While a printed color image may appear to contain thousands of individual colors, it usually consists of just four inks, referred to as
, yellow, and black (CMYK). The process inks are transparent, so when they are combined on paper, they produce other colors (
). Thus, cyan plus yellow makes green. Cyan plus magenta make
. Yellow and magenta make red, and yellow and magenta combined with cyan makes an unattractive muddy brown. That's still a
small box of crayons. How can you make all the colors you need?
Figure 2.4. The four process inks combine to create much more than just four colors.
In traditional offset printing, the illusion of so many colors is the result of varying sizes of halftone dots, which allow different amounts of the four process colors to interact in a given area. Other printing
use different ruses to fool the eye into seeing more than four colors, but the concept is the same: Use varying amounts of CMYK to approximate a wide range of colors (
Figure 2.5. Varying sizes of halftone dots add up to an optical illusion (section of image greatly enlarged). Note the rosette pattern
by the dots.
It's important to avoid unsightly patterns, called
). To see the moir effect, put one piece of window screen on top of another, and then rotate one piece of screen. It's a challenge to eliminate an obvious pattern. That's why there are time-honored intervals of 30 degrees between the angles of the inks to create the desired rosette pattern (
). Yellow, being the lightest color,
at a 15-degree angle away from other colors.
Figure 2.6. Incorrect intervals between screen angles can result in a distracting moir. This can also be caused by a combination of screen angles and image content such as woven fabric.
Figure 2.7. Optimal screen angles add up to form a rosette pattern. Yes, it's a pattern, but it's usually not noticeable.
preferences vary between print service providers and may sometimes be
to accommodate job content (
Table 2.1. Screen angle combinations are
to minimize patterns. Traditionally, they are 30-degrees apart (for example, 45 degrees and 75 degrees). The angle of each color is chosen to minimize interference with the other colors when all inks are combined in the printed piece.
Screen Angle Combinations
It's easy to imagine that B might be mistaken for
with cyan. But where does the K come from? One theory holds that the K is from
, referring to black being printed first, so other colors can be aligned to its registration marks. Another notion is that the K is lifted from the end of the word black. Whatever the true origin of the
K in the acronym, it's here to stay.
One solution for moir is to eliminate screen angles entirely by using a printing method without the conventional grid of regularly
, also called
(frequency modulation) screening, uses a seemingly random distribution of very small dots (Figures 2.8 and 2.9). If you have an inkjet printer, you have a stochastic output device right on your desk. Look at an inkjet print through a magnifying loupe, and you'll see how the
arrangement of tiny dots creates an image.
Figure 2.8. Stochastic screening
Figure 2.9. Enlarged detail of stochastic screening
After its first appearance in the 1980s, stochastic screening failed to gain much acceptance due to limitations in
and proofing systems of the time. But it is experiencing a
(and cautious) resurgence in the printing industry, thanks to the
of computer-to-plate (CTP) printing, as well as increased implementation of digital (rather than film-based) proofing systems
Stochastic screening offers some interesting advantages over conventional
Reduced chance of moirsince there are no angles, there is almost no chance for an interference pattern to be created.
Ability to use lower resolution images.
Ability to print images containing more than four ink colors without screen angle issues.
Retention of smaller details in images.
Reduced ink usage.
Misregistration on press is less obvious.
of skin tones.
But stochastic screening has not
the old-fashioned halftone dot. In fact, it's used only in a minority of printing. There are some challenges to using FM screening:
The need for extreme cleanliness in plating:
may be bigger than a stochastic dot.
Modifications to RIPs can be expensive ($15,000$25,000).
Slightly increased RIP processing time.
On-press dot gain is higher than with conventional screening.
The possibility of visible graininess in large highlight areas.
Consult your print service provider to determine whether stochastic screening is something they offer and whether it might be appropriate for your job. Be prepared for the possibility of increased job cost because of special handling.
When you need to describe a combination of cyan, magenta, yellow, and black that will print a particular color, such as the dusky blue below (Figure 2.10), the recipe is written in this format: C75M50Y25K0. Think of halftone dots as occupying a square grid, each in its own square of the grid. The
signify a percentage of that square that will be filled. If the square is full, it's 100 percent. If half the area of the square is filled, it's 50 percent, and so on.
Figure 2.10. Dusty blue is 75 percent cyan, 50 percent magenta, and 25 percent yellow.
Limitations of CMYK
range of colors can be rendered with various combinations of cyan, magenta, yellow, and black ink, there's still a limit to what CMYK can create. The human eye can see a huge range of colorslarger than even the large gamut of a computer monitor. But the total
of the process inks is considerably smaller than the human eye can see, or even the range that the monitor can display. Consequently, images that are quite vibrant on your monitor may print disappointingly dull. It's not because your print service provider is incompetent. It's because of the limitations of the printing-ink spectrum. In
, the large,
toe is an
of the range of colors perceived by the human eye. The solid
line corresponds to the range of colors that can be displayed on an RGB (red-green-blue) computer monitor. The much smaller
shape indicates the approximate gamut of CMYK inks. Note that the CMYK blob, while rather constricted, does not fall entirely within the RGB gamut. Some colorsbright yellows and cyan shadesfall outside the range that can be displayed faithfully on a monitor. Even a finely
and color-managed monitor has its limitations.
