In article , Arny
Krueger
wrote:

On Thu, 21 Jun 2007 09:14:41 +0100, Jim Lesurf
wrote:


However IIUC with DVD the information layers are roughly half-way
though the disc - i.e. about 500 to 600 microns from either surface.
But with BluRay they are only around 100 microns from the playing
surface, and over a mm from the label surface. Thus I am wary of
assuming that BluRay will behave as DVD in terms of vulnerability to
scratches or dirt.

There is a significant difference in the hardness and scratch-resistance
of the two layers of plastic on either side of the information layer.
One is polycarbonate, and the other is dried acrylic lacquer.

That's interesting. Although it won't help much if an optical blockage is
due to dirt on the surface

The biggest difference in vulnerability to scratches is the size of the
features of the disc that carry the information.

I assume the main defence will be the way data is interleaved along the
'track' and made redundant. Thus reducing problems due to isolated marks.

I'll be interested to see how this develops as the discs are in use for a
few years. I will also now wonder if there will be some problems with the
dissimilar material popping apart due to thermal cycling effects, etc. :-)

The main reason I mentioned the above was that the 'shallow' infomation
layer is a feature that distinguishes BluRay from previous discs, so may
make the system different in terms of physical vulnerability. It will also
be interesting to see if there are problems with variations in the
thickness of the layer causing prism effects, etc.

Slainte,

Jim

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