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From: dpe@rocket.cc.umr.edu (David Edwards)
Subject: Re: CD player lasers
References: <364bgk$o9@acmey.gatech.edu>
Date: Sun, 25 Sep 1994 18:27:52 GMT
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Richard John Farmer (gt5876b@prism.gatech.edu) wrote:
:   Real bad move, and unfortunately a common one. What makes lasers hazardous
: is the power density W/M^2. Since lasers are by definition coherent light
: sources the are very highly focused beams. There is a standard for what
: is considered dangerous to the eye but I don't know it offhand. Remember
: the size of a bit on a CD is very small so the power density may be several
: Watts per square meter, perhaps several hundred, definitely bad news. Unless
: you know everything about a particular device I would err on the side of
: caution.
Just a comment... at 'several watts per square meter', the laser would be
generating a SIGNIFICANT amount of heat, quite possibly enough to cause
localized distortion of the plastic. At 'several hundred', it would be
cutting completely through the CD... (It's just plastic, folks...
technically polycarb, but still easy to melt.) I've seen demonstrations
of _metal_  cutting/welding using power densities of 'several hundred
watts/M^2'...

The laser diodes used are fractional mW devices, typically .3-1.2 mW...
and the beam diameter starts out pretty small. Power at the point where the
laser hits the disk surface is right around 6-7 mW/M^2... (No, I can't
provide documentation for that... that's just a off-the-top-of-my-head guess.)
The actual LASER is a class I device, despite what folks have been claiming...
Also, the laser is focused for a focal length of about 1 cm or less... if
you look at it from a distance of about 2-3 feet, the beam has spread to a 
radius of SEVERAL meters....

In closing: Looking into a laser deliberately is sorta like deliberately
shocking yourself for fun (?)... it's a really bad idea, it's an even
worse practice, but [in most cases...] it will not cause permanent damage.

					Later!
					David
--
____________________________________________________________________
David Edwards   ||  I will not do it, in my car, I will not speak ||
U of MO, Rolla  || for UMR.  I would not, could not in a plane, I ||
dpe@umr.edu     || would not, could not on a train. I will not    || 
                || speak for UMR, I will not do it, Sam you are.  ||  
                ====================================================