Figure 2.11. A rough comparison of monitor and CMYK gamut to the range of visible light.
This is not intended to plunge you into despondency over the limitations of the printing process. After all, disappointment is all about expectations. If your expectations are
, you're bound to be disappointed. But if your expectations are realistic, you can be prepared for the limitations of CMYK. And, equally importantly, you'll know when you need to step
the world of cyan, magenta, yellow, and black to get what you want.
is used when it is necessary to print colors that fall outside the range of CMYK inks. While it's fairly intuitive that inks such as fluorescent colors and metallics can't be faithfully imitated by process colors, there are also some rather common colors that fall outside the CMYK universe, such as bright orange and navy blue.
You're probably familiar with the Color Formula Guides from Pantone Inc. The terms
and Pantone are often used interchangeably, although that's not
correct. A resource such as the Pantone Matching System swatchbook is actually a recipe book for print service providers. It provides ink-mixing formulas for creating over 1,000 standard colors, many of which cannot be accurately rendered in process colors. It's forgivable that the
Pantone has become synonymous with spot color, since the fan-like Pantone swatchbooks are probably the most common reference for specifying color. But there are other spot-color resources, such as the Toyo Color Finder from Toyo Ink, and the DIC guide from Dainippon Ink and Chemicals, Inc., (used predominantly in Japan). And there are Pantone swatchbooks that
depict spot colors, such as the Pantone 4-Color Process Guide, which contains only colors created by combinations of cyan, magenta, yellow, and black.
While adding a spot color to a four-color job may slightly increase the cost of printing because of the need to create an additional plate and purchase additional ink, it ensures that important colors will print as desired. Using a spot color can also eliminate problems caused by slight misregistration on press. Consider a job containing elements in a burgundy color consisting of a process-color build of C10-M100-Y35-K50. Even the most
pressman can find it challenging to keep fine elements such as small type and narrow rules in register across a large press sheet. It can also be difficult to keep the balance of four inks consistent from one part of the paper to another or from one press sheet to another in a long, multipage job. Any variation will result in color shifts, which would be
pages. Replacing the process build with a single ink, such as Pantone 209,
both registration and color-consistency issues. The small increase in printing costs (as opposed to a four-color job) might be justified by the improved outcome.
It's time to dispel some urban myths about spot-color designations. The terms
, not ink. Pantone 185C is Pantone 185U is Pantone 185M (apologies to Gertrude Stein). The C represents coated paper, U signifies uncoated stock, and M indicates matte paper, whose surface texture falls between that of coated and uncoated. These designations are primarily intended to keep you oriented as you view color on your computer monitor. For example, you may notice that a U version of a Pantone color looks a bit less
compared to the C version. It's just an attempt to
ink behavior on different stocks. In the olden days, CVU meant computer video uncoated, and CVC meant computer video coated. But recent DTP software has simplified this to U and C, and added the enlightened M for matte.
Since its inception, the
from TRUMATCH, Inc., has provided only CMYK builds.
The Pantone Corporation also provides several CMYK-based color specifiers:
Solid to Process. Uses the same familiar Pantone numbers but indicates process builds.
Solid to Process (Euro). Same concept as PC, but with slightly different CMYK equivalents.
. Does not use ancestral Pantone numbers, which may help avoid confusion.
Approximating Spot Colors with Process
It's a widespread practice to pick colors from a swatchbook such as the Pantone Color Formula Guide, even for jobs that are intended to print as process. Just because everyone does it doesn't mean it's right. (Sorry. That sounds like your mother.)
The problem with this approach (as with so many things your mother
you about), is that it can lead to disappointment. Remember that the purpose of spot colors is to render colors that fall outside the range of CMYK. Understandably, process approximations of spot colors are often unsatisfactory.
For example, a CMYK translation of a dark blue such as Pantone 286 can become a purplish blue (
). It's unfortunate, but this is as close as a combination of cyan, magenta, yellow, and black can get to the navy blue that should be used for the Brand X logo. As long as you know to expect this color approximation, you aren't shocked by the printed piece. But the president of Brand X will
Figure 2.12. The Brand X logo is supposed to print in Pantone 286, a navy blue. But approximating that color with CMYK results in an unsightly purple. If you have a Pantone swatch book, compare the real Pantone 286 swatch with the CMYK version at left.
In the interest of
on process jobs, consider selecting colors from a purely CMYK-based swatchbook instead, such as the TRUMATCH Colorfinder or one of the Pantone process guides. If you want a single-source swatchbook showing Pantone spot-color formulas
process equivalents, the Pantone Color Bridge provides helpful, side-by-side swatches.