Laser Pointer Usage

 

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

 

These guidelines and the materials on this page were created by the RASC's former Green Laser Pointer Committee.

The Intensity, Luminance, and Illuminance of Laser Pointers

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 x 106 cd (candelas)

Luminance = 5 x 1012 cd/m2 (candelas per square metre)

To put the latter number into perspective, the luminance of the surface of the Sun = 1.9 x 109 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.


These resources were developed by the RASC's Green Laser Pointer (GLP) Committee 2010-2013

R.A. Rosenfeld, Chair (National Member)
Chris Beckett (National Member)
Dr. Roy L. Bishop, FRASC (Halifax Centre)
Mike Hanes (London Centre)
Dr. Paul Schumacher (Victoria Centre)

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Last modified: 
Saturday, February 13, 2021 - 3:49pm