Tom,
The confusion with the resolving power spec is that resolving power is measured
not in "lines per inch" or "lines per cm", but actually in "Line PAIRS per inch or
cm" If a lens or a media could resolve 2450 line pairs per inch, each pair would
be 10 microns. The black and the white line would each be 5 microns.
This has nothing to do with graphic arts. The term LPI only means the number of
ROWS of halftone dots that are in 1 linear inch. The way this is measured is
always in the dimension perpendicular to the screen angle. For example, picture a
checkerboard pattern. A checkerboard is always a 50% dot at 45 degrees. The LPI of
the checkerboard is measured across the diagonal of the dots. Take a checkerboard
dot which is made up of 10x10 imagesetter pixels, and the imagesetter is running
at 2540 dpi. (the dots are 100 microns by 100 microns). The screen ruling is not
254 lpi. You have to measure the screen ruling across the diagonal so multiply the
254 lpi by .707 and you get 179.6 lpi.
This is to say that an imagesetter cannot possibly produce a screen ruling which
is more than .707 times the DPI of the machine. The maximum screen ruling at 2400
dpi would be 1696.8 lpi. This would be imaging a checkerboard 50% dot at 45
degrees where each halftone dot is only 1 pixel. Of course, you'd only have 3 gray
levels!
Dave
This archive was generated by hypermail 2b29 : Tue Jan 11 2000 - 12:10:49