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

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PROCESS COLOR 129

As images go through

the production process,

the image information

is transformed and dis-

played first as photo-

graphic data in the

original image; second,

as digital information in

the scanned image file;

third, as pixels of red,

blue and green light on

screen; and finally as

printed dots of CMYK

on a substrate. Each of

these steps introduces

color changes.

sible to associate a correction with each

device. The image to be scanned or proofed

or output is stored in terms of L*a*b* values;

that is, the color values in CIELab color

space. Each device then handles the color to

the best of its capability. Implicit in this

workflow is the RGB-to-CMYK conversion.

That is, the color management system will

have a RGB-to-CMYK conversion engine. In

actual practice, the image might be stored in

“tagged RGB.” This means RGB values are

stored along with the profile or characteriza-

tion information for the input device that

captured the image.

In all the workflows shown, the corrections

can take place at different stages of the

process. For example, the correction to the

plate film could be made in the workstation

before being sent to the imagesetter for film

output. Alternatively, it could happen when

the film is output. Similarly, other corrections

can take place at different stages and in dif-

ferent programs in the process. Color man-

agement is a process or function that address-

es the details and decisions associated with

where and when to make the corrections

shown “logically” in Figures , , and .

The color-correction loop is present in all

the workflow diagrams. Even if the CIELab

method were to give acceptable color match-

es the first time around, this loop would still

be required. Many times color corrections

are done not to achieve “match copy” but to

satisfy personal editorial desires of the cus-

tomer. The truth will always still be in the eye

of the beholder.

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

ORIGINAL

MONITOR

CMYK FILM NEGATIVES

LAMINATE PROOF

PRINTED PIECE

RGB DATA

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

CMYK FILM NEGATIVES

85% Magenta

70% Magenta

87% Magenta

84% Magenta

83% Magenta

87% Magenta

89% Magenta

130 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Achieving Optimum

Press Performance

efore any corrections can be

applied in process-color print-

ing, two tasks need to be accom-

plished. The first is press opti-

mization and the second is press

characterization. Press opti-

mization refers to finding the best or optimal

values for the myriad of variables encoun-

tered for a given printing process. It means

printing to a consistent set of specifications

and tolerances. The most comprehensive set

of specifications and tolerances for flexogra-

phy are found in FIRST.

Press characterization, to be covered in

detail later, refers to measuring key print

variables once the variables which affect the

print have been set. This means that the

printing process must be stable, repeatable

and under control before it is characterized.

The purpose of characterization is to quanti-

fy or document the printing process; the pur-

pose of optimization is to improve the print-

ing process.

PRESS OPTIMIZATION

In order to optimize the press, tests need

to be conducted. For example, a banded

anilox test is a good way to find the optimum

anilox configuration for process printing.

This is a test print with different anilox rul-

ings and volumes engraved on the same roll.

Some may choose to combine part of the

optimization effort with characterization. It

is also called fingerprinting the press.

One of the major specifications for the

printed sheet is the solid-ink density. The val-

ues are found in FIRST and reproduced in

Table 21. Other variables to optimize include:

• film: screen angles, D-min, D-max and

screen ruling

• plate: durometer, relief and caliper

• mounting material: density, thickness

• ink: pH, viscosity and density

• substrate: dyne level, tension

• anilox roll: cell count, cell volume, cell

angle

• press settings: impression, speed, dryer

temperature

These individual topics are covered in

detail in the other chapters of this work. It is

vital that the result of optimization is a set of

achievable conditions that can be main-

tained during normal production. It does not

mean the best possible that the press can do

if everything is tweaked to perfection.

Achievable, realistic target values which

represent quality printing are documented in

FIRST.

When running any press evaluations,

PAPER FILM

■ CYAN 1.37 (0.07) 1.25 (0.07)

■ MAGENTA 1.25 (0.07) 1.20 (0.07)

■ YELLOW 1.00 (0.05) 1.00 (0.05)

■ BLACK 1.50 (0.07) 1.40 (0.07)

Note: (+/–) tolerance values in parentheses.

SOLID-INK DENSITY

Table 21

include a control target which will be used

during production to maintain control of the

press. Figure is the FIRST control target.

PRESS CHARACTERIZATION

Press characterization follows press opti-

mization. It accomplishes two goals. One is

to document the values of key print vari-

ables such as dot gain, ink trap, minimum

highlight dot and maximum shadow dot,

L*a*b* values for selected patches (solid-ink

patches, gray-balance patches and overprint

patches).

