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From: rpw3@fortune.UUCP
Newsgroups: net.arch
Subject: Re: Re: seeking information on lasers an - (nf)
Message-ID: <3066@fortune.UUCP>
Date: Sat, 14-Apr-84 07:02:20 EST
Article-I.D.: fortune.3066
Posted: Sat Apr 14 07:02:20 1984
Date-Received: Sun, 15-Apr-84 08:48:18 EST
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Organization: Fortune Systems, Redwood City, CA
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#R:infopro:-5000:fortune:16500009:000:1571
fortune!rpw3    Apr 14 02:53:00 1984

>>The recommended maximum direct viewing power (looking
>>down the beam) is well under ONE MILLIWATT!  (Ref.  some
>>Gov't Std.  or other)      [ Me. ]

>The standard for maximum dose to the eye is 0,2 microjoules/sq.cm
>in this country. That is the same as 0,2 microwatt-seconds/sq.cm.

From playing with very low-power helium/neon lasers in physics lab
(these were rated SAFE at about 0.1 milliwatt), I know (the hard way)
that those things appear **BRIGHT** and cause quite a blink/flinch
reaction. Assuming a slow 150ms. for that blink (eyes are faster than
hands), we're talking about a dose of 0.1*0.15 = .015 milliwatt-second =
15 microwatt-second.

Assuming the beam is well collimated enough to entirely enter the pupil
(diameter about 2 millimeters -- I just measured mine), you could easily
get a focussed spot of 1 millimeter or less (though focussing at true
infinity is not a normal thing). So the dose could easily be as high as
15 uW-sec/(.1)^2 sq.cm. (forgetting pi aren't square) ~= 1500 uW/cm^2 !!!

That's 7500 times the dose you mention. Yet those lab demo lasers were
supposedly "safe". Hmmm... I knew some U.S. standards on radiation
dosage were looser than other countries, but...

(Well, my retinas get looked at once a year whether they need it or not,
and that was 20 years ago and Doc hasn't complained...)

But that's nit-picking. You're right. Lasers AREN'T toys!

Rob Warnock

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