Text 24555, 81 rader
Skriven 2006-11-28 17:31:00 av JAY EMRIE (1:123/140)
     Kommentar till en text av JEFF GUERDAT
Ärende: Re: CDs
===============
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JG>JAY EMRIE wrote:
JG>> The other day I somehow managed to break a CD into two almost equal
JG>> pieces when my chair rolled over it. I noticed the thin covering on the
JG>> top side of the CD was peeling off a bit and was very thin. So everybody
JG>> is in the same boat:
JG>> Top side is the side you can write on with a felt tip marker
JG>> 
JG>> Bottom side is the side the drive actually writes the data on and is a
JG>> clear "plastic" substance.

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

More accurate, admittedly, but "on" is easier for many to understand.
Certainly the writing beam has to go through the plastic "carrier".
The main point of my little treatise was to amplify just how thin the
actual written on surface is - in fractions of an inch so the average
Joe could grasp just how thin it actually is. I doubt that many here can
relate angstroms, nanometers, etc.to actual size/thickness, but most can
relate to 1/1000 of an inch. THIN!
Jay

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

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

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

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

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

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

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

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

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