Page 155 - Graphic Design and Print Production Fundamentals

Graphic Design 143
Colour Separations

When colour separating photographs, we can build in an appropriate amount of GCR or UCR to give us
the right total ink coverage for the imaging technology we are using for reproduction. UCR stands for
under colour removal. It is applied to very dark shadow areas to remove equal amounts of cyan, magenta,
and yellow (CMY) where they exceed the total ink limit. For example, in sheet-fed lithography, a typical
total ink limit is 360. In areas that print 100% of all four colours, UCR will typically leave the full range
black separation, and remove more and more CMY the deeper the shadow colour is. A typical grey
balance in shadows may be 95% cyan, 85% magenta, and 85% yellow. Including a 100% black, that area
would have a total ink coverage of 365. Other imaging technologies have different total ink limits, and
these can vary greatly from one substrate to another within an imaging technology. An uncoated sheet
will absorb more ink than a glossy coated sheet of paper and so will have a different total ink limit.

GCR stands for grey component replacement, and it is intended to help improve grey balance stability in
a print run and save on ink costs. GCR can affect far more colours than UCR as it can be set to replace
equal amounts of CMY with black all the way into the highlight greys. This is particularly useful in
technologies like web offset newspaper production. Grey balance is quickly achieved in the make-ready
process and easily maintained through the print run. Black ink for offset printing is significantly cheaper
than the other process colours, so there are cost savings for long runs as well. GCR is used in photos
and vector graphics produced for other imaging technologies as well. Any process where grey balance
of equal values of the three primary colours could be an issue is a smart place to employ GCR.

You may be wondering how we can check the shadow areas of every photo we use. These GCR and
UCR values can be set in ICC profiles by linking the shadow and neutral Lab values to the appropriate
CMYK recipes. When the ICC profile is applied for a given output device, the shadows get the proper
ink limits, and the grey tones get the prescribed amount of black, replacing CMY values.

Keylines

Black keylines, or outline frames for photos, are common in many documents. This is another place
where a document should have trapping software applied for every imaging technology. Outline strokes
on graphics can also have a ‘hairline’ setting, which asks the output device to make the thinnest line
possible for the resolution of the device. This was intended for in-house studio printers where the
resolution is 300 dpi — so the lines are 1/300th of an inch. But the same command sent to a 3,000
lspi plate-setter will generate a line 1/3000th of an inch, which is not visible to the naked eye. These
commands must be distinguished in PostScript and replaced with lines at an appropriate resolution —
trapping and preflight software will do this.

Knock outs

The use of solid black backgrounds is becoming more popular in documents, which can cause problems
in reproduction with all imaging technologies. The first problem is with filling in details in the graphic
shapes that are knocked out of the solid black background. Fine type serifs, small registered or trademark
symbols, or the fine hairlines mentioned above will all fill in and be obliterated when imaged. The