Back scattered light from laser pointers

[OneOfOne]

Hi guys,

This group seems to best fit my question so I hope I am in the right spot and get some thoughts flowing. I did a search but couldn't find any previous questions about the subject, so here goes.

Last night as I was setting up the scope using my laser a question hit me as I was jogging Sirius to the centre. If you are looking at the beam in say a finder or binoculars or whatever, is the back scattered light a danger? I would expect the light that you see directed up into the sky will probably be no longer be "in phase" as it will be reflected and scattered by dust in the air, mozzies etc and will also be of much lower intensity as most of the light will continue skyward, hitting some unsuspecting Sirian in the eye in several years time! Normally, once I have the laser over the required star, I turn it off and then jog the star into the centre of the finder, then the eyepiece (aligning the GoTo), but some times I forget.

Any thoughts? You wouldn't look at the spot on a nearby tree with binoculars, but maybe even that reflected light would be "safe", unless it was reflected off a window or mirror.

[astrogeek]

Light on the sky would definatly be safe because as you suggested only a small fraction of the light is being reflected. Tree.......maybe. The light reflected off the tree will be much more spread out therefore safer, so yeah probably ok. Mirror.......don't think I would be willing to try that ;).

[acropolite]

There are laser light levels that are considered safe even if light gets directly in to the eyeball. From memory, the safe level is around 1mw, anything greater and permanent damage can occur if the light enters the eye directly, anything less than a total reflection would be unlikeley to cause eye damage, but when I worked on communication equipment with infrared lasers we were taught to assume all laser light was hazardous. Therefore even a 5mw green laser should be used responsibly, but that said it is unlikely that backscattered light presents any danger. Here is a document that outlines possible problems and relevant effects at various powers. http://www.mech.unsw.edu.au/safety/SafetyData/Laser_safety_guidelines.pdf

[janoskiss]

Take it from some one who is supposed to know about these things. What backscatters from the atmosphere is nothing as far as the welfare of your retina is concerned. If it's visible light (which green laser sure is), it has to look very bright before it starts doing damage.