Green Laser Facts See the Light

Wednesday, August 4, 2010 @ 06:08 PM gHale

It seems a simple matter of safety, but green laser pointers have become a popular consumer item, delivering light that’s brighter to the eye than red lasers, but potential hazards exist from the inexpensive models.
In one case, a green laser pointer emitted almost twice its rated power level of light—but at invisible and potentially dangerous infrared wavelengths rather than green, said physicist Charles Clark at the National Institute of Standards and Technology (NIST).
A new NIST technical note describes the nature of the problem as well as a home test using an inexpensive webcam that can detect excess infrared light from green lasers.
Late last year, the research team purchased three low-cost green laser pointers advertised to have a power output of 10 milliwatts (mW). Measurements showed one unit emitted dim green light but delivered infrared levels of nearly 20 mW—powerful enough to cause retinal damage to an individual before he or she is aware of the invisible light. NIST’s Jemellie Galang and her colleagues repeated the tests with several other laser pointers and found similarly intense infrared emissions in some but not all units.
The problem stems from inadequate procedures in manufacturing quality assurance, according to the research team. Inside a green laser pointer, infrared light from a semiconductor diode laser pumps infrared light at a wavelength of 808 nm into a transparent crystal of yttrium orthovanadate doped with neodymium atoms (Nd:YVO4), causing the crystal to lase even deeper in the infrared, at 1064 nm. This light passes through a crystal of potassium titanyl phosphate (KTP), which emits light of half the wavelength: 532 nm, the familiar color of the green laser pointer.
However, if the KTP crystal becomes misaligned, little of the 1064 nm light converts into green light, and most of it comes out as infrared. Excess infrared leakage can also occur if the coatings at both ends of the crystal that act as mirrors for the infrared laser light are too thin.
The NIST team said there could be a solution to this problem incorporating an inexpensive infrared filter at the end of the laser, which could reduce infrared emissions by 100-1000 times depending on quality and cost. Although these filters exist in modern digital cameras and more expensive green laser pointers, they often not installed in the inexpensive models.
The team demonstrated a home test that laser hobbyists could conduct to detect excessive infrared leakage, by using a common digital or cell phone camera, a compact disc, a webcam and a TV remote control.
Regardless, they said owners of the devices should never point the lasers at the eyes or aim them at surfaces such as windows, which can reflect infrared light back to the user, a particularly subtle hazard because many modern energy-saving windows have coatings designed specifically to reflect infrared.

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