Pixels 101

ppi = PIXELS PER INCH
Usually confused with dpi (see next). Best used in reference to actual scan resolution. A pixel is a unit of unmeasurable length. How many of these units you utilize per linear inch is your ppi.Typical Scan ppi is 300 ppi for a qood quality scan. Web graphics or screen images are typically 72 ppi.

dpi = DOTS PER INCH
Usually confused with ppi (see above). Best used in reference to the resolution of an imagesetter such as a Lino 330. Typical resolutions are 1270, 2540 and 3386.

lpi = LINES PER INCH
This refers to the number of lines per inch for which things will be screened for purposes of actual printing. Typical screens are 175, 150, 133, 120, 110, 100, 85 and 65. 150 is general standard for good quality on a coated stock of paper. 85 is most commonly used in newspapers where the ink tends to spread (gain) .

So how do you know what ppi you should scan at for the expected line screen?

Use this simple rule:
ppi should be 2 x your expected lpi.

For example:
Images in TIME magazine printing at 150 lpi,
your scans should be 300 ppi.
Images used in Post-Dispatch printing at 85 lpi,
your scans should be 170 ppi.

But what about the dpi?

The imagesetter resolution is typically one of only three choices:
3386 - very high quality, process separations, above 175 lpi.
2540 - most common for general purpose separations of 150 lpi and less.
1270 - use only for text, line art or simple spot separations.

How Big is BIG?

How to calculate the file size of a scan before you scan it:

File Size (Mb) = width x height x [dpi x dpi]x channels ÷ 1 million

File Size (Kb) = width x height x [dpi x dpi]x channels ÷ 1 thousand

Width & Height in inches
dpi2 = Line Screen X 2 if Contone'; scan res if line work
Channels = 1 gray, 3 RGB, 4 CMYK, 1/8 for B/W line work

Example (150LPI): 8.5" x 11" x [300 x 300]x 4 ÷ 1 million = 33.6 Mb

If scaling required multiply result by scaling percentage.

Separation Parameters

Use these settings as a guide for your own scanner if your software allows your input. These settings can also be found in Photoshop and can make a difference when converting from RGB to CMYK.

Standard S.W.O.P. printing conditions

Total Ink = 300% max. on any one area of the page

Highlight Dot = 5%C, 3%M, 3%Y, 0%K
– This is the least they can hold

Shadow Dot = 75%C, 65%M, 65%Y, 95%K
– This is the most they can hold; added together this equals our 300%

Dot Gain on web = 25%
Dot Gain on sheet fed = 15%
Dot Gain example, if 50% dot on film, then measure that same 50% dot after it has
been put on press. The resulting reading less 50% equals the dot gain.

UCR = Under Color Removal = Equal values of CMY = Gray

GCR = Gray component removal, same as UCR but does it in colors

UCR replaces equal % of CMY with K in neutrals

GCR removes equal % of CMY in neutrals AND colors