"Two-tone" discs

[Gesticulates Very Expressively]

On Fri, 4 Jan 2002 21:14:34 -0500, K Henderson <henderson.120 at OSU.EDU>
wrote:

>No, no...I'm talking about something entirely unintentional and non-
uniform,
>more like a 'discolored blob' effect that can only be the result of uneven
>processing.
>Two shades of silver with slightly different reflectivity...that's all.
>
>Nothing at all to do with marking differences, or colorization of CDR's.

I think I know what you mean and it's vaguely bugged me in the past.
Lower quality CDs seem to have it more than higher, which is why it's often
most visible on CDRs.
I don't *know* why it is and can't seem to find out but I can take a
running guess. CD groove spacing does spoony things to light anyway - hence
the flashing rainbow colours of a turning disc, as it acts as a reflective
diffraction grating. You sometimes get blank CDs, with the grooves but no
metal foil, used as spacers in blank CD stacks, which I use at work as
transmission diffraction gratings - very handy. So they're some sort of
optically active to start with.
Then there's the plastic they're made of. Clear plastic, polycarbonate,
perspex type things in general, will show their internal stress patterns
and plane shifts as coloured fringes, like an oil film, if you look at them
under polarised light, where all the waves are organised into the same
plane. I can imagine a blob of hot plastic being dropped onto the metal
layer of the CD and pressed out into shape. If the plastic is hot enough as
it goes on it would flow evenly and you'd get a top surface with no
stresses between itself. If it cooled too much as it spread out, bits of it
would flow at slightly different speeds and you'd get permanent internal
stresses in the plastic.
If the reflected light is coming back polarised, organised, to some extent,
it would pass through the stressed parts and possibly show up the stress
patterns in the plastic in that way.
That would explain why the ones with the most obvious marks like that (I
always thought of them as water stains, though not literally) don't write
very well, as they've probably been badly produced generally if the top
plastic layer was too cold as it went on.

This is all pure speculation of course. Who let a lab technician in here?

Sherm