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
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:
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?
> Don Cox
> [log in to unmask]