FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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108 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Appendix B

1. CHECK FPOs

2. LIVE IMAGES PLACED ■■ YES ■■ NO

Comments

3. FONTS SUPPLIED ■■ YES ■■ NO

Comments

4. IMPORTED EPS SUPPLIED ■■ YES ■■ NO

Comments

5. IMAGES IF SUPPLIED ■■ HI RES DPI _____ ■■ RGB ■■ CMYK

Comments

6. RULES (SMALLEST ALLOWED) IN SPEC

0.007 POSITIVE ■■ YES ■■ NO

0.005 REVERSE ■■ YES ■■ NO

Comments

7. TEXT (SMALLEST ALLOWED) IN SPEC

POSITIVE ■■ SERIF ■■ SANS SERIF 6 PT ■■ YES ■■ NO

REVERSE ■■ SERIF ■■ SANS SERIF 6 PT ■■ YES ■■ NO

8. FONTS – MISSING

Screen Printer

PREFLIGHT CHECKLIST

CHAPTER 3

Process Color

110 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ACKNOWLEDGEMENTS

Author/Editor: Michael Wiest, FFTA

Contributors: Tony Bart, DuPont Company

Nick Lena, GretagMacbeth

Mark Samworth, PCC Artwork Systems

Pantone and PMS is a registered trademarks of Pantone, Inc. Apple, Macintosh are registered trademarks, and

TrueType is a trademark of Apple Computer, Inc. Adobe, Adobe Acrobat, Adobe Dimensions, Adobe Distiller, Adobe

Illustrator, Adobe Pagemaker, Adobe Photoshop and PostScript are trademarks of Adobe Systems Incorporated or

its subsidiaries and may be registered in certain jurisdictions. QuarkXpress is a registered trademark of Quark, Inc.

FreeHand is a trademark of Macromedia, Inc. DOS and Windows are trademarks of Microsoft Corporation. All other

trademarks are the property of their respective owners.

All trademarks have been used in an editorial fashion with no intention of infringement.

PROCESS COLOR 111

n the past, flexography was seen as a

low-cost and low-quality printing

process. That image has changed with

advances in materials, equipment and

techniques. The simplicity, efficiency

and consistency of flexography’s ink-

metering system continues to improve.

Printing plates continue to see new advance-

ments, including digital output directly to

plates. Anilox rolls are available in higher

rulings and take advantage of high-strength

inks. Presses have better control as well as

on-line inspection and feedback systems.

The entire production process, from design

to press, is being specified by FIRST.

These are a few of the trends which enable

flexo to achieve consistently high-quality

results. The ultimate test of a major printing

process is its ability to print consistent, high

quality process color. That it can be done is

demonstrated daily on the shelves of stores

and by the ever increasing quality of entries

in flexo printing competitions worldwide.

Still, process-color printing with flexography

remains a challenge for many printers. This

section is intended to present material that

is designed to facilitate better control of the

process.

“Process printing”, or “process-color print-

ing”, refers to the full-color reproduction of a

subject by recreating the original’s full, con-

tinuous-tone color. Subjects can range from

paintings or color transparencies to full-color

photographic prints. In today’s environment,

the subject can also be the image captured

electronically by a digital camera.

Process printing is achieved by first con-

verting the continuous-tone copy to

halftones, separating the color into yellow,

magenta and cyan process colors and print-

ing these colors sequentially in register with

each other. This “three-color process” is fre-

quently converted to a “four-color process”

by using black to improve the contrast

and/or tone balance of the reproduction. It is

essentially the same for all printing meth-

ods—flexography, offset lithography, or

gravure—but corrections are made for the

different mechanics of a given method.

Obviously, printing four colors in register

is more difficult than printing only one color

and should be attempted only when the

printer has the equipment, materials and

skills to achieve good-quality, single-color

flexo printing.

To print process color successfully with

flexography, it is important to understand

each step in the process and how to perform

it. This includes a basic understanding of

color theory; what it is and how it is mea-

sured and controlled. The control of color

applies to the entire process, from the origi-

nal color object, to it’s conversion to process

colors, to the final printing on the press. The

press itself needs to be optimized, character-

ized and controlled, in order to achieve con-

sistent, quality process color.

