T3D Re: CD ROM Lifespan

[Greg Erker <erker@xxxxxxxxxxxxxx>]

>I have a natural tendency to compare CD's with holograms, because in both
>cases
>you have the same degree of high resolution embossing and a metallic coating.

  Well, the mylar foil holograms do but film based (like Agfa)
and dichromated gell based holograms don't.

>Well, but my main question is:  Which is the material employed for the
>coating?
>
>For the gold colored disks, is it gold?

  Mobile Fidelity Sound labs (www.mofi.com) makes 24karat gold
coated CDs. The others are just tinted I believe.

>For the silvered colored disks, is it silver?

  Aluminum.

>I was surprised to see holograms losing its reflectity, becoming transparents
>in irregular paths, closely resembling that of fungus. These two holograms I
>saw in two opportunities, maybe were exposed to humidity, I never saw that in
>well stored holograms.

  A film based hologram has gelitin in the emulsion
layer so it is succeptible to fungus like slides
and negatives in warm humid environments.

  The dichromated gell holograms are sealed because
humidity quickly damages them. If it was this type
perhaps the sealing had failed.

  I'm not sure if humidiy or fungus will attack embossed
mylar holograms. Perhaps it was just a manufacturing flaw
in the mylar.

>Would degradation of CDs be caused by fungus?  Where could we find technical
>information on this important subject?

  Early laser discs suffered from what was
called "laser rot". On laser discs the two
sides are glued together. But early ones allowed
moisture to get in through the edges causing
oxidation of the aluminum and thus noise in
the picture (these are analog video disks).
They later improved the glue/sealing and claimed
the laser rot was cured.

  Mobile Fidelity claims that their 24K discs
are more immune to degredation from heat and
humidity than aluminum discs. But then they
are selling something so buyer beware.

Greg E.


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