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Old November 20th 10, 09:09 PM posted to rec.photo.equipment.medium-format
Geoffrey S. Mendelson
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Default Sending film in for developing.

Ric Trexell wrote:
I'm
familar with the drum scanner process a little bit, but couldn't explain it
if my life depended on it.


Maybe this will help:

Take a picture and and make a tube out of it. Take bright flashlight and shine
it at a spot on the tube. Look at that spot with a magnifying glass. Now rotate
the tube a little bit and look again. Keep doing that until you've looked at
every spot around the tube. Then move the light and magnifying glass a little
to the side and start all over again.

Eventually you've seen every spot on the tube, which happens to be every spot
of picture.

Now why is this better than a regular scanner? A regular scanner uses a
row of photocells. The resolution across is limited to how close the
photocells are placed. The smaller a photocell, the more noise it
produces (meaningless information), the more light it needs to see
and the higher the cost.

Since you are using a highly focused beam of light and a lens on the
photocell, a drum scanner can use a much more sensitive, but large cell
and still have a smaller spot. The number of spots across it can "see"
is determined by how narrow the beam is and how precise the postioning
mechanism is.

The postioning mechanism is basicly a screw on a stepper motor, so for any
given resolution it's cheaper than squeezing photcells together.

The spot size around the drum is determined by how far around it moves
during each scan cycle. More prescision can be obtained electricaly by
using faster photocells and analog to digital converters, or it can be
obtained by slowing down the drum.

Did that help?

Geoff.


--
Geoffrey S. Mendelson N3OWJ/4X1GM
To help restaurants, as part of the "stimulus package", everyone must order
dessert. As part of the socialized health plan, you are forbidden to eat it. :-)