More than anything else, successful

process-color printing demands a dedicated

team effort between the color separator, ink

maker, platemaker, printer and print buyer.

Clear communication among team members

is essential.

Introduction

olor has been defined as “the

perception of light that has been

modified by an object.” This def-

inition actually refers to more

than color. It alludes to what

determines color; a light source,

an object and observer. These elements are

illustrated in Figure

Note: Light comes from a source and is

modified not only by the object being

observed, but also by the surroundings.

The first element to examine is light itself.

The light we see is part of a natural phenom-

enon that includes x-rays, ultraviolet radia-

tion, visible light, infrared radiation, televi-

sion and radio waves. The key word is

waves. All are a class of what is called elec-

tromagnetic radiation and the key difference

is in the wavelength. X-rays have the short-

est wavelength and radio the longest.

Visible light ranges in wavelength from

approximately 400 to 700 nanometers (nm).

White light contains an equal amount of all

of these wavelengths. It can be broken out or

dispersed, such as with a glass prism, into

light of the separate wavelengths that make

up the “colors of the rainbow” (Figure

All visible light is a combination of these

wavelengths.

PERFECT SPECTRA

Beside the wavelength of the light, intensi-

ty is a key attribute. Light is composed of a

combination of intensities of the visible

wavelengths. A graph of this distribution is

called the spectrum of the light. White light

is composed of equal intensities of all wave-

lengths, shown in Figure

. The vertical

axis is the intensity of the light and the hori-

zontal axis is the wavelength. The intensity

scale goes from 0 to 100 and light that con-

tains a uniform intensity of 100 at all wave-

lengths is white light. At a lower intensity,

but still equally distributed, the light is gray,

and then at a zero intensity, black or no light.

Figure

shows the spectra of “perfect”

neutrals.

Spectra are extremely useful when talking

about color. The visible spectrum can be

divided roughly into thirds, with each third

representing one of the colors: red, green or

blue. Figure

shows a “perfect” red. It

would have no intensity for the first two

thirds of the visible spectrum and then full

intensity from about 600 to 700 nm. Similarly,

Figure

shows a “perfect” green, which

has intensity in the middle of the spectrum

from about 500 to 600 nm and zero every-

where else. Finally, Figure

shows a “per-

fect” blue, which has intensity in the first

third of the spectrum up to about 500 nm.

PROCESS COLOR 113

Color Theory

Elements that

determine color: light

source, object and

human observer. All

are influenced by the

surroundings.

Light

Source

Object

Surround

Human

Eye

114 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Additive Color

The three spectra in Figures

and

are for the three additive primaries of red,

green and blue. If we were to take all three

spectra and add them together, the result

would be the spectrum of Figure

, that is

white. Of course adding together the spectra

is nothing more than combining or adding

the light itself. It is the same as shining three

beams of different colored light onto one

area. The primary colors of red, green and

blue combine as shown in Figure

Subtractive Color

The spectrum for the addition of red and

green light, which produces yellow, is shown

in Figure

. This spectrum can be thought

of in two ways. One is as was just described.

It is the addition of red and green light.

Another way of describing the exact same

spectrum is to say it is the subtraction of

blue light. That is, instead of starting with no

Dispersion of white light

into the constituent

wavelengths

Spectrum of three per-

fect neutrals: white, gray

and black.

400 500 600 700

100

Wavelength (nm)

Intensity

Wavelength (nm)

780

700

600

Visible Light

500

400

380

Broadcasting

Shortwave Radio

Television

Radar

Infrared

Ultraviolet

X-Rays

Gamma-Rays

Cosmic Rays

Visible Light

-2

-4

-6

-8

-10

-12

-14

Wavelength (m)

light and adding red and green, we start with

white light and take away blue. Similarly,

Figure

shows red and blue light combin-

ing to give magenta. This can be alternative-

ly thought of as starting with white light and

subtracting green. Finally, Figure

shows

the case of blue and green combining to pro-

duce cyan. This can be thought of as starting

with white light and subtracting red.

