From: lancelot@spock.uucp (Thor Lancelot Simon)
Subject: Re: Photo CD
Organization: Choate Rosemary Hall
Date: Wed, 27 Jan 1993 03:49:48 GMT

In article <1jq2llINN9hc@chnews.intel.com> agough@sedona.intel.com writes:
>
>CD Storage:
>~~~~~~~~~~
>There has been a lot of controversy on the lifetime of Compact Discs.  Properly
>manufactured, they should last a long time (decades), but they won't last forever.
>The principle mechanism of degradation appears to be chemical change of the 
>aluminum reflecting layer.  Oxygen, water, and other reactants (I assume), diffuse 
>through the lacquer coating (or through a pinhole) and react with the aluminum, to 
>form Al2O3 (otherwise know as safire) that is translucent and not as reflective.
>Thermal cycle stress (due to different coefficients of thermal expansion between the
>aluminum, lacquer, and polycarbonate) introduces cracks that can 1) facillitate
>contaminant diffusion, 2) act on the bit data itself.
>      One can greatly extend the life of a CD by:
>	1) using gold (Au) as the reflective layer, as gold is less reactive
>	2) a thick, pinhole-free, lacquer coating
>	3) inks with low diffusion constants through the lacquer.
>	4) storing the CD at low temperature and low humidity, with a slow ramp-up
>	   to room temperature before use.
>
>I saw some data from Mobil Fidelity (makers of the UltraDisc series) that compared
>an UltraDisc (a well manufactured disc, with a gold reflecting layer) to a regular
>CD through two types of stress test:  steam and temperature cycling.  As I remember
>it, the regular CD lasted only two or three cycles before substantial error rates were
>present.  The UltraDisc sailed right through, with little degradation--orders of magnitude
>less.  I suppose one can still get the data from Mobil Fidelity.
>     Now, if one is concerned about future generations being able to read the "lost
>knowledge of our era" from a degraded compact disk (i.e., in 20,000 years), I would
>assume that, if the disc is intact, that an electron microscope could be used to read
>the presence/absence of pits in the aluminum layer.  This could be kindof slow.




WRONG!  Unless I'm extremely mistaken, the PhotoCD uses photographic DYES to
alter the reflectance properties of the underlying metal layer.  Think about
it, how would you go about burning pits in a CD, then lacquering over them,
in the type of drive they use to write a PhotoCD?

In fact, the _other_ vendors of PhotoCD compatible drives admit that the disks
really aren't anywhere nearly as archival as Kodak claims ("like an audio CD")
beacuse of dye fading issues.  Kodak, of course, keep their mouths shut and
blather about archival qualities. 

So where does that leave us?
>
>The Arrhenius Equation:
>~~~~~~~~~~~~~~~~~~~~~~
>     t1 = t2 * exp[(Ea/K)*(1/T1 - 1/T2)]
>       where:
>	t1 = mean time to failure at T1
>	t2 = mean time to failure at T2
>	K  = Boltzmann's constant (8.62E-5 eV/K)
>	T1 = temperature 1 (in Kelvins)
>	T2 = temperature 2 (in Kelvins)
>	Ea = thermal activation energy (eV)
>
>     To me, the bottom line for preserving film and compact discs is to store them at
>low temperature--say in a refrigerator at 50 'F.  The equation above shows that the 
>rate of a chemical reaction will decrease exponentially as temperature is decreased.  So,
>one can apply this knowledge and extend the life of a compact disc (say, normally 40 years
>at room temperature) and by storing it at low temperature multiply its lifetime by two
>or three orders of magnitude.
>      Of course, with low temperature storage, one must be careful that:
>	1) Humidity is controlled in the refrigerator
>	2) No water condensation occurs on the disc during cool down or warm-up
>	3) That the disc is gradually cooled and warmed up (to reduce thermal stress)
>
>This is more work than most people will be willing to put up with, though.
>
>Regards,
>Andy
>-- 
> Andy Gough			|  Internet: agough@sedona.intel.com
> Intel Corporation  CH3-36	|            agough@az.intel.com
> 5000 W. Chandler Blvd.		|      "Knowledge is power."
> Chandler, AZ  85226		|                -- Francis Bacon
>-------------------------------------------------------------------------------
>


-- 
*******************************************************************************
*Thor Simon             * Okay, just a little pin-prick...There'll be no more-*
*tls@panix.COM          * Aieeeeaaaugh!-but you may feel a little _sick_.     *
*tls@spock.UUCP         *   ---Pink Floyd                                     *
*******************************************************************************

From: myron@hpwrce.mayfield.hp.com (Myron Cox)
Date: Fri, 22 Jan 1993 00:56:03 GMT
Subject: Re: New Public Access Show:suggestions?
Organization: the HP Mtn.View Resp. Cntr.