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.

I am sorry - I often make assumptions about the knowledge of my
correspondents which are not supported in fact.

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. 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

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