By Patrick Murphy and Greg Makhov
As the costs of consumer lasers drop, and as beam powers increase, there is growing concern over misuse of pointers and of more powerful hand-held lasers. Pilots and others harassed by lasers are often worried about potential injuries to their eyes. Fortunately, it is more difficult than the general public might think, for a consumer laser to cause eye damage.
Take for example a police helicopter pilot hovering at an altitude of 500 feet. She is suddenly “lased” by a blue flash that lights up the windscreen. She directs ground forces to the location, where the perpetrator is arrested — but now she is worried. When she reads up on lasers, she finds that the most powerful consumer blue laser has a “Nominal Ocular Hazard Distance” of 733 feet. She may interpret this as meaning that being closer than 733 feet will cause instant blindness. Fortunately, this is not at all true, for two main reasons:
1. The NOHD is a laser safety concept which has a built-in safety factor. For consumer-type continuous-wave visible lasers, the NOHD is about three times the “Exposure Dose 50” point. Summarized, this is the irradiance at which there is a 50/50 chance of a laser causing the smallest medically detectable retinal lesion (burn) under laboratory conditions where the laser and the subject’s eye are both fixed in place.
This means that the pilot would have had to have been at about 1/3 of the NOHD, or 244 feet, for her to have a 50% chance of the smallest observable lesion under optimum conditions. At the NOHD (733 feet), the chance of such an injury is “vanishingly small”, according to one expert.
2. Many of the highest-powered lasers are claimed to be stronger than they really are. For example, a blue laser widely advertised to have a 1 watt beam with a tight 1 milliradian divergence (spread) — giving an NOHD of 733 feet — actually has a weaker and wider beam. The best samples can only reach 0.8 watts with a 1.5 mrad beam; this reduces the NOHD to 437 feet. The ED50 point is 1/3 of this, or 138 feet.
The pilot now understands that even if she was lased by the most powerful readily available consumer laser (0.8 W, 1 mrad), she was already beyond the NOHD when flying at 500 feet. And she would have had to have been closer than 138 feet to have a 50% chance of the smallest detectable lesion, if the laser beam and the helicopter were completely motionless relative to each other.
These factors are two key reasons why, as of late 2012, both US and UK aviation safety officials state that there have been no documented permanent eye injuries from any of the over 16,000 civilian laser-aircraft incidents since 2004. A few pilots had temporary injuries that healed in the same way as a minor skin burn; fortunately they all returned to flight status.
In summary, the NOHD is a perfectly valid concept in a lab, industry, military or other situation where personnel can be prevented from approaching the NOHD to avoid any chance of harm. But there are occasions when police, Coast Guard and other first responders may need to be closer than the NOHD. By better understanding the actual laser hazard (as opposed to the “nominal” hazard), they can better weigh the relative risks of laser exposure vs. the benefits of performing their public safety mission.
[Authors’ note: It should be emphasized that all laser beams should be treated by consumers as potentially hazardous. Lasers should never be aimed at the face or eyes of a person or pet, and they should never be aimed at an aircraft. The ILSC paper’s goal was to help reassure 1) persons who have already been lased and 2) first responders who may need to go after laser perpetrators.]