Graphics Programs Reference
In-Depth Information
specified when building a CMYK profile such as
total ink limits
, something
called
UCR/GCR
,
black start
, and
minimum/maximum black ink usage
. Don't
panic, I'll explain each.
Total Ink Limits or TAC
Total Ink Limits, also known as TAC (Total Area Coverage), specifies how
much ink is used when printing. If we have four inks and print each at
100 percent capacity, the total ink would be 400 percent. On a press, this
is an impossible amount of ink to use since doing so would produce
output that would be dripping wet with ink. In addition, inks are expen-
sive so printers want to use as little as possible while still producing the
quality output necessary. Total ink limits are built into the output profile
by specifying how much of each ink to use. Most products allow the total
ink to be divided by the individual inks, usually CMY versus K. Coated
or less absorbent paper stocks often have higher total ink limits gener-
ally ranging from 280 to 340 percent. Newsprint uses significantly more
absorbent paper stock and often uses a TAC value around 200 to 240
percent. Ideally you would find out what the printer prefers for total ink
limits and enter that value into the profile building software. If such a
value is not available, an educated guess would be approximately 300
percent for coated stock and 220 percent for newsprint. Those values will
get you in the ballpark. For ink-jet printers, dye subprinters, and most
toner-based printers that will output CMYK, a 400 percent total ink limit
is an appropriate starting point. The RIPs or drivers that control these
printers can play a role in ink limits. In Fig. 7-3 I've selected two preset
defaults in ProfileMaker Pro for Offset and Newsprint. You can see the
total ink limits reflect the basic guidelines shown earlier, and the overall
distribution of CMY inks in relationship to K.
UCR/GCR
There are a number of ways to specify a color by mixing different com-
binations of CMY with K. For example, you can produce a neutral gray
with just CMY inks if the correct combination is produced. Alternatively,
some black can be added to the mix. It's rare that only black ink is used
to print anything but a solid black item like text or a drop shadow. This
brings us to part of the separation processes known as
UCR
(
Under Color
Removal
) and
GCR
(
Gray Component Replacement
). The two are used to
control the mix of CMY along with K in certain portions of an image.
UCR is able to identify where CMY produces neutral values in addition
to some colors that contain a large mix of neutral values. An example of
the latter is a dark brown. UCR then replaces some combination of CMY
with the correct amount of black ink. In other words, UCR is the con-
trolled removal of a specific ratio of CMY inks with a replacement of black
ink. Far less CMY inks are used since black ink is substituted. Why use
expensive CMY inks to produce a neutral gray or color when one lesser
