Recent reports of aviator crews being "blinded" by visible green laser beams have led to speculation in the media of a possible security issue. The Department of Homeland Security issued a press release in January 2005 reporting incidents, and noting concerns. On January 12, Secretary of Transportation Norman Mineta stated that FAA and Law Enforcement would work together to prevent "careless people making stupid choices" and putting aviators at risk. Many have asked why the flurry of new reports after a period of relative quiet on the misuse of laser pointers.
Visible laser beams created a stir about a decade ago in the aviation community following a flurry of incidents of laser beam illumination of airline cockpits at night during final approach at several airports around the United States. At that time, laser lightshows, using multi-watt argon and krypton lasers, had come into common use to attract customers to casinos in Las Vegas and elsewhere around the country. Xenon-arc searchlights had been used for many decades at the opening of events or at special events to attract notice of people in the surrounding countryside at night. Pilots had grown accustomed to such searchlights, however, and there had been no strong efforts to regulate the use of searchlights in regulated airspace. Federal Aviation Administration (FAA) officials became particularly concerned in the 1990s, however, after casino laser incidents at airports, such as the McCarran Airport in Las Vegas. Pilots even had to abort landings because of severe visual disturbances, such as flash blindness with after-images.
The laser illumination incidents in the 1990s did not cause any documented instances of permanent retinal injuries, but the FAA updated their guidance (FAA 7400.2D, now 7400.2E) for use of lasers in the airspace to address the important safety issue of distracting aircrew during take-off and landing. Basically, the airspace around an airport became "off limits" to laser use. A specialized ANSI Standard, ANSI Z-136.6 (2000) for Safe Use of Lasers Outdoors, was developed in part to address these incidents. This guidance for a consensus standard provides for laser operations in the outdoors such as might be conducted by the military, scientific research, atmospheric monitoring, or laser light shows.
Pilot concerns
A survey of pilots was conducted by the Southwest Airline Pilots Association in the 1990's and reported at the International Laser Safety Conference in 1997 (Sliney, 1997). Incidents of illumination by searchlights were compared with lasers and typically most pilots felt that they had never been bothered by any high intensity searchlight at distances of much more than 2 to 3 kilometers; whereas, afterimages distraction and similar visual disturbances were experienced out to 5 or even 9 kilometers from laser beams (see Figure 1, below). The duration of afterimages or distraction after laser illumination was greater than reported due to searchlights.
Current concerns of the aviation community center on the issue that a nighttime laser exposure causing temporary adverse visual effects might also cause distraction and confusion in the cockpit, and in the worst case, loss of aircraft control. Those who have been frequently illuminated, such as some low-flying urban helicopter pilots, have learned to maintain aircraft control, not to stare into a laser beam and to limit any direct viewing of the laser. Proper training and situational awareness can apparently reduce startle and distraction from visual effects. Aviators who are familiar with occasional searchlight illuminations have come to accept these, where the actual illumination levels are similar to light-show lasers and certainly greater than from laser pointers. In addition, dirty or scratched windscreens tend to increase scatter of light and can produce a more dramatic visual effect in some instances.
The red reputation
When the ubiquitous, inexpensive ($8 - $15.00) GaAlAs diode laser pointers became widely available, a rash of reports of alleged "laser blinding" of teachers, basketball game spectators, and other students ensued. But a thorough review of these incidents clearly showed that there were only two or three documented retinal injuries from class 3R laser pointers and these only occurred when the person steadily fixated into the laser at close range.
Bruce Stuck and colleagues nicely demonstrated the nature of any realistic exposure when they asked a group of individuals to attempt to point a laser at a detector positioned to simulate the human eye at a distance of a few meters, to see how long the laser beam would remain within the detector's 7-mm aperture (Stuck, et al 2003). The study showed that a hand-held beam only occasionally and momentarily entered the detector, such that cumulative exposures never exceeded the applicable exposure limit. Ophthalmologists, such as Professor Martin Mainster at the University of Kansas Medical Center, have carefully reviewed many incidents and have published a number of papers and editorials in ophthalmology journals, noting the sensationalism that is frequently associated with laser exposures. Mainster explains the concerns expressed by many when dazzled, and why some people think that they have been "blinded" when in fact they have only experienced temporary visual disturbances.
