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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]