Starting with white and taking away one

third of the light at a time is utilizing the sub-

tractive primaries of yellow, magenta and

cyan. This is what happens in printing. We

start with a white (or at least highly reflec-

tive) substrate and the inks we use (cyan,

magenta, yellow) each take away roughly

one third of the visible spectrum. They, com-

bine as shown in Figure

Note: The () symbols in Figure

mean

combine, and connote adding or increasing

something. “Adding” subtractive primaries

means taking away light. All printing is a

subtractive process (Figure

Using this concept of taking away, gives

the same result as shown in Figure

Combining magenta and yellow inks takes

away green and blue light, leaving red.

Combining magenta and cyan inks, takes

away green and red light, leaving blue.

Combining yellow and cyan inks, takes away

blue and red light, leaving green.

REAL-WORLD SPECTRA

Figure

shows three examples of real

world cyan: a flexo cyan ink, an offset cyan

PROCESS COLOR 115

Spectrum of perfect red

showing light intensity

in upper third of spec-

trum.

Spectrum of perfect

green showing light

intensity in middle third

of spectrum

Spectrum of perfect blue

showing light intensity

in lower third of spec-

trum

The combination of the

additive primaries, red,

green and blue.

400 500 600 700

100

Wavelength (nm)

Intensity

400 500 600 700

100

Wavelength (nm)

Intensity

Red + Green = Yellow

Red + Blue = Magenta

Green + Blue = Cyan

Red + Green + Blue = White

Yellow

Cyan

Magenta

RED

GREEN

BLUE

400 500 600 700

100

Wavelength (nm)

Intensity

116 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ink and a cyan from a digital proofing sys-

tem. Regarding these spectra:

1. The perfect cyan of Figure

, com-

pared to the real cyan has some light in

the red portion of the spectrum. This

means the cyan is not as pure a color as

the perfect cyan. It is contaminated with

some red.

2. The curves are different, as might be

expected for three different types of

cyan. The bigger the difference in spec-

tra, the bigger the difference in the

appearance or color of the ink. The off-

set and proof curves are closer together

than the flexo curve. This is evidence of

the fact that proofing systems have

been optimized for offset printing, not

for flexo. The proofing cyan is closer to

the press cyan for offset than for flexo.

3. The peak in the blue and green portion

of the spectrum is not as high as in the

perfect cyan. This means the color is

less saturated.

Spectrum of perfect yel-

low showing NO light

intensity in lower third

of spectrum.

Spectrum of perfect

magenta showing NO

light intensity in middle

third of spectrum.

Spectrum of perfect

cyan showing no light

intensity in upper third

of spectrum.

The combination of the

subtractive primaries,

yellow, magenta, cyan.

Printing is a subtractive

process where the inks

take away light.

Start with white light and take light away. That is:

1. Cyan ink

takes away

red light

(leaving blue and green)

2. Magenta ink

takes away

green light

(leaving blue and red)

3. Yellow ink

takes away

blue light

(leaving green and red)

Substrate

1. 1.

2. 2.

3. 3.

Red

Green

Blue

400 500 600 700

100

Wavelength (nm)

Intensity

400 500 600 700

100

Wavelength (nm)

Intensity

400 500 600 700

100

Wavelength (nm)

Intensity

Magenta + Yellow = Red

Magenta + Cyan = Blue

Yellow + Cyan = Green

Magenta + Yellow + Cyan = Black

Blue

Green

Red

MAGENTA

CYAN

YELLOW

Figure

shows the same three cases for

magenta. Notice how much lower the peak

is in the blue part of the spectrum for all

three cases. The magenta ink takes away a

lot of the blue light. Remember, magenta is

supposed to take away only green. This dra-

matically demonstrates some of the limita-

tions of the printing process. It leads to

gamut compression, which will be covered

in detail later. Again, the proof matches the

offset curve much better than the flexo.

Figure

shows the case for yellow. In

this case, the match of the proof is better to

the flexo ink. Unfortunately, yellow is the

least visible and the mismatch in the cyan

and magenta means that for matching

process colors, the proof is a better match

for offset printing.

