Dieder wrote:
> Ok, what with all the discussion about CCDs, A/D conversion
> etc, what is the difference between the CCDs of a high end
> scanner and the photomultiplier tubes of a drum scanner. How
> do they compare, what are their differences?
CCD stands for "charge coupled device". It's an element on a chip which
generates a response to light striking the upper surface. I know little
about how photomultiplier tubes work - I believe it's a similar principle
except that photomultipliers are vaccuum tube technology. Photons entering
the photomultiplier generate a shift in charge which can be measured. The
net effect is much the same. As far as I'm aware, it's easier to make extremely
sensitive photomultipliers than CCDs, and photomultipliers are still often
used in extremely low light situations like telescopes.
> Why is a drum scanner such a high resolution device?
Engineering. :) Sorry, it's a strange response, but if you're making a
device that costs over $10,000 you can afford the kind of mechanical components
required to get higher resolutions. It's hard to make a cheap, reliable
device which accurately scans at resolutions over 4000dpi. 8000dpi is roughly
0.003mm per pixel, which is a miniscle amount of movement - the slightest
vibration could cause problems.
BTW AFAIK some drum scanners use lasers as their light source which makes
it much easier to scan dense films due to the intensity of the light.
Anyone with more detailed knowledge of all these things feel free to correct
me - I'm just trying to provide a precis.
Rob
Rob Geraghty [EMAIL PROTECTED]
http://wordweb.com
