Digital BW, The Print

Scott McLoughlin wrote:

> Stupid question, but why *are* drum scanners so much more expensive
> than the alternatives?
>
> Scott

They are harder to make.

With a CCD flatbed, you need one degree of motion. You scan a CCD array 
down the length of the film. Parallel capture.

With a drum scanner, you need two degrees of motion. You scan a single 
point at a time. Serial capture.

For serial capture to work, you have to move the film past the aperture 
(turn the drum past the scan point) to scan a line one pixel at a time. 
Then you have to index the drum to the next line and do it again. In 
order to space the scanning samples (pixels) just right, much precision 
is required, and different methods are used for the two degrees of 
motion. On top of that, a drum scanner is typically a higher precision 
machine to begin with because many of them are designed to try to get 
8000 ppi or greater. At that level, vibration is a serious problem 
(which is one reasons they are so big and heavy). All of this, because 
PMTs are expensive to make and pretty big on their own. That is, you 
can't afford to make a line of, say, 4000 PMTs like you can CCDs, and if 
you could afford it, you couldn't fit them together closely enough. 
Finally, since they are big and heavy and fairly expensive, only service 
bureaus wanted to buy them (individual photographers typically did not), 
which means that the production runs where small, which in turn ran the 
cost up even higher. There is little automation in the making of a drum 
scanner because of the small production runs. They are typically 
assembled completely by hand.

You can probably see where all this goes - with every step the price 
spirals upwards.
--
Bruce Watson