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From: sam@colossus.stdavids.picker.com (Sam Goldwasser)
Subject: Re: CD player lasers
In-Reply-To: dpe@rocket.cc.umr.edu's message of Sun, 25 Sep 1994 18:27:52 GMT
Message-ID: <SAM.94Sep25171346@colossus.stdavids.picker.com>
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References: <364bgk$o9@acmey.gatech.edu> <1994Sep25.182752.7217@umr.edu>
Date: Sun, 25 Sep 1994 22:13:46 GMT
Lines: 36

Comments on the characteristics of Lasers in CD players.

First problem: CD Lasers are IR - usually around 780 nm.  I don't know why
anyone would deliberately look into one close up.  You will not see an
IR Laser beam until it is too late.  I know there are people who claim to be
able to see what IR remotes put out.  Most cannot.  Having said that:

At the surface of the CD the beam is about 1 mm in diameter.  It then
travels through the clear plastic to the information surface - the pits.

Approximately .8 mm later, at the surface of the recorded pits, the diameter
is less than 1 um.  (Recall that the track pitch is 1.6 um.).

Assuming a radiated power level at the CD of 1 mW (on the high side, but
a nice round number) then 1 mW/1 um**2 or 10**9 watts/square meter.
(Ignoring round vs. square area and a whole bunch of other stuff.)

However, the focal length (if my memory serves me correctly) of the optics
is around 4 mm.  The distance of the CD to the objective lens is 1 or 2 mm.
For one thing, this means that the beam is highly divergent and will not
be focused by the eye to a small spot (assuming that you haven't mucked
with the CD Laser's optics).  The human eye can accomodate from
parallel (infinity) to slightly divergent (close up).

Despite this, why take the chance?  You cannot make a light show with
a the Laser from a CD player.  If you really want to see it, use the
camcorder or get one of those cards that are used to test for the correct
functioning of IR remotes.

--- sam

Samuel M. Goldwasser
Technical Director, Visualization, Computed Tomography
Picker International, St. Davids, PA 19087
sam@stdavids.picker.com

