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Led Lenser Torches for night birding

 To: "'Robert Inglis'" <>, "'Peter Shute'" <>, <> Led Lenser Torches for night birding "Roger Giller" <> Mon, 21 May 2012 22:50:22 +1000
 ```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 m("tpg.com.au]","[inglisrc"); Sent: Sunday, 20 May 2012 3:20 PM To: Peter Shute; m("vicnet.net.au","birding-aus"); 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: m("tpg.com.au'","'inglisrc"); ; m("vicnet.net.au'","'birding-aus"); 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 -------------------------- Sent using BlackBerry ```
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