Figure

shows the spectra for the most

saturated yellow possible, that is a solid yel-

low patch. What happens when a yellow dot

is printed? Very simply, less yellow and more

white light, resulting in the spectra shown in

Figure

. The numbers above the curves

represent the dot percentages printed. The

zero-dot percentage, which is nothing more

than the substrate, indicates that the sub-

strate itself is not a perfect white as was

shown in Figure

Finally, Figure

depicts the spectra of a

black and an overprint of an equal combina-

tion of cyan, magenta and yellow. In the fig-

ure, the black line is the spectrum for the

black ink and the brown line is the spectrum

for the three-color overprint. As was the

case for the perfect neutral of Figure

, the

PROCESS COLOR 117

Spectrum of real cyan

showing the curves for

flexo, a digital proof and

offset.

Spectrum of real

magenta showing the

curves for flexo, a digi-

tal proof and offset.

Spectrum of real yellow

showing the curves for

flexo, a digital proof and

offset.

Spectra of real yellow

showing the curves for

different dot percent-

ages.

400 500 600 700

100

Flexo

Proof

Offset

Wavelength (nm)

Intensity

400 500 600 700

100

Flexo

Proof

Offset

Wavelength (nm)

Intensity

400 500 600 700

100

Flexo

Proof

Offset

Wavelength (nm)

Intensity

400 500 600 700

100

Wavelength (nm)

Intensity

100

118 FLEXOGRAPHY: PRINCIPLES & PRACTICES

black is nearly a horizontal line. It has equal

intensity at all wavelengths, which gives a

neutral gray or black. For the three-color

overprint, there is more light intensity in the

red end of the spectrum. If a more neutral

black is desired, more red light needs to be

taken away. What takes away red? Cyan. This

is the reason why the cyan dot needs to be

larger than the magenta and yellow to print a

neutral using cyan, magenta and yellow.

QUANTITATIVE COLOR –

CIELAB COLOR SPACE

In the last section, spectra were presented

without taking into account the other two

elements of what determines color: the light

source and the human eye. They too can be

described in terms of spectra.

Light Sources

Different light sources emit light which

have different spectra. Table 18 lists CIE

standard light sources or illuminants, while

Figure shows the spectra of “A”, D50 and

D65. “A” is for a tungsten filament bulb (i.e.,

an ordinary light bulb) at a color temperature

of 2,850° K. D50 and D65 represent light at

color temperatures of 5,000° K and 6,500° K

respectively. Degrees Kelvin is a temperature

scale much like degrees Fahrenheit (° F) and

degrees Celsius (° C). The different tempera-

tures mean that a well-defined material heat-

ed to that temperature will emit light of a

given spectral composition. This is called

black body radiation. In the graphic arts, D50

or 5,000° K light is standard for making color

evaluations. The light sources themselves are

special types of fluorescent light bulbs.

Note: As with any specification, nothing

is ever exact, there is always a tolerance.

For a D50 source, one measure of this tol-

erance is called the color rendering index

(CRI). The higher this number, the more

Spectra of real black

(shown in black) and an

overprint of cyan,

magenta, yellow (shown

in brown).

Spectra of CIE standard

illuminants A, D50, D65.

D50 is the graphic arts

standard for making

color evaluations.

400 500 600 700

100

Wavelength (nm)

Intensity

CMY

400 500 600 700

Wavelength (nm)

Relative Intensity

D65

D50

ILLUMINANT

■ A Incandescent lighting at a color

temperature of about 2,850° K

■ B Direct sunlight at about 4,874° K

■ C Tungsten illumination simulating day-

light at about 6,744° K

■ D50 Graphic arts standard viewing condi-

tions at about 5,000° K

■ D65 Used by textile, paint and ink indus-

tries, about 6,500° K

■ F2 Cool-white fluorescent lamp at about

4,200° K

■ F7 Broad-band daylight fluorescent lamp

at about 6,500° K

■ F11 Narrow-band white fluorescent lamp

at about 4,000° K

CIE STANDARD ILLUMINANTS

Table 18