FTA (изд-во). Flexography: Principles And Practices. Vol.1-6
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measured L*a*b* values are then input to
profile building software. The profile is used
to make the corrections to the proof.
Some sample numbers are shown in Tables
25 to 28. Table 25 shows the C, M, Y and K
values of the first five patches of the FTA
press characterization target. Table 26 shows
the values from a press sheet (using a default
cutback curve) and a digital proof. After the
profiles were generated, the proof was cor-
rected to match the press sheet and the target
was again output on the digital proofer using
these corrections. Table 27 shows the results.
Also listed in Tables 26 and 27 are the
CMC (2,1) color difference values calculated
for each patch between the press and proof.
As a comparison, Table 28 shows the degree
of match achieved using dot-gain compensa-
tion. Of course, with the full 1,344 overprint
patches, a more useful metric is needed for
PROCESS COLOR 139
Table 26
PRESS PROOF
OVERPRINT L* a* b* L* a* b* CMS(2, 1)
1A 46.03 62.03 51.07 49.02 71.85 32.78 11.91
1B 53.45 –49.54 27.35 63.74 –52.94 13.33 8.39
1C 38.70 –31.06 33.21 48.22 –37.51 42.36 6.09
1D 44.31 27.44 –7.21 55.18 31.05 –19.14 8.53
1E 18.07 –11.20 8.40 21.26 –19.47 –0.32 9.63
PRESS VS. PROOF
Table 27
PRESS PROOF
OVERPRINT L* a* b* L* a* b* CMS(2, 1)
1A 46.03 62.03 51.07 47.71 59.94 47.84 1.45
1B 53.45 –49.54 27.35 53.51 –49.80 30.71 1.50
1C 38.70 –31.06 33.21 42.61 –33.96 27.99 3.76
1D 44.31 27.44 –7.21 42.87 26.49 –8.09 1.12
1E 18.07 –11.20 8.40 17.62 –5.50 7.00 5.53
PRESS VS. PROOF (CIELab CORRECTED)
Table 28
PRESS PROOF
OVERPRINT L* a* b* L* a* b* CMS(2, 1)
1A 46.03 62.03 51.07 49.11 70.23 45.73 5.53
1B 53.45 –49.54 27.35 61.00 –57.84 24.18 4.82
1C 38.70 –31.06 33.21 45.53 –36.52 36.07 4.41
1D 44.31 27.44 –7.21 50.78 32.90 –21.80 8.93
1E 18.07 –11.20 8.40 23.38 –16.85 6.03 5.99
PRESS VS. PROOF (DOT GAIN CORRECTED)
140 FLEXOGRAPHY: PRINCIPLES & PRACTICES
the degree of match. One such metric is the
average color difference for all 1,344 patch-
es. For this case, the averages were 2.5 using
CIELab and 6.9 using dot gain.
GRAY BALANCE
One of the key parameters in process
printing is gray balance. Recall that printing
equal dot percentages of cyan, magenta and
yellow results in a brown color, not a neu-
tral. In order to print neutrals and process
images without a cast, it is important to
know the correct combination of cyan,
magenta and yellow that gives the best neu-
tral for the particular printing process. The
information is used in the conversion to C,
M, Y and K. When a neutral color is convert-
ed to C, M, Y and K, the proportion of C, M,
Y is adjusted to the gray-balance value.
Table 29 shows some typical values for
gray balance (from FIRST). The values are
the dot percentages in the electronic file that
will result in a neutral color when overprint-
ed. For example, a combination of 30% cyan,
24% magenta and 24% yellow would print as
a neutral, equivalent to a 35% black. These
are the dot-percent values in the electronic
file before output to film, platemaking and
printing on the press. Once the press has
been characterized and all corrections
applied, these values can be adjusted to give
a visual neutral in gray-balance test patches,
such as those found in the FIRST control tar-
get (Figure , ).
