ISO 13656:2000

INTERNATIONAL ISO
STANDARD 13656
First edition
2000-02-01
Graphic technology — Application of
reflection densitometry and colorimetry to
process control or evaluation of prints and
proofs
Technologie graphique — Application de la densitométrie par réflexion et
de la colorimétrie pour la maîtrise ou l'évaluation des procédés des
imprimés et épreuves
Reference number
ISO 13656:2000(E)
©
ISO 2000

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ISO 13656:2000(E)
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ISO 13656:2000(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative references .1
3 Terms and definitions .2
4 Requirements.7
4.1 Introduction.7
4.2 Control strip .7
4.3 Preparations for measurements.8
4.4 Measurement of reflection density and quantities derived therefrom.8
4.5 Measurement of colorimetric parameters .9
5 Test methods.9
5.1 Introduction.9
5.2 Deviation between the coloration of the solids on the OK print of the production run and on
the proof print .10
5.3 Density or relative density of a process colour solid .10
5.4 Tone value on a print.10
5.5 Apparent ink-trap.11
5.6 Doubling and slur .11
5.7 Variation of the coloration on a single print .11
5.8 Variation of the coloration of the solid over the finished production run .12
Annex A (informative) Gravimetric verification of ink-trap .13
Annex B (informative) Polarisation in densitometry.14
Annex C (informative) Tone value on a printing forme .15
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ISO 13656:2000(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 13656 was prepared by Technical Committee ISO/TC 130, Graphic technology.
Annexes A to C of this International Standard are for information only.
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ISO 13656:2000(E)
Introduction
Reflection densitometers and reflection colorimeters (of tristimulus photometric or spectrophotometric type) are
both reflectometers measuring the reflectance factor of reflection copy materials. Densitometers conforming to
ISO 5-4 and ISO 14981, and colorimeters conforming to ISO 13655 possess a common geometry type, namely
either 0�/45� or 45�/0�. It is further specified in ISO 5-4 that densitometric measurements shall be made on a
specified black backing; ISO 13655 specifies the same condition for colorimetry in graphic arts. Finally, it is noted
that reflectometers of the spectrophotometer type can, in principle, be used as both a densitometer and as a
colorimeter.
Notwithstanding the similarities of the instruments, there are fundamental differences between them. The first of
these is that the typical densitometer used in graphic arts, as its name implies, displays density values (logarithm of
the reciprocal of a weighted average of the spectral reflectance factor) although it may also display other
parameters calculated from these values. A colorimeter, on the other hand, normally displays differently weighted
averages of the spectral reflectance factor, although frequently it can also display various transformations from
these values which may be required for a number of reasons. One such reason is the need to define a more
uniform colour space such as CIELAB.
ISO 5-3 requires that for reflection densitometry the incident flux has a spectral power distribution that conforms to
CIE illuminant A. In colorimetry, ISO 13655 specifies a spectral power distribution that conforms to CIE illuminant
D50 but accepts that such a source is not easily realisable. It requires that D50 be used to calculate the tristimulus
values which, together with the weighting functions specified, effectively defines the spectral response whether it be
achieved by the use of filters or calculation from spectrophotometric data. In practice most colour measurements in
graphic arts today are made with spectrophotometers using a source with a spectral power distribution similar to
illuminant A. The measured spectral reflectance data is used to calculate both densitometric and colorimetric data
and illuminant D50 is used to calculate the tristimulus values as specified in ISO 13655. The implication of this for
colour measurement is that it gives erroneous results when samples fluoresce.
The aim of colorimetry is to provide an instrument response which simulates, as well as possible, that of the
standard observer. In graphic arts, colorimetry serves mainly for colour matching and the establishment of colour
standards. The availability of inexpensive, hand-held colorimeters, with small sampling apertures, has also
permitted the use of colorimetry in process control as a complement to densitometry.
Densitometers are primarily designed for indirect measurement and control of the amount of colorant material of a
specified type present in, or on, a substrate. ISO 5-3 defines a number of statuses, each of which is deemed
appropriate for a particular application. The primary aim of densitometry for graphic arts is to monitor the amount of
colorant per area on a print or proof. For a half-tone print this is a function of the ink film thickness and tone values.
However, densitometry is also used for the determination of other process control quantities. A distinctly different
task is the evaluation of the density ranges of colour separation input material; this type of densitometry is not
covered by this International Standard.
