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
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