[Suzy_A]

Originally Posted by programmer

Some good info, thanks. But I think it's reality it is quite hard to cause 'instant and permanent' damage with a 20mW laser. You'd have to hold them down as the blink reflex would kick in. You'd also have to focus it in the one spot for maybe half a second (guesstimate) and be up very close, say < 1m. The NOHD (Nominal Ocular Hazard Distance) would probably be 30-40m, but focusing it at any more than a couple of meters would be very hard.

Also a true laser pointer (1mW) is almost impossible to cause eye damage with so probably harmless for the kids, but still not a great idea to let them loose.

About 90% of laser pointer sold ARE well over 1 mW. Having actually tested many with a laser power meter, most are 3 - 5 mW and it is not hard to find a 10 - 20 mW laser pointer for $5 - $10 in a shop, although it almost certainly will be labelled "<1 mW".

As for eye damage - it doesn't happen very often as the 0.25 second blink reflex is the worst case - most people will blink and divert their eyes much faster than this, so even a 5 mW will usually not cause damage. However a 10 mW probably will and a 20 mW certainly will.

While these are very low power, their power density is actually very high.

A 1 mW laser may have a beam width of 1 mm or 0.78 mm2. This equates to a power density of (1 mW x ((1000mm*1000mm)/0.78mm2)) = 1274,000 mW/m2 or 1274 W/m2. This is almost exactly the same as the sun at it's zenith and which will cause permanent damage after about 10 seconds exposure - which of course doesn't usually happen as most people have the common sense not to look directly at the sun that long or to blink and divert before there is any damage.

The eye will focus 1 mm beam down to about 20 um, giving about a 400 x increase in power density.

A 20 mW is about 20 x brighter than the sun and will usually cause permanent eye damage before the blink reflex can work.

While not that rigorous, http://en.wikipedia.org/wiki/Laser_safety has some quite a good info.

Incidentally, a number of RAAF (and probably other military) aircrew have suffered permanent eye damage while flying past foreign military naval vessels, particularly those of a former (current?) world super-power, although they would not have been using run-of-the-mill laser pointers...

Susan
Radiation Safety Officer
Senior Physicist