The pointers favored by stargazers use a neodymium diode laser and emit a green beam at a wavelength of 532 nanometers. At a given power setting, such lasers appear much brighter than the older helium-neon lasers, which produce a red beam at wavelengths longer than 630 nm. The reason is simple: the human eye is much more sensitive to green light than red light.
Except on nights of exceptional clarity and near-zero humidity, if you shine a green laser pointer into the sky, you can follow the beam hundreds of meters up. To you and anyone standing around you, it looks like the beam ends at whatever star or planet you're aiming at. This makes it really easy to show someone a particular celestial object. Just point the laser at it and say, "Look there!"
Most laser pointers shine only as long as you hold down a button. But some models have a "constant-on" setting. These are becoming popular as pointing aids for telescopes. Once the laser is mounted and coaligned with the scope, you just move the scope around till it's pointing at your target star, and when you look in the eyepiece, there it is!
The green laser pointers in common use among astronomers and the general public have a power output of 5 milliwatts. Compared with incandescent light bulbs, which draw tens to hundreds of watts (not milliwatts), this sounds like very little. But laser light is highly concentrated into a very narrow beam. Moreover, this beam is highly collimated, meaning it diverges (spreads out) very slowly. Laser pointers typically emit a beam about a millimeter in diameter. Even as far as a kilometer away, the beam is no more than a meter across.
According to engineer Samuel M. Goldwasser, who maintains an extensive Web site about lasers called Sam's Laser FAQ, if you were to look directly into a laser-pointer beam from a mile away, it would appear as bright as a 100-watt bulb seen at a distance of less than 100 feet. Most people would find such a bright light very uncomfortable and would instinctively blink and/or turn away.
Lasers used in industrial applications and outdoor light shows are vastly more powerful than personal handheld units. There is no question that such lasers can be harmful to pilots, so their use is regulated, and airline pilots are trained to avoid them. The problem with personal laser pointers is that there are millions of them in consumers' hands, and there is no way to guarantee their safe use. In many states, it is already a misdemeanor to shine a laser pointer at a person or vehicle. Now that these devices are being talked about in the context of airplanes and terrorists, there's a very real chance that they will be outlawed. Indeed, 5 mW green lasers are already banned in the United Kingdom.
Safety First
Direct viewing of a laser-pointer beam, even briefly and at a distance of a kilometer or two, has the potential to cause temporary flashblindness — the same effect you get right after a flash photo is taken — or afterimages. These effects last anywhere from seconds to minutes. Glare, which is a reduction or loss of central vision, lasts only as long as exposure to the beam. All these effects could be disastrous if they struck a person operating machinery, driving a car or truck, or flying a plane. Fortunately, there have been no reports of laser-related accidents of this type so far.
To help keep it that way, we offer the following safety tips for amateur astronomers and anyone else using a laser pointer. These are based on suggestions from the Laser Institute of America:
Laser-Pointer Safety Tips
Laser pointers are designed to illuminate inanimate objects. Never shine a laser pointer toward any person, aircraft, or other vehicle.
Never look directly into the beam of a laser pointer of any type.
Do not allow children to use a pointer unsupervised. Laser pointers are not toys.
If your telescope is equipped with a laser pointer that has a "constant-on" setting, do not leave the instrument unattended with the laser switched on.
Do not aim a laser pointer toward mirrors or other shiny surfaces. The reflected beam may inadvertently strike someone in the eye.
Do not aim a laser pointer skyward if you hear or see an aircraft of any kind flying overhead.
Be aware of irresponsible uses of pointers so that the psychological effect will be minimized if you happen to be illuminated by one.
Do not purchase a laser pointer if it does not have a "caution" or "danger" sticker on it identifying its class. Report suspicious devices to the authorities.
"Green laser pointers are the best way we have found to point out objects in the night sky to the public," says Monty Robson, a commercial airline pilot and avid amateur astronomer. He regularly conducts observing sessions for high-school students and their families at the John J. McCarthy Observatory in Connecticut. "I would be very disappointed," Robson adds, "if these useful educational tools became restricted by law." Amateur astronomers can help minimize the risk of such an outcome by using common sense, following the safe practices outlined above, and educating others about the safe use of laser pointers.
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Rick Fienberg is editor in chief of Sky & Telescope. He enjoys using a laser pointer to introduce the constellations to friends and relatives who visit his dark-sky observatory in New Hampshire. |
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