"Airbus" wrote

Amazing that this 1930's technology strill holds its own, and indeed
offers the
"high end" solution in today's digital world. . .

There is nothing else that combines the sensitivity and speed.

Since only a very tiny spot about 200 1/1,000,000th's of an inch across
is being scanned the total amount of light from this spot is pretty weak --
even with the bright light sources and fancy condenser systems. To keep the
scan times reasonable the sensor can't dwell on any part of the image
for very long.

If speed weren't an issue then a photodiode would make a better sensor.

The CCD's in a scanner do image the same spot size but as there are several
thousand acting in parallel the amount of time a sensor can dwell on a part
of the image is considerably longer.

If a photodiode were to be used in a drum scanner the scan time would
be 4,000 times longer (gross, wild-arsed-guess estimate).

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/da-main.htm
n o lindan at ix dot netcom dot com