RE: [Leica] Scanning

["Austin Franklin" <darkroom@ix.netcom.com>]

> Austin writes:
>
> > I believe your premise is flawed.  You need to
> > sample at slightly more than 2x the maximum frequency
> > you want to acquire.  A scanner IS a data acquisition
> > system.  53 lp/mm is 106 lines/mm, which, as
> > you say, is ~2700 SPI.  In order to assure that
> > the scanner can actually "acquire" all the lines,
> > both black and white (which is what a line pair is),
> > you would need to sample at > 5400 SPI.
>
> No.  2700 dpi is sufficient.  2700 samples per inch = 1350 cycles
> per inch =
> just over 53 cycles (i.e., line pairs) per millimetre.

If you want to detect a line that has a width of 0.009mm, which is the width
of the line in a 53 lp/mm set, which is 106 lines, you need to sample at
slightly more than 2x that, or < .0045 mm/sensor in order to detect the line
RELIABLY, ie, every time.  Your are flawed in thinking that you can detect a
.009mm line with a .009mm wide sensor RELIABLY.  With your "thinking" you
can detect it only some of the time, and it depends on the alignment of the
lines with the sensor elements.

Draw a picture for your self, and you will see that if you make your line
pairs which are 0.009mm per line, straddle the .009mm sensors, such that 1/2
of a line is over one sensor, and 1/2 over the other, you may not detect any
lines at all, you will get gray.  Each sensor element is covered by 1/2 of a
black line and 1/2 of a white line.

Please think about this before replying.  I've spend the past 20+ years
designing digital imaging systems...so this is not just an academic exercise
for me.