Arnold Krille wrote:
On Sunday 25 April 2010 01:34:12 Fons Adriaensen wrote:
Writeable CDs don't have pits, either the color or the
reflectivity of the surface is modified by heating it.
Both require the laser power to be controlled in function
of write speed.
Afaik writeable CDs _do_ use pits. They are created when the laser heats up
the colour-bubbles so they expand and raise the reflective layer.
From Wikipedia (http://en.wikipedia.org/wiki/CD-R#Writing_methods):
"A CD recorder writes data to a CD-R disc by pulsing its laser to heat
areas of the organic dye layer. The writing process does not produce
indentations (pits); instead, the heat permanently changes the optical
properties of the dye, changing the reflectivity of those areas. Using a
low laser power, so as not to further alter the dye, the disc is read
back in the same way as a CD-ROM. However, the reflected light is
modulated not by pits, but by the alternating regions of heated and
unaltered dye."
Another (older, 2003) article I found said that there are 3 different
ways of marking the track (without mentioning what the 3 different ways
WERE), and the marks are called "pits" whether or not there's a pit
there, an area of changed dye, or a "bubble".
Non-writable optical discs use different technology. From
http://electronics.howstuffworks.com/cd1.htm:
"A CD is a fairly simple piece of plastic, about four one-hundredths
(4/100) of an inch (1.2 mm) thick. Most of a CD consists of an
injection-molded piece of clear polycarbonate plastic. During
manufacturing, this plastic is impressed with microscopic bumps arranged
as a single, continuous, extremely long spiral track of data. We'll
return to the bumps in a moment. Once the clear piece of polycarbonate
is formed, a thin, reflective aluminum layer is sputtered onto the disc,
covering the bumps. Then a thin acrylic layer is sprayed over the
aluminum to protect it. The label is then printed onto the acrylic."
And continuing on to page 3:
http://electronics.howstuffworks.com/cd3.htm
"The elongated bumps that make up the track are each 0.5 microns wide, a
minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a
billionth of a meter.) ... You will often read about "pits" on a CD
instead of bumps. They appear as pits on the aluminum side, but on the
side the laser reads from, they are bumps."
So, since non-writable CD media appeared first and actually had
pits/bumps, I guess it was logical to apply the same terminology to the
newfangled CD-R and CD-RW media even when they don't have pits.
I always thought lasers actually burned holes in the dye layer, so the
scanning laser's beam would be reflected back. But, then, I always
thought an interesting laser printing technology would be to use a laser
that actually burnt dots on the paper (no need for toner cartridges or
passing the printed page through a hot press to melt the plastic onto
the paper).
Disclaimer: I've been found wrong in the past...
But neither of us have ever been found wrong in the future... ;-)
--
David
gnome@xxxxxxxxxxxxx
authenticity, honesty, community
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