Re: palladium vs. cyanotype sharpness / Resolution

The difference here is that the discussion refers to "printer"
resolution, not scan resolution, scan resolution is generally thought
of as "pixels per inch".

As far as an imagesetter is concerned, the resolution is generally
express as "dots per inch" or "machine spots per inch"and the fineness
of a conventional halftone screening is expressed as "lines per inch".
Each halftone line is composed of a given number of halftone cells and
each cell is composed of a given number of "machine spots". So, in the
example that was given in an earlier post, the imagesetter is capable
of printing 3600 "machine spots per inch", the machine spots are then
grouped ( by the output RIP) into cells of 16 spots across and 16 spots
  down to create a halftone cell, each row of halftone cells is
designated as a "line". This halftone cell is now able to represent 256
tones of gray (16x16) depending on how many machine spots are directed
to print within the halftone cell. We will assume that 256 levels of
gray is the maximum available for printing 8-bit grayscale. So, if you
wish each halftone cell to be capable of reproducing 256 levels of
gray, the maximum line screen (lpi) is 225 "lines per inch" in this
configuration (3600 "spots per inch" divided by 16 spots per line) .
If your image doesn't require the maximum reproducible number of grays
per halftone cell - many images don't - you can print at a higher line
screen, say 400 lpi, which will allow only 81 levels of gray per
halftone cell (3600 dpi divided by 400 lpi = 9 dot x 9 dot halftone
cells = 81 possible tones per halftone cell).

I hope this helps. Resolution terms can be very confusing because
expressions like "dpi" are used interchangeably across different
devices, when you think about it, there are no "dots" in your scans
until they're ripped for printing.

Jim Morris

On Sep 21, 2005, at 10:10 AM, BOB KISS wrote:

> Please help me understand...
> I always thought "dpi" was referring to scanning resolution and "lpi"
> was
> referring to the frequency of lines (which result in dots) of printers
> including the Imagesetter. Both my local service bureau which has made
> Imagesetter negatives for me in the past and a printer who also has an
> Imagesetter stated that they only printed up to 300 lpi.
> In many of these sharpness postings there are references to the
> Imagesetter
> being set at 3,600 dpi. From what I can tell, the digital file you
> feed
> into the Imagesetter may be up to any resolution you like but it is
> only
> capable of giving you no more than a 300 lpi negative...right?
> Or have I once again been misinformed.
Received on Wed Sep 21 11:30:34 2005