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From: rstevew@armory.com (Richard Steven Walz)
Subject: Re: Broken CD Rom drives
Organization: The Armory
Date: Thu, 29 Sep 1994 17:07:09 GMT
Message-ID: <CwwI80.Gso@armory.com>
References: <35sr1e$gco@handler.Eng.Sun.COM> <367uk4$jvp@hemp.imel.kyoto-u.ac.jp> <SAM.94Sep27090926@colossus.stdavids.picker.com>
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In article <SAM.94Sep27090926@colossus.stdavids.picker.com>,
Sam Goldwasser <sam@colossus.stdavids.picker.com> wrote:
>RE: red light from CD Lasers.
>
>There is a small amount of red light emitted by the Lasers in CD players.
>However, if you look into this thinking that you are seeing the main
>beam, you are mistaken.  DON'T DO THIS!!!  The main beam is invisible.
>Just compare that red light intensity to a Laser pointer or 1 mW HeNE
>Laser.  It is much much less.  The power output of a CD Laser is in
>the .3-1.0 mW or so range.  I have a Sony Discman Laser that
>says .3 mW, for example.  WORM and MO drive Lasers are higher power since
>they need to heat the disk material to record data.
>
>CDs are designed to work best with IR, usually 780 nm.  If my memory serves
>me correctly, the depth of the pits on the CD are 1/4 wavelength of
>780 nm light in the polycarbonate plastic so that you get destructive
>interference at the pit edges and enhanced SNR for the returning data.
>--- sam
-----------------------------------
Oh boy have people got this issue screwed up!!!

The solid state lasers are so poorly collimated that they barely work in
CD's!!! Without the plastic lens which focuses them on the CD at the
aluminum layer, they are usually so spread out that 3mW of anything
couldn't hurt you at any reasonable range, and by the way, they are this
way because they are not very coherent either or monochromatic! They have
lines in the visible as well as the IR, and when the CD focusing lens is
over them, they cone all their output to about a two foot circle at a one
foot distance!!! If you do the math, you will find that it would be damned
hard to hurt anything with them! The W/m^2 is simply minute!! Even the best
solid state laser diodes issue light in a wedge shaped output from the
substrate junction that is hard pressed to have much of it reasonably focus
by lens into anything like a parallel beam! I would love to see how little
these actually put out, but I bet they are below all safety limits and not
sufficiently concentrated without their conizing lens to harm tissues even
on the retina!!! Any solid state laser people to confirm this? This is what
I learned in a good solid state device course! In other words, mere
amateurs are ruling on the safety of basically glorified LEDs!!!!!
-Steve Walz   rstevew@armory.com
P.S., And yes, I DO know that they CAN and DO make solid state lasers that
can be harmful. My contention is that these are NOT THEM!!!! Unless someone
had a special focal length lens made to focus this spread out light precisely
on their retina, harm to the eye from them would be impossible!!! But this
is as dumb as warning college students not to look at a 100W bulb with a
magifying glass!!!! Of course they shouldn't! But in this case they would
nearly have to grind their own special lens!
-Steve

