18 Nov
2014
18 Nov
'14
3:30 p.m.
[John Foust, quoting me]
The goal is changes in reflectivity on a fairly small scale.
There's no reason in principle this couldn't be done with a smooth
surface by making the surface not all the same material. (A
larger-scale example of this is ink on paper: inked paper reflects less
light than un-inked paper does.) I was speculating that, on the scales
of interest here, it's easier/cheaper to produce a non-smooth surface
of a uniform material (how pressed CDs are actually made) than to
produce a smooth surface of a nonuniform material (the alternative).
I don't
understand why this technique was used. Perhaps it's
easier/cheaper to produce a nonsmooth surface made of a uniform
material than to produce a smooth surface of a nonuniform material?
I'm not
quite sure what you mean here. Wikipedia has an explanation of the process:
Yes, but it does not discuss why that process was chosen over various
alternatives.
> Is it possible that it's clear only in visible
light, with some sort
> of reflective layer present in the (infrared, IIRC) wavelengths
> used?
But as you mentioned, detection normally uses the
phase shift, and
that's dependent on the frequency of the light.
Yes, but I think the light normally used for CD reading is outside the
visible range. This raises the possibility that the disc might be
clear in the visible range but not in the range used for reading.
Your experience trying to read it argues against that, though. Oh
well, it was a nice theory while it lasted.
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18 Nov
18 Nov
3:48 p.m.
At 09:30 AM 11/18/2014, Mouse wrote:
I was speculating that, on the scales
of interest here, it's easier/cheaper to produce a non-smooth surface
of a uniform material (how pressed CDs are actually made) than to
produce a smooth surface of a nonuniform material (the alternative).
I see... you're asking why CDs couldn't be made with a printing
process (ink on paper being effectively smooth) as opposed to
the non-smooth method of pits and lands. On one hand, it almost
sounds like you've re-invented CD-R, where a dye gets zapped and
changes its reflectivity - but I was surprised to read that even
CD-Rs have a single spiral "pre-groove" pressed into the polycarbonate.
You'd need a rather precise printing process to make tiny spots.
Yes, but I think the light normally used for CD reading
is outside the
visible range. This raises the possibility that the disc might be
clear in the visible range but not in the range used for reading.
Your experience trying to read it argues against that, though. Oh
well, it was a nice theory while it lasted.
At this point, I only hold out hope for finding an existing reader
that can see the pits, and the best candidate seems to be the oldest
readers, not the newest.
I can't help but think this is an important topic for data recovery.
A peeled reflective layer is a common failure mode for discs. If
there was a way to dissolve everything above the pits (the label and
aluminum) and recoat with acrylic for protection, and read a clear CD,
you could rescue failed CDs.
- John
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jfoust＠threedee.com -
mouse＠Rodents-Montreal.ORG