Historically, colour densitometers for reflection type material were first used in preparation for colour separation for
determining the density ranges of continuous-tone, coloured original artwork, as measured through the wide-band
filters used for colour separation. As the quality of the printed products improved, however, reflection densitometers
were also applied to process control in printing. Here, the areas measured consist typically of single-colour patches
contained in control strips, printed with the process colours cyan, magenta, yellow and black.
For the control of the chromatic colours, especially yellow, it was later discovered that measurements made with
narrow-band filters, each centred on the main absorption maximum of one of the process ink colorants, provided
features which can be advantageous for certain control applications. These are:
� reduction of the influence of slight hue shifts on density,
� bringing the yellow densities and tone values within the range of those of cyan and magenta,
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ISO 13656:2000(E)
� improvement of inter-instrument agreement,
� extension of the linear relationship between density and ink film thickness to higher densities,
— reduction of the magnitude of density additivity failure.
It was also found that the readings obtained from densitometers with a means for cross-polarisation to minimise the
influence of first-surface reflection were less affected by ink dry-back. Polarisation also contributes to the last two
features above. The need for the instrument designer to correct for it in computing the spectral response is
described in ISO 5-3 and also in annex B of this International Standard. Standardisation of the minimum efficiency
of polarisation is covered by ISO 14981.
The wide range of applications for which densitometry is used, mean that both wide and narrow band instruments,
as well as the optional use of polarisation, are in common use in graphic arts. Furthermore, colorimetry is becoming
increasingly widely used and all of these options present many alternatives for process measurement within the
industry. It is for this reason that this International Standard has been produced. Since the industry increasingly
needs to communicate process control information between various participants in production it is essential that
this be defined unambiguously. By defining terms, specifying preferred test methods and the requirements for
control strips, and defining reporting procedures, such ambiguity should be kept to a minimum.
Many of the parameters measured or calculated in graphic arts process control, including some of those defined
later in this International Standard, do not require any specific spectral response to be effective. They are
comparative measurements and are in many cases calculated directly from the reflectance data from which density
and colorimetric parameters are themselves derived. In isolated production environments various parameters, each
of which may be derived from any reasonable spectral product, can be equally effective for process control. It is not
the intent of this International Standard to preclude their continued use in such a situation. However, in some
situations there are advantages in using specific parameters or spectral products and, furthermore, to aid
communication in a distributed production environment it is essential that graphic arts metrology is based on
agreed procedures. It is in this context that this International Standard specifies colorimetric and densitometric test
methods for the most common process control procedures in graphic arts and specifies the reporting procedures to
be employed.
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INTERNATIONAL STANDARD ISO 13656:2000(E)
Graphic technology — Application of reflection densitometry and
colorimetry to process control or evaluation of prints and proofs
1 Scope
This International Standard applies to process control and evaluation of single and multi-colour proofing and
printing in the graphic arts using densitometry and colorimetry. This International Standard:
� defines terms;
� specifies minimum requirements for control strips;
� specifies test methods;
� specifies reporting procedures for the results.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 5-3, Photography — Density measurements — Part 3: Spectral conditions.
ISO 5-4, Photography — Density measurements — Part 4: Geometric conditions for reflection density.
ISO 12647-1, Graphic technology — Process control for the manufacture of half-tone colour separations, proof and
production prints — Part 1: Parameters and measurement methods.
ISO 12647-2, Graphic technology — Process control for the manufacture of half-tone colour separations, proof and
production prints — Part 2: Offset lithographic processes.
ISO 12647-3, Graphic technology — Process control for the manufacture of half-tone colour separations, proofs
and production prints — Part 3: Coldset offset lithography and letterpress on newsprint.
1)
ISO 12647-4 , Graphic technology — Process control for the manufacture of half-tone colour separations, proof
and production prints — Part 4: Gravure processes.
1)
ISO 12647-5 , Graphic technology — Process control for the manufacture of half-tone colour separations, proof
and production prints — Part 5: Screen printing.
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic arts images.
1) To be published.
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ISO 13656:2000(E)
1)
ISO 14981 , Graphic technology — Process control — Optical, geometrical and metrological requirements for
reflection densitometers for graphic arts use.
