Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!bellcore!petrus!karn
From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.audio
Subject: Re: cleaning cd-player optics
Message-ID: <414@petrus.UUCP>
Date: Sun, 21-Jul-85 21:59:22 EDT
Article-I.D.: petrus.414
Posted: Sun Jul 21 21:59:22 1985
Date-Received: Wed, 24-Jul-85 20:57:04 EDT
References: <412@petrus.UUCP> <4034@alice.UUCP>
Organization: Bell Communications Research, Inc
Lines: 23

> When my CDP-101 is switched on, I can peer into the drawer
> and see red light.  Are you sure that the laser operates
> in the infrared?  (actually, I suppose the frequency is
> up to the manufacturer...)

Depending on which article you read, the wavelength of the laser in a CD
player is 780 or 800 nm. This is dictated by the depth of the pits in the
metal layer of the disc (.11-.13 micron) and the refractive index of the
polycarbonate plastic (1.55). In the plastic, the wavelength becomes 500 nm,
and the pit is one quarter this value. Light traveling down the pit is still
reflected, but when it arrives back at the metal surface it has travelled an
extra 1/2 wavelength and is therefore 180 degrees out of phase with the
light being reflected from the area around the pit (the beam is larger than
the pit.) The two reflections interfere with each other, so that less light
is able to travel back up to the photodetector than when the beam hits an
unpitted area.

Technically, the visible spectrum ends at 750 nm, so I suppose it's possible
for some people to see the CD laser as a deep red. I can't see any visible
light out of either of my players, nor can I see the IR LEDs in my VCR
remote control (although they probably operate at a different wavelength).

Phil