Those of us who use green lasers for astronomy outreach are always worried about law enforcement cracking down on these devices. As the lasers get cheaper and more available they inevitably get into the hands of those who do not use them responsibly. Worse, the lasers are easily available at power levels that are truly dangerous.
The problem has continued to escalate, each year there are more reported incidents of aircraft being illuminated by the laser of some idiot (yes, the correct term) who thinks it might be cool to tempt fate and the law. In 2010 there were 2836 incidents reported to the FAA, up from only a few hundred a few years before. With this sort of trend it seems inevitable there will be some sort of official reaction.
Illuminating an aircraft with a laser can be prosecuted under federal law. Not because there is any specific statute addressing lasers, but as it is deemed “Interference with a Crewmember” using an interpretation of a pre-existing 1961 federal law, specifically 14 CFR 91.11.
The FAA has put together a new webpage on lasers and aircraft safety. The page organizes and links some informative resources. This includes a couple reports on the possible effects of laser illumination on aircraft crew, as well as the legal and regulatory recommendations of the FAA. I urge anyone who uses these devices to follow the link and do a little reading.
Used responsibly these lasers are extraordinarily useful in astronomy education. Nothing grabs the crowd’s attention so quickly as that brilliant green beam. Everyone can follow along without confusion as objects are pointed to across the sky. From the constellations to the Milky Way, satellites, planets and zodiacal light, on to star clusters and galaxies, everyone knows right where to look. I do prefer lasers in the 20-30mW range, bright enough to be seen by a crowd, even under moonlight. Not powerful enough to easily injure in the case of a brief exposure to the beam.
How bright a laser beam is needed to allow good public presentations and a good astronomy education experience? In the United States legal green lasers are limited to 5 milliwatts (mW) by the Food and Drug Administration. Many of the lasers sold as 5mW are actually 3-4mW as it is necessary to stay below that limit if you wish to import the laser into the United States or to sell the laser across state lines. 5mW units are fine if you are under fully dark skies, but often you are not, there are city lights, or moonlight and the 5mW beam ceases to be usably visible. This is worse if you are working with a larger group and the distance from the presenter is larger, also making the beam less visible. In practice I have found a beam in the 20-30mW range is about ideal. These lasers are available from a number of sources for around $100. Good visibility in moonlight, good visibility in light polluted surroundings and good large group utility. But importantly, not powerful enough to be truly dangerous.
Where does the 5mW limit come from? The FDA, being a very conservative organization, set the safety threshold at 5mW based on animal and human studies and medical injury data. The 5mW limit is a national standard and applies to interstate or customs transactions, not all states have adopted it. Hawai’i does not appear to have strict laws regarding lasers, licensing or use, so my 30mW unit is legal. This contrasts with my previous home in Arizona, which is more typical of many states. The use of a class IIIb device was subject to licensing, training requirements, and yearly fees. The agency responsible being the Arizona nuclear regulatory office. Sheer bureaucratic overkill that made it practically impossible to legally use a device in the power range I needed, just a few milliwatts over the limit. I am always surprised there is no intermediate class, that a 20-30mW device is classed with devices of up to half a watt, devices that are quite dangerous.
How do I come up with the 20-30mW number as being relatively safe? Being an engineer I understand safety margins and over specification and figured there must be a compromise. The trick was to find undistorted data, without the bias of overcautious bureaucrats. The data is out there on the web, but took some digging to find, many medical journal articles are hidden behind subscription services. Fortunately there are sources like PubMed that are freely and publicly accessible. Eventually I found and read several very informative papers on lasers and eye damage, looked at pictures of laser damage on monkey retinas and looked a damage done by visible lasers in the same sort of power class as those available for public work. I was surprised by actual human damage studies, done on patients who were having eyes removed due to conditions like cancer, where the researcher could do damage without harming the patient. What I was looking for was just what level of laser radiation is dangerous in practical use.
The conclusion I came to was that a 20-30mW unit could cause damage, but was not excessively risky. The data in the papers showed you could generate damage to a retina with even a 5mW device given a long (60s) exposure. But there were a lot of caveats, the beam had to be well focused, the eye could not be moving and it had to be focused for a substantial length of time. In reality a number of things protect the human eye, the first is the fact that the eye is constantly moving, this spreads the power over more than one spot, not allowing cumulative damage at one site. The second is that a bright visible laser will initiate a blink reflex, keeping any exposure in the tens of milliseconds time scale. To prevent these protective effects the tests in the references were sometimes done with anesthetized subjects. At the 100mW power level damage occurred with very short exposures and was quite dramatic.
A 20-30mW laser needs to be treated with respect as injury is possible, but no more than any other dangerous tool we use every day. In practice damage with a laser at a sensible power level (20-30mW) would require prolonged exposure (>0.5sec) on a single site on the retina, requiring staring into the beam. The bright green light would elicit both a blink response and an aversion of the head and eye, particularly in a visually dark adapted environment. I would strongly discourage use of any laser 50mW or greater for public astronomy work. Some of the references showed significant damage inflicted with 50mW lasers and sub second exposures. Lasers at all power levels should be kept out of the hands of anyone too young to understand the implications of the danger posed by a laser.