The second is to provide the data used to

calculate the corrections necessary for

matching color. The procedure is to print a

characterization target using the optimized

conditions. The target is evaluated both visu-

ally and by measurements. The measure-

ments are used to develop the corrections by

either conventional cutback curves or

CIELab-based corrections (ICC profiles).

Target

There are numerous targets available.

Figure depicts the FIRST press charac-

terization target. The largest number of target

elements are the overprint patches arranged

in 42 rows by 32 columns for a total of 1,344

patches. Included are single-color step

wedges used to calculate cutback curves.

The patches are all combinations of six tint

values arranged in random order. This

arrangement serves to distribute any local

press variations throughout all color values.

Spectrophotometric (L*a*b*) measurements

of these overprint patches provide the data

for the CIELab-based corrections.

Additonal elements in the target, used for

different types of characterization, include:

• slur target;

• linear blends to determine minimum

and maximum dot;

• positive and reverse lines; and

• microdots and register marks.

TYPES OF CHARACTERIZATION

Characterization can be broken out into

different types: visual, line, screen and

process. Clearly, for process printing,

process characterization needs to be done.

For completeness, the other types will be

mentioned and briefly described.

Visual Characterization

The main purpose of characterization is to

quantify the process. Nevertheless, visual

PROCESS COLOR 131

Components of a FIRST

control target.

Ink

Trap

Patch

Solid

Process

Patches

Exposure

Guide

Solid

Equivalent

Patches

Slur

Patch

Shadow

Gray

Balance

Highlight

Gray

Balance

Characterization

Dot Gain

Values

Tonal

Scale

FIRST

Logo

Reference

Code

3 A good tutorial on the subject of press characterization is available on CD

from the FTA.

132 FLEXOGRAPHY: PRINCIPLES & PRACTICES

examination of the press sheet is always part

of the characterization. It would be foolish

to spend a great deal of time and effort quan-

tifying a press using a press sheet that

exhibits unacceptable slur, or is in misregis-

ter. Perhaps this is stressing the obvious, but

before any quantitative analysis is done, the

press sheet needs to be carefully examined

to assure that no defects are present. Faults

which are best examined visually include:

• misregister;

• sharpness;

• slur;

• ink trap;

• streaking;

• ghosting;

• solid coverage; and

• clarity of the graphics.

FIRST press

characterization target.

123456789101112131415161718192021222324252627282930313233343536373839404142

ABCDEFGHI JKLMNOP

3Electronic File Values 5 10152025303540

QRSTUVWX

45 50 55 60 70 80 90 100

3Cutback Values (film) 5 10 15 20 25 30 35 40 45 50 55 60 70 80 90 100

Y Z AA BB CC DD EE FF

024686 88 90 92 94 96 98 100 024686 88 90 92 94 96 98 100

Line Characterization

Line characterization is aimed at quantify-

ing the growth of positive lines and reduc-

tion of reverse lines. The information can be

used to calculate bar-width reductions for

printing bar codes and to determine the min-

imum type size and fonts which can be print-

ed. The positive and reverse lines in the

FIRST target (Figure ) show how small a

line, in points, can be held and what size will

fill in. To quantify the growth or reduction,

the lines are measured with a 50x or 100x

magnifier that has a built-in scale. An alter-

native method is to print actual bar codes

and type and visually examine the result.

The bar codes can also be tested with a bar-

code verifier. Specifications and test plates

can be found in the second edition of FIRST.

Screen Characterization

Screen characterization is used to deter-

mine cutback curves when printing screen

work only. The procedure for process-color

screens is the same as it is for process char-

acterization using conventional cutback

curves (described in the next section). It is

usually not practical to develop cutback

curves for spot colors because of the large

number of spot colors used. The curves

developed for process colors can be used as

a starting point, and the cutback curves can

be adjusted on subsequent print runs using

the same spot color.

It might be practical to develop a cutback

curve for a specific spot color which is used

frequently, or one which is critical, such as a

customer’s logo color. Keep in mind that the

spot colors referred to here are those made

up of screens of a spot-color ink. Spot colors

printed as a solid are controlled by the ink

formulation and achieve target solid density.

Process-color Characterization

Often referred to as fingerprinting a press,

the goal of this process is to measure and

analyze the press sheet in order to develop

the corrections used in the workflow dia-

grams presented in Figures , and .

