At 23:50 15/01/01, 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? Why is a drum scanner such a high resolution device?
I am not greatly into this technology, so I'll make a guess about a couple
of things and those who know can correct me.
Photomultipliers differ from CCDs in that they have internal amplification
and hence can be more sensitive. The only ones I have ever seen are bulky
and thus could not be used in arrays like CCDs, so my GUESS is that drum
scanners use only one or a limited number of photomultipliers, and move it
across the image to cover the whole territory. I have never seen a drum
scanner so this is a guess, pleas correct me if this is not true. I am
assuming that the purpose of the drum is to make it easy to spin the image
past the sensor for one line of resolution, then move the sensor up one
line and read the next line.
If that is true, then because you only have one sensor, you can engineer it
to greater tolerances, and read a smaller spot size and thus get better
resolution. Because you only have one sensor too, you can design the
amplifier and subsequent circuitry in a more expensive way and thus get
better performance - or at least you don't need the switches that you would
need to read an array of sensors. And because you only have one sensor you
don't have the problem of matching the response of thousands of different
sensors and their associated switching circuitry etc., as you do for CCD
array scanners.
Hope this helps, or elicits more accurate information,
Julian
Julian Robinson
in usually sunny, smog free Canberra, Australia
