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Light is not "scattered" in the polycarbonate substrate. Diffraction and interference modulate reflected light from pits and lands of a replica. Recorded CD-R dye or CD-RW alloy layers directly modulate the reflected light intensity. Media Sciences does not attempt to evaluate specific media brands or permutations, but does offer to test a disc at no cost. If you are not familiar with our site, details are at: http://www.mscience.com/test.html#FRETST Jerry Media Sciences, Inc. > -----Original Message----- > From: Association for Recorded Sound Discussion List > [mailto:[log in to unmask]] On Behalf Of Don Cox > Sent: Saturday, August 21, 2004 1:38 PM > To: [log in to unmask] > Subject: Re: [ARSCLIST] ? about Burning CDs for archives > > On 21/08/04, Mike Richter wrote: > > At 10:28 PM 8/20/2004 +0000, Don Cox wrote: > > > >> I cut a sliver off the centre hole and examined it on a lightbox. The > >> dye is in the plastic - it is definitely black polycarbonate. > >> > >> The other colours might be just layers. I haven't seen them. > > > A write-once disc consists of several layers. from the bottom surface, > > the one through which it is illuminated, they are: > > > > 1. A thick, clear polycarbonate layer > > 2. A thin polycarbonate layer with light-sensitive dye > > 3. A metallic overlay which reflects the illumination > > 4. A thin acrylic lacquer > > 5. Any overcoating, printing or related surface. > > > > The decorative pigment is in the plastic, 1 above. > > As I said. > > Your previous mail said it was an extra layer between the 1 & > 2 as listed above. > > > > The illumination > > passes through that layer before passing through 2 on its way to 3; > > then after passing through 2 a second time. While essentially opaque > > to visible light, the pigment in 1 is approximately transparent to the > > infrared illumination. Yes, it appears to be through the polycarbonate > > since the dye layer is quite thin; in fact, the pigment may also > > permeate the dye layer, though I believe it does not. Regardless, the > > dye layer would be extremely hard to see in cross section - it is thin > > so that it is within the depth of field of the illumination where the > > pigment is ideally perfectly out of focus. > > > > Note the distinction between the light-sensitive dye and the > > decorative pigment. > > Dye in your layer 1 would be effective in reducing scattered light. > Dye in both 1 & 2 would be even more effective. > > I didn't attempt to see the pigment layer - I think a good section to > show this would need an ultramicrotome with a glass blade, which I don't > have here. > > The msciences site, which I was already familiar with, has no mention of > black disks. > > Do you have a link to authoritative tests on such disks? > > Regards > -- > Don Cox > [log in to unmask]