The methods fall into two categories: densit-

ometric and perceptual. Densitometric

refers to the measurement of densities and

dot gain in order to calculate a cutback

curve. Perceptual refers to spectrophoto-

metric L*a*b* measurements of the over-

print colors. The data is used to calculate

CIELab-based corrections (ICC profiles).

It is important to keep the goal of the char-

acterization in mind. In the workflow sec-

tion, it was pointed out that a cutback curve

matches the press to the proof. For CIELab

there can be different goals depending on the

particular workflow used. One is to match an

absolute L*a*b* value. That is, if the desired

output L*a*b* value is known, the press can

be adjusted or corrected to print that value

subject to gamut limitations. The second

approach is to match the proof to the press.

CUTBACK CURVE

The general objective of a cutback curve is

to apply a correction to the dot percentage

of one device, so that the measured size of

the dots match that of a second device. This

will lead to color matching, provided some

of the other print variables – notably the hue

of the process inks, ink trap and the sub-

strate – are similar. Relative density mea-

surements of single-color step scales

(Figure ) facilitate calculation of dot per-

centages. The cutback curve is essentially a

gradation curve applied to each process

color. The process of generating the curve is

the same, whether the curve is applied to a

file going to a proofing device, platesetter,

imagesetter or any other output device.

The specific place and method where it is

actually applied can vary depending on the

particular workflow, software and hardware

involved. The usual application of cutback is

illustrated in Figure , which is similar to

Figure and shows a conventional work-

PROCESS COLOR 133

134 FLEXOGRAPHY: PRINCIPLES & PRACTICES

flow path where the electronic file is modi-

fied by a cutback curve before output to

films for platemaking. The correction step is

called “total cutback curve” because a

default cutback curve could have been used

originally when the press characterization,

or fingerprinting, was carried out. Example 1

and Example 2 will show the details with

and without the use of a default cutback

curve. In these examples, the press is

matched to the proof.

Example 1: Table 22 shows the measured dot

percentages for the proof and press sheet.

These are the dot-gain curves for the proof

and press sheet. The values are shown

graphically in Figure .

It is a common mistake to calculate the

cutback required by taking the difference

between the measured dot-percent values

between the proof and press sheet at a par-

ticular value of the electronic file dot per-

cent. In Table 22, for example, at a value of

20% in the electronic file, the proof has a 31%

dot and the press a 43% dot (points A and B

in Figure ). This would give a correction

of 12 and a cutback curve value of 8 (20

minus 12). This incorrect process for the

entire curve is detailed in Table 22. The rea-

son for showing the entire curve generated

is to highlight the fact that this incorrect

method yields negative dot-percent values

for the cutback curve.

The key to the correct procedure is to ask

the following question:

For a given dot percent in the electronic

file, what dot percent must be sent to the

Single-color step

scales are used to mea-

sure dot percentages

for cutback curve.

A cutback correction

called “total cutback

curve” is applied in the

conventional workflow.

This correction is

applied to the electronic

file before outputting

to film for platemaking

or outputting direct-to-

plate.

Contract

Proof

Press

Sheet

Electronic

File

Press

Intermediate

Steps for

Printing

Plates

Total

Cutback

Curve

Contract

Proof

0 3 5 7 10 15 20 25 30 35 40 45 50 60 70 80 90 100

press to get the same measured result as on

the proof?

In Figure , the 20% dot prints as a 31%

dot on the proof (Point A). The question is,

which dot-percent value in the electronic file

also prints as a 31% dot on the press sheet?

Examination of Table 22 reveals there is no

measured dot-percent value of 31% in the

press dot percent. To get the answer, a hori-

zontal line is drawn at the 31% output value

as shown in Figure with the line labeled

13-20. The press prints a 31% dot for an elec-

tronic dot-percent value of 13%. The 13% was

obtained by reading it from the graph in

Figure which shows the same procedure

for all dot-percent values in the electronic

file from 0 to 100 in steps of 10.

Put in a slightly different way, Figure

indicates that in order to print a 31% dot, a

20% dot needs to be sent to the proofing

device and a 13% dot needs to be sent to the

press. This implies a correction to the press

of 7% (20 minus 13). All values are listed in

Table 23.

