CYMK conversion issues:
Low-end printers convert RGB data to CYMK internally, but if
you prepare images for high-end imagesetting or proofing
systems it is necessary to convert the RGB files to CYMK
before output. Conversion in PhotoShop is done via the
color management engine or CMM. Several are available when
the advanced options box is checked on the Edit > Color
Settings screen. If you use QuarkXpress for layout you
should use the Apple ColorSync CMM, which is the CMM
QuarkXpress uses. The rendering intent setting also
controls how the colors are mapped from RGB to CYMK. The
most critical factor is the mapping of the white tones
Relative Colormetric is recommended by PhotoShop for most
applications and it is the only rendering intent supported
by QuarkXpress. There are other CYMK issues such as dot
gain, 300% maximum combined percent dot for all four colors,
and grey component removal (GCR) which require coordination
and consultation with your imagesetting and printing
suppliers before CYMK conversions for offset printing are
made. In general it is best to let the printer do the CYMK
conversions using profiles created from test subjects
printed with various paper stock / ink combination.
CYMK SWOP
One of the first printing industry color standards
was SWOP, which standards for "Standard for Web Offset
Printing". Developed in the 1970s this standard requires the
use of CYMK printing inks of a standard strength and color,
and specifies the ranges of ink densities, dot gain, and
paper reflectances which are acceptable. It also specifies standard viewing
conditions, including the color temperature of the light the
proofs are viewed under and the gray color of the
surrounding walls.
When I worked for a large web offset printer in the
late 1970s CYMK separation films were received from a wide
range of sources for the editorial content and
advertisements in the mangazines we printed. Each
submission of film was accompanied by a press proof with
separate CYMK ink patches (i.e., color bars). A quality
control technician would read the color bars with a
densitometer and compare the proof with the SWOP standard
densities. The dots on the film would be examined with a
magnifying glass and compared with the proof to identify any
excessing dot gain. If the proof densities were within the
range specified by SWOP and the hue of the inks also
matched, then there was a reasonable expectation the films
could be printed without a problem when run in tandem with
other color subjects. If a proof was found to be outside the
SWOP standards the proof and film would be rejected. In the
1980s Imation Matchprint proofs made with SWOP standard
colors replaced press proofs as the "gold standard" for
evaluating color separations.
CYMK inkjet technology, more than anything else in
recent years, has revolutionized the graphic arts industry.
It is now possible to produce a photo quality printout from
a ink jet printer costing less than $300. An entirely new
and rapidly growing segment of the graphic arts industry has
been created around the rapid quality advances and
decreasing cost of wide-format inkjet printers. The move to
a completely digital workflow will eventually eliminate the
traditional step of imagesetting to film and making a
Matchprint and a calibrated inkjet proof will instead be
used, increasing the importance of an accurately calibrated
color workflow.
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