The most popular program used on desktop computers for color separation
is Adobe Photoshop software. Photoshop has the ability to convert from any
of its many color spaces to CMYK, and with excellent results Out of the
boxs the color conversion from RGB to CMYK can be unacceptable but this
is usually because Photoshop owners don't take the time to set up the program
to make pleasing color separations
Photoshop's "Preferences" windows are critically important to
the process of correct and successful color conversion Take the time to
set the preferences correctly and the results will improve. First of these
is "Monitor Set-up."
The settings for monitors in Photoshop software include many of the industry
standards as well as settings for custom phosphor and color temperature
You must know the correct values to enter into the custom window to be successful
Some monitors will come with phosphor color values and color temperature
information but this is unusuah In the absence of such datas try entenng
the known manufacturer of the monitor useds and set the color temperature
to 6500K. Recent advances in monitor technology have made it possible for
manufacturers to deliver monitors for graphic-arts applications that have
a 5000K white point Most existing monitors are balanced to the 6500K standard
though and changing the setting does not change actual color temperature
as that is set in the manufacture of the monitor
"Printing Inks Set-up"is the second preference that must be addressed
in Photoshop software In this window
are settings for standard printing ink standards such as SWOP (Specifications
for Web Offset Publications) and SNAP (Specifications for Newspaper Advertising
Printing). These standards are designed to establish methods by which ads
destined for web offset printing can be proofed. Most sheet-fed printers
are equally comfortable with SWOP standards.
In the Printing Inks Set-up window is a custom option where actual ink color
performance can be entered to customize the settings. For this to work,
a spectrophotometer or colorimeter must be used to measure a series of solids
in progressive order (cyan, magenta, yellow, red, green, blue, CMYK, white
and black).
Once the readings are made, the color values must be converted from their
spectral values to CIExyY values for this window. Instruments such as the
X-Rite 938 Spectrophotometer, Gretag's Colorimeter and Lightsource's ColorTron
can make these conversions, delivering the values in the right mathematical
system for success.
Dot gain is also entered in "Printing Inks Set-up." The value
in the window is measured at 50 percent (how much the 50 percent dot gains
on press - in percent). If you don't know the dot gain value, it is important
to either make a press test, or get a valid average dot gain value from
your printer. And dot gains vary with different presses and especially different
papers. Coated sheets have significantly less gain than the uncoated sheets,
and newsprint and other porous papers have radically different performance
than other sheets.
The last element in the Inks Set-up is values for individual colors on the
press. Usually unnecessary, these settings are multipliers that can compensate
for radical ink variations on a certain press. If cyan is consistently 5
percent high on one press (due to some condition of the press or inks) you
can compensate for this by using a .95 multiplier value in the cyan window.
"Color Separation Set-up" is the next important preference setting
that one must address when planning to make dolor separations from Photoshop
software. The Adobe 'factory default" setting is for GCR separation
with medium settings. Interestingly, GCR was less common before Photoshop
software, and the option has spawned considerable discussion in the printing
industry.
GCR is an acronym for gray component replacement and is also known as polychromatic
color replacement by some manufacturers. The basic concept is to enhance
the balance of neutral grays on a fast-moving web press. Components of cyan,
magenta, and yellow ink - all of which add up to neutral gray - are subtracted
from each of those colors, and a small amount of density is added to the
black printer in the same area. Since black is just one color, maintaining
balance on-press with one color is simple compared to the same feat with
three or four colors. (SWOP specifications define neutral gray balance as
C25 Ml6 Y16 for 25 percent, C50 M39 Y39 for 50 percent, and C75 M63 Y63
for 75 percent.)
Using GCR for sheet-fed printing is fine, as long as the printer has a say
in the decision - ask your printer to determine their standards for color
separations, and determine what level of GCR they like (medium is usually
adequate).
The screen printing industry is finding that GCR separations are making
the process of printing on fabrics dramatically easier. Maintaining neutral
gray balance - quite difficult in screen printing - is much easier with
GCR separations, resulting in better jobs and less spoilage.
SWOP standards are usually easier to achieve using GCR, so most of the available
software for SWOP standards will implement GCR as a default.
UCR is the other option in Separation Set-up. UCR is an acronym for Undercolor
Removal, which is a standard color separation with ink limits applied. While
UCR and GCR are related Photoshop software treats them as opposites; you
must choose either one or the other. In either case it is helpful to know
the total ink coverage value for the printing process and press you will
he using as this value is entered in both GCR and UCR setups. Total ink
coverage is the measure of how much ink can be piled up without introducing
drying and set-off problems on the press. Values in the 250-to-300 range
are typical; ask your printer!
Once these set-up options are set in Photoshop, the color separation process
will be possible. Without taking the time to make these settings, the conversion
of either RGB or CIELAB to CMYK is effectively impossible. And even after
making these settings, the individual production situation will dictate
an occasional adjustment that makes the process better.