A more systematic way of determining the
gray-balance values is to print a special test
target. If the proof has been matched to the
press as outlined earlier, this target can be
printed on the proofing device as opposed to
the press – a much more cost-effective
method. Figure shows an example of a
target used to determine the magenta and
yellow values needed to combine with a
cyan of 30 in order to print a neutral. The tar-
get is arranged as a set of overprints where
every patch has a cyan value of 30. Along the
columns, the yellow values increase with the
magenta at a constant value. Along the rows,
the magenta values increase with the magen-
ta at a constant value. The net effect is a sys-
tematic combination of many different C, M,
Y values, all with cyan of 30. Once the target
has been printed, a visual determination can
be made as to which patch is the best neu-
tral. The C, M, Y values can then be read
directly from the target. Many times, partic-
ularly if the dot-percent increment is small, it
is difficult to tell which patch is the best neu-
tral. Using a spectrophotometer, the L*a*b*
values of the patches can be measured. The
patch with the a and b values closest to 0 and
0 is the most neutral patch.
Note: Figure is not meant to represent
flexo printing and flexo gray balance. It is
for illustration only, not for color accuracy.
Using spectrophotometric measurements
This sample target
visually evaluates the
best combination of C,
M and Y to give a gray
balance for a cyan value
of 30.
FILM DOT PERCENTAGE
■ CYAN 510307090
■ MAGENTA 3 7 24 58 78
■ YELLOW 3 7 24 58 78
■ BLACK 814357698
GRAY BALANCE
Table 29
Y30C30
M30
M28
M26
M24
M22
Y28 Y26 Y24 Y22
and CIELab ICC profiles, gray-balance val-
ues are inherently evaluated. An ICC profile
links a L*a*b* value with a particular CMYK
value for a given device, such as the press.
Once the profile is generated, CMYK values
for any given L*a*b* value are available.
Gray-balance values are the CMYK values
where K equals to zero, corresponding to
L*a*b* values with a* and b* of 0 and 0.
PROCESS CONTROL
It was mentioned in the beginning of this
section that process-color printing requires
consistency, first and foremost. Once the
press and the entire process have been opti-
mized and characterized, it is imperative to
keep all the variables to specification and
within tolerance.
A control target should be printed on
every job. The FIRST control target is
shown again in Figure . After press char-
acterization, the tint values next to the tint
patches should be changed to the actual val-
ues determined. This will allow the press
operator to simply read the values and veri-
fy that the values remain within specifica-
tion. Likewise, actual minimum and maxi-
mum dot should be used, as well as the actu-
al gray-balance values. With these changes,
the control target becomes specific for the
particular press that was characterized. The
target has an area to put an identification
name or number (Cutback #1 in Figure ).
The control target gives a continual visual
indication and with simple densitometric
measurments assures that the press is still
running to specifications. The elements,
along with their method of measurement,
are summarized in Table 30.
Some of the elements listed can be
checked both visually and with a densitome-
ter. The ink trap and gray-balance values are
measured during characterization and serve
as the reference point during production.
The consistency of the values is more impor-
tant than the actual values. The gray-balance
patches are a good example of a key visual
indicator of press variation. The gray in the
three-color overprint patch is extremely sen-
sitive to even small shifts in values of the
cyan, magenta or yellow. Even small varia-
PROCESS COLOR 141
This FIRST control
target indicates values
which are changed for
the specific press.
B
L
A
C
K
O
N
L
Y
B
L
A
C
K
O
N
L
Y
T
H
R
E
E
C
O
L
O
R
T
H
R
E
E
C
O
L
O
R
Change these values to match
actual dot gain values for each color
16, 34, 95, 100
Change to actual
gray-balance values
Change to
actual minimum dot
Change
Identification
Change to
actual maximum dot
ELEMENT METHOD OF MEASUREMENT
■ Slur visual
■ Dot Gain densitometer
■ Density densitometer
■ Ink Trap visual, densitometer
■ Gray Balance visual, densitometer
CONTROL TARGET ELEMENTS
Table 30
142 FLEXOGRAPHY: PRINCIPLES & PRACTICES
tions, not readily visible in other colors, will
be apparent in these patches.
