Text 24499, 63 rader
Skriven 2006-11-28 08:36:42 av Jeff Guerdat (1:275/311)
     Kommentar till en text av JAY EMRIE
Ärende: Re: CDs
===============
JAY EMRIE wrote:
> The other day I somehow managed to break a CD into two almost equal
> pieces when my chair rolled over it. I noticed the thin covering on the
> top side of the CD was peeling off a bit and was very thin. So everybody
> is in the same boat:
> Top side is the side you can write on with a felt tip marker
> 
> Bottom side is the side the drive actually writes the data on and is a
> clear "plastic" substance.

Well, "writes through" rather than "on" would be a better description.

> Looked terribly thin to me. Got my trusty micrometer out and measured
> the thickness. Turned out to be exactly one one/thousandths (.001")
> of an inch thick. Assuming the colored surface one writes on with a
> felt tip pen is half of the total thickness means that that layer is
> .0005" thick and the layer that is actually written on is also .0005"
> thick.

I don't know for sure (and have no way to quantify it anymore) but I 
think the dye layer is measured in angstroms.  The rest of the thickness 
is made up of the reflective metal layer, various protection layers and 
the writable surface. Here's a link to a document that describes a 
patent from Kodak (I worked with the folks listed as inventors although 
they were in the research labs working on media while I was in the 
design area for hardware):

http://www.freepatentsonline.com/5882760.html

Note that the reflective layer is 60-80nm (nanometers).  IIRC, the 
thickness of the medium itself is 1/2 the wavelength of the laser so 
that the "pit" would be 180 degrees out of phase compared to a "land". 
This may be blatantly wrong, however, but the medium thickness is still 
very thin.  I found a document that talks about dye thickness in 
relation to reflectivity for DVDs here:

http://www.eta-optik.com/pdf/wpDVDRInlineControl.ec.pdf

It would appear that a ~100nm dye thickness is used.  I don't pretend to 
understand all of what they're talking about and may be misinterpreting.

> The extreme thinness of the writable coating is why a CD can be easily
> damaged on the top side beyond use and why felt tip pens should be used
> when writing there on. The bottom of the CD can be scratched up rather
> badly before it causes read errors.

For the Great Unwashed, the substrate (the clear piece of floppy plastic 
that constitutes the majority of the disc thickness) is not written on. 
  The writing layer is actually on top, under the label, as you state. 
The reason you shouldn't use a ball point pen or a chemical that may 
penetrate the surface is that the dye layer is very nearby.  The 
substrate provides protection from dirt and scratches, putting them very 
out of focus relative to the actual medium layer.  Of course, 
dirt/scratches also reduce the light level so there's only just so much 
that it can protect from.

I had a damaged CD purchased from me since it had a scratch on the top 
side that reduced signal level below the detection threshold 
(recalibrating the detection level allowed the drive to read the damaged 
area).  It was a real-world defect that a lab would be hard pressed
duplicate or to even think of...
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