Figure shows the values above 10%,

while Table 23 lists some of the values below

10%. This area needs some special consider-

ation, especially considering the minimum-

dot value that will be printed. Figure

shows an enlarged part of the highlight area

of Figure . The dashed line shows the

line drawn in for the dot-gain curve while the

solid line signifies the actual curve, assum-

ing a minimum printing dot of 3%. Below 3%,

the output is zero-dot percent or a drop out.

Table 23 reflects this drop out and keeps the

value at 3% for all electronic dot values of 3%

or less.

A similar cutoff can be applied in the shad-

ow end. In this case, the value of 93% would

PROCESS COLOR 135

This chart compare a

dot gain curve for proof

and press sheet with no

default corrections

applied.

Reading cutback values

from the dot-gain

curves of Figure 36.

Table 22

PRESS DOT %

FILM PROOF WITH WITHOUT

DOT% DOT% DEFAULT DEFAULT

00 014

3 5 13 14

5 9 17 14

7122114

10 17 26 20

15 24 34 27

20 31 43 33

25 38 50 39

30 44 56 43

35 49 62 48

40 55 67 53

45 60 73 57

50 65 78 62

60 75 86 71

70 83 92 80

80 91 96 87

90 96 99 94

100 100 100 100

DOT-GAIN CURVE

0 102030405060

ProofPress

70 80 90 100

100

Electronic File Dot %

Printed Dot %

010

10093

20 30 40 50 60

Proof

Press

70 80 90 100

100

Electronic File Dot %

Printed Dot %

136 FLEXOGRAPHY: PRINCIPLES & PRACTICES

not present a problem if the maximum print-

ing dot is 93% or higher. Figure shows

the resultant cutback curve. The graph

reveals the corrected values (those to be

output to press) vs. the original electronic

file dot-percent values.

Example 2: It is assumed that the cutback

corrections derived in Example 1 have been

applied. That is, the cutback curve derived in

Example 1 is the default cutback curve used.

This could be the case where a press char-

acterization has been used to generate the

cutback curve and perhaps a different but

similar press is being characterized. The

task is to generate a new cutback curve

(Total Cutback Curve in Figure ).

As before, the proof and press sheets are

measured, the dot-gain curves generated and

the horizontal lines drawn in (Table 22 and

Figure ). At first glance, this may seem

like a strange dot-gain curve. The press has

more gain than the proof in the quarter tones

and less gain than the proof in the mid tones

and higher. It seems to hold dots all the way

to 100% and in the highlight, prints a 14 % dot

all the way to 7%. The answer is, of course,

that a cutback curve has already been

applied to the electronic file before it went

to press. Ideally, the two dot-gain curves of

Figure would overlap; this is simply a

correction to that cutback curve.

A magnified section

of dot-gain curves of

Figure 36 shows the

drop out (no printed

dot) in the press sheet

below an electronic file

dot of 3%.

The resultant cutback

curve shows the

original electronic file

dot on the horizontal

axis and the corrected

file to be output to

plate-making film on

the vertical axis.

The dot-gain curve

for the proof and press

sheet with default

correction applied.

010

20 30 40 50 60

Proof Press

70 80 90 100

100

Electronic File Dot %

Printed Dot %

01053152520 30

Electronic File Dot %

Printed Dot %

ProofPress

0 102030405060708090100

100

Electronic File Dot %

Corrected Dot %

Table 23

WHAT TO DO WHAT NOT TO DO

FILM CORRECTION CUTBACK INCORRECT INCORRECT

DOT % (FIG. 37) CURVE CORRECTION CUTBACK

00 3 0 0

30 3 8 -5

52 3 8 -5

74 3 9 -2

10 5 5 9 1

20 7 13 12 8

30 8 22 12 18

40 12 28 12 28

50 12 38 13 37

60 13 47 11 49

70 13 57 9 61

80 11 69 5 75

90 8 82 3 87

100 7 93 0 100

CUTBACK

The procedure is exactly the same as

before. One thing to be cautious about is to

make sure the horizontal lines in Figure

are keyed to the proof curve since the aim is

to match the press to the proof. Table 24

shows the numbers and Figure the

default and total cutback curves.

Note: Throughout this section and in

Figures through , the output dot per-

centage are plotted on the vertical axis.

This is the correct procedure if the solid-ink

density of the proof and press sheet are the

same. The same methodology can be

applied and cutback curves can be calculat-

ed if the solid-ink densities are not the

same. In that case, density would be plotted

on the vertical axis. Everything else follows

in the same manner as described.