Certain press configurations or package
types do not have trim areas where control
targets fit. In these cases, a smaller, more
limited target, called a run target should be
placed in an inconspicuous place in the
image area. Figure shows the run target
specified in FIRST. Good places include the
back panel, flaps which will not be visible in
the final product or even in the nutrition
information area. These targets provide the
minimum information required to maintain
dot gain and density. On special colors, they
can be used to provide CIELab color data.
Each production run should have a pro-
duction run spec sheet. An example is
shown in Figure . In this example, the
target viscosity and density for each ink is
specified. There is room on the sheet to
record the actual values during the produc-
tion run. The actual values of viscosity and
density can then be plotted on a control
chart to monitor the variables for many jobs.
On a long production run, the variables
should be frequently measured and a control
chart created for the job itself. Details of
control charts are covered in the Quality
chapter, Book 3.
An important part of production control
that should not be overlooked is the calibra-
tion of all measurement instruments.
• Densitometers should be periodically
checked against reference standards
supplied by the manufacturer. Each
instrument comes with calibration pro-
cedures in case adjustment is needed.
• Spectrophotometers come with a white
standard and a table of what the read-
ings should be on that standard. They
too need to be periodically checked.
• As simple as it sounds, even a microme-
ter needs to be checked. This is as sim-
ple as making sure it reads zero when it
closes with no sample.
A FIRST run target is
used instead of the
control target, when
limited space on the
package is
available.
In this typical produc-
tion run spec sheet,
the target viscosity and
density for each ink is
specified.
ABC Printing Company
Production Run Spec Sheet
Order #: 3064A
Customer: America’s Favorite Bread
Linear Feet: 35,000
Substrate: 140#/liner board
Print Line
Deck # Metering Volume Count
1 Two-Roll 5.1 360
2 Chambered 1.9 600
3 Chambered 1.9 600
4 Chambered 1.9 600
5 Chambered 1.9 600
6 Chambered 5.1 360
Print Aim Acutal Aim Actual
Station Color Viscosity Viscosity Density Density
1 White 30 0.19
2 Cyan 25 1.35
3 Magenta 25 1.25
4 Yellow 25 1.00
5 Black 28 1.50
6 Varnish 22 n/a
C
M
Y
K
PMS 259
2% Mininum
Dot Percentage
95% Maximum
Dot Percentage
PROCESS COLOR 143
Appendix A
ANSI/CGATS.4-1993
Graphic Technology – Graphic Arts Reflection Densitometry Measurements – Terminology, Equations,
Image Elements and Procedures.
ANSI/CGATS.9-1993
Graphic Technology – Graphic Arts Transmission Densitometry Measurements – Terminology, Equations,
Image Elements and Procedures.
ANSI/CGATS.5-1993
Graphic Technology – Spectral Measurement and Colorimetric Computation for Graphic Arts Images.
ANSI/IT8.7/1-1993
Graphic Technology – Color Transmission Target for Input Scanner Calibration.
ANSI/IT8.7/2-1993
Graphic Technology – Color Reflection Target for Input Scanner Calibration.
ANSI/IT8.7/3-1993
Graphic Technology – Input Data for Characterization of 4-Color Process Printing.
ANSI PH2.30-1989
Graphic Arts and Photography – Color Prints, Tranparencies and Photomechanical Reproductions –
Viewing Conditions.
ISO 3664-1999 (replaced ANSI.PH2.30-1989)
Viewing Conditions for Graphic Technology and Photography.
A: REFERENCE RESOURCES
TRANSMISSION:
REFLECTION:
In all these calculations, the appropriate filter needs to be used for the process color (CMY) being mea-
sured. Refer to Chapter 3, Table 2.
■ DOT PERCENT
(MURRAY-DAVIES EQUATION):
The equation shown is for the case of D-max
greater than 3.0 with the densitometer zeroed
on clear film. A black-and-white densitometer
is used.
