From: jeh@cmkrnl.com
Newsgroups: sci.electronics
Subject: Re: Laser gunsights (was: Info on CO2lasers wanted
Date: 8 Jan 93 17:28:21 PST
Organization: Kernel Mode Consulting, San Diego, CA

In article <6899@tuegate.tue.nl>, wouter@blade.stack.urc.tue.nl (Wouter Slegers) writes:
> jeh@cmkrnl.com wrote:
> : In article <6802@tuegate.tue.nl>, wouter@blade.stack.urc.tue.nl (Wouter Slegers) writes:
> : > I'd like to get some info CO2-lasers... I read the following on a BBS..
> : > If anyone has a sequel on this, please email it to me..
> : > [...]
> : >                LASER WEAPONRY / PART 1 / LASER SIGHTS
> : > [...]
> : 
> : I don't know about the circuitry provided... but the stuff in there about a 
> : 2 mW HeNe laser being dangerous to human eyes is complete bunk.  
> : 
> This is not quit true: a 2 mW laser is enough concentrated to damage the eye
> tissue if your looking right into it..

Oh, rubbish.  True, the BRH would appear to think so, but they're being extra
careful.  In real life, it simply isn't the case.  I've taken several
accidental hits, right in the eye, from much larger HeNe's with absolutely no
problem.  So have lots of people. The effect is about like having a red-colored
camera strobe go off from just a few inches away.  You have a bluish afterimage
for a few minutes, and that's it. 

Perhaps if you stared into a 2 mW beam for a long time, like tens of seconds,
AND your head and eyeball couldn't move, you might heat up one spot of retina
enough to lose a few cells in that spot.  But I doubt it.  

As for real-world dangers, your eye has this marvelous ability when it suddenly
encounters light that's too bright... it closes!  Within about a sixtieth of a
second.  Which means we are talking about a total energy of 1/120000
watt-second. That isn't much! 

Now, for a first approximation, human tissue can be treated like water, and the
mechanism by which a laser will do damage to human tissue is through heat.  Why
don't you do the math and tell me by just how few degrees 1/120000 watt-second
of energy will raise the temperature of, say, a cc of water. 

Why so much water?  Because we won't be heating just that tiny spot on the
retina.  Some of the energy that reaches the eye will be reflected off the
cornea; of what's left, some will be absorbed by the lens and by the fluid in
the eye, and of the part that actually hits a spot on the retina, a lot of it
will be reflected and not absorbed at that spot.  This is because the retina is
reddish-colored!  (Yes, the danger levels for non-red lasers are quite a bit
lower than for good old orangy-red HeNe.)

Which is NOT to say that you should go around pointing even a 1 mW HeNe at 
unsuspecting victims.  Even though it won't hurt anyone's eyesight, it will
startle the hell out of anyone who catches it in the eye, and if they happen
to be doing something that requires concentration (driving a car, for instance)
you could well cause an accident.  

Hmm, maybe people are better off *believing* that the 2 mW beam is dangerous
to eyesight, even though it isn't so. 

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
Uucp:  ...{crash,eisner,uunet}!cmkrnl!jeh