Normally red diode-laser pointers, the type most commonly available, can be seen at a distance but are not very dazzling at the typical standoff distances between occupied areas and commercial landing strips. Laser dazzle potential varies with distance as the beam diverges and the illumination level decreases. At very close distances as may occur with police helicopters or some military aircraft, however, pilot complaints have been reported. Certain aircraft were even grounded temporarily in Bosnia by the US military in October 1998, while a specific situation related to laser pointers was clarified. After repeated exposures to small laser pointers, however, pilots may find them less distracting. One pilot in the Los Angeles Police Department claimed that such exposures no longer distracted her.
Mean green beams
In the last several months, numerous incidents related to green laser exposures (presumable green laser-pointers) have been reported. Green laser pointers are perceived as brighter than red at the same power and have created new concerns in the aviation community. The green laser-pointer wavelength—generally 532 nm—is near the peak visual response at 555 nm for daylight or photopic vision, when the cones in the eye are the primary visual receptors. In a dark-adapted eye (scotopic vision), the rods, which are far more sensitive to green and blue light than to red light, become the primary visual receptors. So the effect on the dark-adapted eye is much more pronounced. Commercial laser pointers do not pose a problem during daylight; the startling temporary visual effects are possible only during nighttime or twilight illumination. And the effect upon the aircrew depends upon the level of dark-adaptation prior to the exposure.
The green laser pointers at 532 nm are far less common and are also more expensive than the typical red-diode laser pointers, which emit at 635 to 670 nm. Lasers are classified sequentially for safety from Class 1 to Class 4, and the US Federal Government regulates laser manufacturers, permitting only Class 1, Class 2 and Class 3R lasers to be sold for demonstration, alignment and leveling (to include laser pointers). Foreign Governments have similar regulations, but enforcement is often minimal.
Class 3A laser products are now also termed Class 3R laser products and have an output power emission between 1 and 5 mW. These lasers are in a transitional category between power levels where the aversion response (blinking and turning away) provides protection against injury and power levels where the aversion response is inadequate and where retinal injury becomes more likely (Class 3B). The warning labels differ with the different classes. Even Class 1 "eye-safe" lasers can produce dazzling visual disturbances such as after-images at close enough range; however, there is no chance for a permanent injury under normal use of Class 1 lasers. The upper emission limit for a Class 1 laser product is 0.4 mW in the red end of the spectrum.
Amateur astronomers are now using green laser pointer beams with output powers of 5 mW and even higher, to point out objects in the night sky. Most are unaware of the potential risk to aircrews should they directly illuminate a plane. Websites posted by amateur astronomers talk about their experiences with laser pointers and how some have obtained more than 5 mW power outputs. The US Army CHPPM acquired a number of commercial laser pointers and indeed confirmed that some lasers labeled as Class 3A were indeed 3B with output powers of the order of 10–20 mW. The beam appears to reach 300–500 feet into the air and is described by many amateur astronomers as "awesome". A quote on one website described a 58-mW green laser: "... I had two friends wait back at a white water tower that was seven miles from my location ... the large green circle was easily visible on the tower ...."
Temporary visual effects on aircrew vary, but they range from startle or distraction to discomfort and disability glare with dazzle and even after-images or flash blindness. Startle refers to an interruption of a critical task due to the unexpected appearance of a bright light such as a laser beam, whereas glare and dazzle refer to a bright light that makes it difficult to see, such as oncoming headlights or momentary laser pointer exposure. These visual effects last only as long as the light is present. As the sun sets and backgrounds become darker, a small laser at a distance becomes more visible. It is unlikely that real after images would be produced by the recently reported aircraft incidents related to ground-based laser pointers. After-images may occur with laser light show beams, which are a hundred to a thousand times brighter.
Based on data that have been collected to date, momentary laser pointer exposure does not produce a risk of permanent injury. In most individuals the aversion response will terminate a commercial laser exposure to less than a quarter of a second. So there is no realistic risk of eye damage from accidental viewing of Class 3R commercial laser pointers even at relatively close distances. Brief exposures during the performance of critical tasks, however, can be very disruptive and could cause accidents.
The opinions or assertions contained herein are the views of the author and are not to be construed as official or as reflecting the views of the Department of the Army of the Department of Defense.
David Sliney is a physicist and program manager at the US Army Center for Health Promotion and Preventive Medicine, Aberdeen Proving Ground, MD 21010-5422. E-mail: [email protected]; URL: http://chppm-www.apgea.army.mil/laser/laser.html.