Many consumers like green laser pointers; their popularity increased since they are much brighter than red lasers. However, recently stories about the potential hazards of these inexpensive models swamped the Internet.
Now, a new study published by a team from the National Institute of Standards and Technology (NIST) tries to estimate the real amount of occurrences in which people were hurt from using such devices. The study, led by Charles Clark, describes the nature of the problem, and details the experiments made using an inexpensive webcam that can detect excess infrared light from green lasers.
The experiments involved using three low-cost green laser pointers, advertised to have a power output of 10 milliwatts (mW). Measurements showed that out of these three units, only one emitted dim green at levels of nearly 20 mW. The researchers say this is powerful enough to cause retinal damage to an individual before he or she is aware of the invisible light.
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, 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. The team reports in their study that the problem stems from inadequate procedures in manufacturing quality assurance.
When the KTP crystal is misaligned, little of the 1064 nm light is converted 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. Although misalignment does not occur often, one time is enough to cause users serious damage.
The NIST team proposes several solutions. One is incorporating an inexpensive infrared filter at the end of the laser, which could reduce infrared emissions by 100-1,000 times depending on quality and cost. Such filters exist in modern digital cameras and more expensive green laser pointers, but manufacturers tend to leave them out of the inexpensive models.
The average Joe could perform simple tests to detect excessive infrared leakage; by using a common digital or cell phone camera, a compact disc, a webcam, and a TV remote control, they can “feel” the presence of infrared by the varying performance of these devices. Regardless, the study stresses that users should never aim laser pointers toward human or animal eyes. Moreover, one should never point the laser towards surfaces such as windows, which can reflect infrared light back to the user. This is a particularly subtle hazard because many modern energy-saving windows have coatings designed specifically to reflect infrared.
For more information about the dangers of dim laser pointers, see NIST’s press release.