Torches, spotlights etc. do not follow the inverse square law. The Inverse
Square Law only applies to light emitted from a "point" source such as a
bare light bulb, a candle in a room, a star in space etc. All torches,
spotlights, searchlights etc. work as they do because the light is focused
into a beam by one or more lenses and/or mirrors. In a perfect (but
unobtainable) system a perfectly parallel beam of light would have the same
intensity at all distances, neglecting a few lesser know effects such a
Rayleigh scattering and atmospheric absorption. Constraints of cost and
practicality mean that what we get is an approximation to a perfect beam.
Roger.
-----Original Message-----
From: Robert Inglis
Sent: Sunday, 20 May 2012 3:20 PM
To: Peter Shute;
Subject: Led Lenser Torches for night birding
Light reduces in intensity in accordance with a formula known as "The
inverse square law" the meaning of which is "the intensity of light from a
point source falling upon a subject is inversely proportional to the square
of the distance from the source".
Simply put, the light at 2 metres from the point source would be one quarter
the intensity of the same beam of light measured at 1 metre. At 4 metres it
would be one sixteenth the intensity at 1 metre.
Obviously some of these new LED torches are very powerful if they can throw
a beam which illuminates a subject at 200 metres. It is unlikely, however,
that an animal's eyes would be damaged by the light from such a torch at
that distance, or half that distance. I am sure that many animals would be
annoyed by the light beam, though.
Of course, aiming the beam from such a torch into one's own eyes or the eyes
of an animal at arm's distance would be quite foolish and could be
classified as deliberate self harm or animal cruelty. The warnings which
come with those torches are probably just the manufacturer's way of covering
themselves from liability for the foolish actions of the end user.
No one in their right mind would shine a 200 to 1000 lumens beam of light
into their eye from a distance of a few centimetres, would they?
A 60 watt incandescent light bulb produces about 800 lumens. Imagine looking
at one of those from a distance of a few centimetres.
When considering the inverse square law it should be remembered that the
effect of the intensity of the light from a constant source such as a torch
will be different to that of a momentary source such as a camera flash. I
have seen it suggested that birds may not be affected to any great degree by
camera flash because the flash duration is so short the bird's eye is able
to absorb the light without any damage being done. Some birds seem to be
able to look at the sun for a significant period of time without any adverse
effect on the eye or vision.
Bob Inglis
Sandstone Point
Qld
-----Original Message-----
From: Peter Shute
Sent: Sunday, May 20, 2012 2:17 PM
To: ;
Subject: Led Lenser Torches for night birding
I'm equally interested in the warning that these torches can damage human
eyesight. Is it true, and from what distance? Is it any less true of
traditional powerful torches?
I imagine the warning might apply to people shining it into their own eyes
from close up, but surely the danger diminishes further away.
Peter Shute
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