% dot 100 1 10
-DT
where
DT is the density of the tint
B: DENSITY-BASED MEASUREMENTS
144 FLEXOGRAPHY: PRINCIPLES & PRACTICES
Appendix B
■ DOT PERCENT
(MURRAY-DAVIES EQUATION):
% dot 100 1 10
–DT DP
1 10
–DS DP
where
DS is the density of the solid
DP is the density of the paper
or substrate
DT is the density of the tint
■ TRAP (use filter for second down ink)
% Trap 100 D
OP
D
1
D
2
where
D
OP
is the density of overprint
D
1
is the density of first down ink
D
2
is the density of second down ink
■ GRAYNESS
% grayness 100
D
L
D
H
where
D
L
is the density using the filter which
gives the lowest reading
D
H
is the density using the filter which
gives the highest reading
■ HUE ERROR
% hue error 100 D
M
D
L
D
H
D
L
where
D
L
is the density using the filter which
gives the lowest reading
D
M
is the density using the filter which
gives the middle reading
D
H
is the density using the filter which
gives the highest reading
■ PRINT CONTRAST
% Contrast 100 D
S
D
T
D
S
where
D
S
is the density of the solid
D
T
is the density of the shadow tint;
typically 70%
COL0R DIFFERENCE EQUATION – DELTA E (∆E)
Note: These calculations are in the L*a*b* color space. For clarity, the (*) have been omitted.
■ L*a*b*
∆E
Lab
L
1
L
2
2
a
1
a
2
2
b
1
b
2
2
■ CMC
∆E
CMC
L
1
L
2
2
C
1
C
2
2
H
1
H
2
2
where
land c are adjustable parameters (usually set to 2 and 1)
C
1
(a
1
2
b
1
2
)
C
2
(a
2
2
b
2
2
)
H
1
H
2
[(∆E
LAB
)
2
(L
1
L
2
)
2
(C
1
C
2
)
2
]
S
L
0.040975L
(1 0.01765° L)
if L is greater than 16
S
L
0.511 if L is less than or equal to 16
S
C
S
H
SC (Tf 1 f)
f
T 0.56 ABS[0.2cos(h168)] for h equal to 164° to 345°
T 0.36 ABS[0.4cos(h35)] for angles not 164° to 345°
h arctan(b/a)
In the equations starting with S
L
, non-subscripted values refer to the standard. The result of the CMC
calculation depends on which of the two points is the standard.
■ CIE’94
∆E
CIE’94
L
1
L
2
2
C
1
C
2
2
H
1
H
2
2
where
K
L
, K
C
and K
H
are adjustable parameters (usually set to 2, 1 and 1)
S
L
1
S
C
1 0.045C
S
H
1 0.015C
C refers to the C value of the standard, as in the CMC case.
CONT’D ON FOLLOWING PAGE
C
4
C
4
1900
冢冣
0.638
1 C
1 0.0131C
冢冣
C: COLORIMETRIC CALCULATIONS
PROCESS COLOR 145
Appendix C
K
L
S
L
K
C
S
C
K
H
S
H
lS
l
cS
C
S
H
Additional material on press characterization is available from the FTA.
146 FLEXOGRAPHY: PRINCIPLES & PRACTICES
■ METARISM INDEX (MI)
MI
∆L
1
∆L
2
2
∆a
1
∆a
2
2
∆b
1
∆b
2
2
where
∆L
1
Difference in L value between the two samples under illuminant 1.
∆L
2
Difference in L value between the two samples under illuminant 2.
∆a
1
Difference in a value between the two samples under illuminant 1.
∆a
2
Difference in a value between the two samples under illuminant 2.
∆b
1
Difference in b value between the two samples under illuminant 1.
∆b
2
Difference in b value between the two samples under illuminant 2.
