Over the last decade, the striking ethereal glow of Green Laser Pointers (GLPs) has become a familiar sight at astronomy education and public outreach (EPO) events, from star parties for beginners to sky-at-night tours around astronomical installations. With the beam of a GLP an astronomy educator can direct an audience unerringly to a specific object - planet, meteor shower radiant, star, asterism,constellation, or nebula. The very light of the GLP itself can seem to confer an enhanced authority on the EPO guide. The GLP remains one of the coolest tools in the amateur's instrument case - but only if used responsibly. GLPs are tools not toys, and their misuse can lead to serious harm.
Under the Canadian Aeronautics Act, conviction for laser flashing an aircraft carries maximum penalties of a five-year prison term, and a $100,000 fine:
The RASC counsels its members at all times to cooperate with law enforcement officials, and to remain within the law.
In keeping with the RASC's commitment to informative, inspiring, and safe EPO, it encourages its members to follow the following guidelines for the use of GLPs:
- Ensure that GLPs are operated only by designated, responsible adults, preferably RASC members who are familiar with the potential hazards of laser light. During public events, consider assigning one or more RASC members to help GLP operators spot oncoming air traffic;
- Take special care not to shine GLPs in the direction of any person, vehicle, aircraft, or wildlife;
- Avoid using GLPs near an airport or airport runway approach. We suggest that GLPs not be used within 10 km of any airport;
- Use the minimum power to do the job: if a 5 mW laser is bright enough, why use a stronger one?
- Be aware that distraction and distress can be experienced by anyone illuminated by green laser light, even if the level is well below that which would cause physiological damage;
- Use good sense in storing GLPs. Don't leave lasers accessible to children. Consider removing the batteries when you are done using a GLP.
By following these guidelines (which are simply common sense) RASC members will reduce the chance of an unfortunate incident involving GLPs, and will demonstrate due diligence while leading public astronomical activities.
Remember, GLPs are useful if you're SMART about using them.
Safe = place safety foremost
Mature = keep GLPs in responsible hands
Astute = use GLPs skilfully and economically
Rational = match your GLP use to your scientific approach
Tactical = think ahead - plan your GLP use
As more information becomes available to us, we will pass it along.
- Randall Rosenfeld (Unattached), Chair
- Chris Beckett (Regina)
- Dr. Roy Bishop (Halifax)
- Mike Hanes (London)
- Paul Schumacher (Victoria)
The Intensity, Luminance, and Illuminance of GLPs
by Roy Bishop
I note in the recent flurry of emails on Green Laser Pointers, that some people seem to underestimate the hazard these devices pose if aimed at someone, particularly from a distance of several kilometres. I did some calculations for my 5 milliwatt green laser and arrived at these results:
Intensity = 3.8 exp 6 cd (candelas) (That is: 3.8 multiplied by 10 raised to the power 6).
Luminance = 5 exp 12 cd/m2 (candelas per square metre)
To put the latter number into perspective, the luminance of the surface of the Sun = 1.9 exp 9 cd/m2, so my green laser has 2,600 times the luminance (surface brightness) of the Sun.
The power required to light an incandescent light bulb (sending light in all directions) that appears as bright as my green laser (when aimed at someone) = 3 megawatts (Three megawatts will power a sizeable town).
At a distance of 3.5 km my green laser will provide the same illuminance (surface illumination) as does the full Moon.
From a distance of 320 km (for example, from the ISS) my green laser will appear as bright as the planet Jupiter (neglecting atmospheric absorption). Thus, at "everyday" distances (several kilometres or less) a 5 mW green laser is going to be damn bright, and a major hazard to anyone whose attention is needed elsewhere, like a pilot or the driver of a car, especially at night.
It is the extremely small divergence of its beam that gives the laser these impressive figures.
And mine is only a 5 mW laser. 600 mW (!) hand-held green lasers are being advertised for sale to the public, few of whom understand physical concepts like solid angles and luminance.
If anyone would like to check my results, here is the input information:
- Power: 5 milliwatts (Advertised)
- Beam diameter at the laser: 1.0 millimetre (I measured it.)
- Beam divergence: 1.0 milliradian (Twice the half-angle) (I measured it.)
- Wavelength: 532 nanometre (Advertised, and I measured it.)
At the wavelength of the peak sensitivity for bright-adapted (photopic) vision (555 nm): 1 watt produces 683 lumens (p. 36 of the Observer's Handbook).
At a green laser's wavelength (532 nm), the response of the eye is about 0.88 as great (p. 70, Observer's Handbook).
Sun's radius, power, and luminous intensity: p. 34 Observer's Handbook.
Photometric efficiency of an incandescent light bulb: p. 36 Observer's Handbook.
For the illuminance provided by a light source of a certain visual magnitude: p. 35 of the Observer's Handbook.
For the visual magnitude of the full Moon: p. 35 Observer's Handbook.
For the relations between the units lm, cd, lx, and sr: p. 36 of the Observer's Handbook.