On 23/08/04, Jerome Hartke wrote:

> "Color" in the polycarbonate layer relates to the visual part of the
> spectrum, while the CD laser operates near 780 nm in the
> near-infrared. Consequently, the visual properties of the
> polycarbonate are immaterial.

Yes, but when a dye absorbs strongly in the red there is a chance that
it will also absorb in the near infra-red. Do we know the spectrum of
this black dye? Visually, it has the same purplish hue as the dye used
in markers. It may be the same.

> Modification of the infrared properties
> of the polycarbonate would degrade the read/write capabilities of the
> disc, and is not done.

It would reduce the sensitivity. So does the anti-halation dye in a
film. That also increases the resolution, which is the issue here.

The advantages I am suggesting are possible (not necessarily actual as I
have seen no scientific tests) are

1. better resolution - i.e. more accurate lengths of the "dashes"
written on the disk

2. reduced action of light on the disk (causing ageing)

> I do not pretend to be always right, but the above information is
> accurate, as a bit of common-sense thought would confirm.

Common sense can be misleading.

Don Cox
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