C: COLORIMETRIC CALCULATIONS
CONT’D
A
additive color, 114
analog proofs
laminate, 96
overlay, 96
single-color, 96
anilox roll, 38
B
bar codes
bar-width reduction, 43
color, 43
orientation, 43, 86
bitmap image
converting, 35
defined, 35
resolution of, 35, 68
rotating before importing, 37
blends, 31-32, 45-46, 47, 77, 99
brand identification, 11
C
central-impression press, 28, 29
chroma, 120, 122
CIE, 118, 119
CIE’94, 121, 145
CMY color model, 114, 118, 121, 140
CMS, see color management system
color
defined, 113
differences, 139
gamut, 117, 121-122
maintaining consistent, 128
matching, 133
metarism, 121, 126
proofing, 116-117, 122, 127, 128-129, 133-
141
properties of, 119-120
systems for managing, 127-129
specifying, 73
spectrum, 113-114
spectra, 113
color management system, 56, 128
color matching system, 132
color measurement
color matching, 137
density, 120-121
color model, see CMY, RGB,
(CMYK) process color
color proofs, 49, 127
color rendering index, 100, 118
color separations
flexo vs. offset, 69
color space, 119-121
combination screening, 40
comprehensive roughs, 22
computer software
drawing, 47, 51
page layout, 52
raster image, 37, 46, 53
trapping, 38
concept proof, 93
continuous-tone art
defined, 37
scanning, 43
contract proof
analog, 95
digital, 95
profiled, 95
control target, 131, 140-141
conventional screening, 40, 68, 91
corrugated press, 28
cropping bitmap images, 37
customer service
estimating, 105
quoting, 105
cutback curve, 88, 93, 133
D
DCS
(desktop color separation) file format, 59-
60, 81
delta E/(∆)E, 75, 120-121
densitometer, 100, 101-102, 123
PROCESS COLOR 147
Index
148 FLEXOGRAPHY: PRINCIPLES & PRACTICES
density, 70, 90, 100, 101, 124
solid-ink, 100, 130, 137
design (packaging)
consumer considerations, 14-16
definition, 3
development, 17-18
for flexo, 36, 55
merchandising considerations, 10-11, 13
objectives, 3, 8-9, 10, 19, 21
presentation, 23, 24
production conderations, 13, 18-19, 26
design elements
die line, 32, 50
halftone images, 37
illustrations, 32, 55
layers, 50, 52
pattern fill, 34
photography, 36
type, 26
digital photography, 37, 71-72
digital proofs
continuous ink-jet, 99
drop-on-demand ink jet, 97
dye sublimation, 98
electrophotography, 97
wax transfer, 98
dot gain, 36, 39, 70, 87, 88, 100, 127, 133-135,
142
dot shape, 90, 91, 99, 102
E
EPS
simplifying art in, 53
working with, 52, 60, 82
F
file formats
for graphics, 57
film
properties, 90-92
fingerprinting, see press characterization
FIRST, 42, 61, 80, 82, 89, 91
fonts, 27, 29-30, 58, 60, 61, 78
Postcript, 29
TrueType, 29
G
gamut, color, see color gamut
GCR, (gray component replacement), 41,
53, 70, 72, 80, 82
gradations, see blends
gravure, 13
gray balance, 141
H
halftone cell, 42
halftone dot, 42, 99
halftones
reproducing, 42-43
halftone screen, 43, 68
defined, 37, 90
high-fidelity color printing, 41
hue, 76, 101, 120, 122, 124
hue error, 124
I
ICC profile, 56, 70-71, 80, 95, 128, 133, 137
illustrations
preparing for imaging, 34
simplifying, 34
illustration techniques, 32-33
imaging
errors, 29, 30, 34, 38, 40, 46, 55
preparing files for, 55
reducing time for, 57
ink trap, 124, 125, 131, 133, 137, 141
in-line press, 29
J
job assembly, 65, 79, 80, 84-88
K
K factor, 87
L
L*a*b*, 119-120, 125, 128, 129, 131, 133, 137-
138, 139, 141
L*C*h°, 119, 120, 122, 125
lightness, 119, 120, 122
light source
standard, 118
D50, 118
A, 118
D65, 118
line screen, see screen ruling
M
microdots, 91
moiré, 36, 90, 91, 99
N
narrow-web press, 27, 28, 43
O
object-oriented graphics, 33-34
offset lithography, 13