CIELab CORRECTION

(ICC PROFILES)

The objective of CIELab correction is to

match the colors of one device to another.

The colors are measured with a spectropho-

tometer in CIELab perceptual color space.

Because the colors are measured as the eye

perceives them, all variables, such as sub-

strate, ink trap and changes in the hue of the

process inks are taken into account.

Measurements are taken in absolute mode,

not relative to the substrate.

Corrections are made using ICC profiles.

Basically, an ICC profile identifies or maps

the device-independent L*a*b* color values

to the particular color values of the specific

device. For a press, it means for a given

L*a*b* value, the press must print a specific

CMYK combination. For example, in order to

achieve a color specified by L*a*b* values of

46-62-51 the press needs to print CMYK val-

ues of 0-100-80-0. For a proofing system, to

print these same L*a*b* values, it needs to

print CMYK values of 3-100-75-0. This means

the electronic file to be output needs to be in

L*a*b* values, or the equivalent “tagged

RBG.” When color 46-62-51 needs to be print-

ed, 0-100-80-0 is sent to the press and 3-100-

75-0 is sent to the proofing device.

ICC profiles can also be used when the

starting point is a CMYK file. Suppose we

start with a CMYK file which has been sepa-

rated for a flexo press using a cutback curve.

The goal is to match the proof to the press.

The CMYK values to be sent to the proofing

device need to be modified so that for a

given L*a*b* color printed on the press, the

PROCESS COLOR 137

The default cutback

curve (same as the

curve of Figure 39)

is corrected to total

cutback curve.

TOTAL CUTBACK

Table 24

TOTAL

FILE DEFAULT CORRECTION TOTAL CUTBACK

DOT % CUTBACK* (FIG. 40) CUTBACK CURVE

00 303

30 303

52 323

74 343

10 5 5 6 4

20 7 13 8 12

30 8 22 8 22

40 12 28 10 30

50 12 38 9 41

60 13 47 9 41

70 13 57 9 41

80 11 69 7 73

90 8 82 6 84

100 7 93 7 93

0 102030405060708090100

100

Electronic File Dot %

Corrected Dot %

Total Cutback Curve

Default Cutback Curve

138 FLEXOGRAPHY: PRINCIPLES & PRACTICES

same L*a*b* color is printed on the proof.

The modification or correction is shown in

Figure , which is similar to Figure .

As before, printing 0-100-80-0 on the press

gives Lab values of 46-62-51. When output to

the proofing device, the CMYK values need

to be modified to 3-100-75-0 to print the

same L*a*b* values of 46-62-51.

Details specifying how and where to apply

the profiles depend on which of the correc-

tions are done. Corrections can be done at

different stages of the process, whether it be

at the final output stage (RIP) or within an

image-editing program such as Adobe

Photoshop. This can be confusing. There are

many different profiles to deal with and dif-

ferent corrections can be applied at various

stages of the workflow. True device-indepen-

dent color management (i.e., images are

stored in L*a*b*) is still in its infancy and will

undoubtedly experience some growing pains

before gaining full acceptance. CMYK is cer-

tainly attainable and is a good place to start.

The best advice is to have a clear understand-

ing of the goal, which is to match the proof to

the press. Procedurally, a press sheet is mea-

sured and the proof is corrected to match that

sheet The process is verified by outputting a

second proof with the correction applied to

verify the match. Example 3, which follows,

illustrates the process with real-world data.

Example 3: The overprint patches of the

press characterization target are measured

with a spectrophotometer. Recall, that these

targets have many of these patches. The FTA

target (Figure ) has 1,344 of them. While

these measurements can be made manually,

it is much more reliable and efficient to use

a spectrophotometer that reads strips or one

mounted on an x-y table (Figure ). The

An ICC profile

correction is applied to

the proof in order to

match the press sheet.

The original is an elec-

tronic CMYK file .

Readings of a FTA

press characterization

target taken by a

spectrophotometer

mounted on an x-y

table.

Contract

Proof

Press

Sheet

Electronic

File

Press

Intermediate

Steps for

Printing

Plates

Total

Cutback

Curve

Contract

Proof

Correction

L= 56

a= -6

b=-29

L= 87

a= -1

b=-46

L= 55

a=-17

b= -1

OVERPRINT C M Y K

1A 0 100 80 0

1B 80 0 60 0

1C 80 20 100 20

1D 20 60 0 20

1E 100 40 80 80

ELECTRONIC FILE VALUES

Table 25