1)
ISO 15790 , Graphic technology and photography — Reflection and transmission metrology — Documentation
requirements for certified reference materials, procedures for use, and determination of combined standard
uncertainty.
DIN 16536-2:1995, Colour density measurements on on-press or off-press prints; Part 2: Instrument specifications
for reflection densitometers and their calibration.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply; they are given in
alphabetical order. Where appropriate they have been taken directly from the CIE International Lighting
Vocabulary, ISO 5-1 to 5-4, ISO 12637-1 and ISO 12647-1 as indicated at the end of the definition.
3.1
aperture
see sampling aperture
3.2
colorimeter
instrument for measuring colorimetric quantities, such as the tristimulus values of a colour stimulus
[International Lighting Vocabulary 845-05-18]
NOTE A tristimulus colorimeter achieves this by the analogue integration of the spectral product of object reflectance or
transmittance factor, illuminant and filters which are defined by the standard observer functions. A spectrophotometric
colorimeter achieves this by calculation from the spectral reflectance or transmittance factor data.
3.3
control patch
area produced for control or measurement purposes
[ISO 12647-1]
NOTE This definition is independent of whether the control patch is produced on film, a printing forme or a print substrate
by conventional or direct methods.
3.4
control strip
one-dimensional array of control patches
[ISO 12647-1]
3.5
core density (on a half-tone film)
transmittance density in the centre of an isolated opaque image element such as a half-tone dot or line. Unit: 1
[ISO 12647-1]
3.6
density
See reflection density
NOTE (Optical) density can be defined for both transmitting and reflecting samples. However, in the context of this
document it is usually only appropriate to reflecting samples. The document uses the term density freely; it should be
understood to be reflection density unless otherwise specified.
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ISO 13656:2000(E)
3.7
doubling/slur patch
control patch for the assessment of the true rolling condition
3.8
film (separation)
image carrier which contains black and white information in analogue form
NOTE Although normal use of the word film would also include colour film materials they are not included in this definition
which is restricted to monochrome half-tone film separations, of which there is one for each colour ink to be printed.
3.9
film polarity
polarity (of a film)
positive if clear and solid areas on the film correspond to unprinted and solid areas on the print, respectively;
negative if clear and solid areas on the film correspond to solid and unprinted areas on the print, respectively
[ISO 12647-1]
3.10
fringe width (of an isolated opaque image element)
average distance between the density contour lines corresponding to 10 % and 90 % of the minimum core density
specified for the printing process under consideration
[ISO 12647-1]
NOTE Fringe width is expressed in millimetres.
3.11
half-tone
image composed of dots which can vary in screen ruling (number per centimetre), size, shape, or density, thereby
producing tonal gradations
[ISO 12637-1]
NOTE Used especially with printing processes like offset lithography where the ink film thickness or the amount of colorant
per unit area is constant throughout the image. However, it is sometimes used in gravure printing where the half-tone dot
density may also vary.
3.12
image element
See half-tone
3.13
incident flux
flux incident on the sampling aperture defining the specimen area on which the measurement is made
[ISO 5-1]
3.14
ink-trap
I
for an overprint, a relative measure for the average amount of colorant per unit area of the second-down colorant
layer that is deposited on to the first-down colorant layer
NOTE 1 Ink-trap is expressed as a percentage.
NOTE 2 Not to be confused with trap employed in colour separation to attenuate mis-register effects.
NOTE 3 Apparent ink-trap is measured optically; gravimetric ink-trap by weight.
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ISO 13656:2000(E)
3.15
mid-tone balance control patch
a half-tone control patch, containing all three chromatic process inks, used for assessing the balance between the
inks; the cyan tone value is normally in the range between 40 and 60 and the magenta and yellow tone values are
selected to approximately produce an achromatic colour
3.16
non-periodic half-tone
image in which the elements composing it do not have a regular frequency
3.17
OK print
OK sheet
during production printing the production print singled out as reference for the remaining production run
[ISO 12647-1]
3.18
overprint
condition where two or more layers of colorant, usually ink, are printed on top of another
3.19
print substrate
material bearing the printed image
[ISO 12647-1]
3.20
process colours (for four-colour printing)
yellow, magenta, cyan and black
[ISO 12647-1]
3.21
reflectance factor
R
ratio of the measured reflected flux from the specimen to the measured reflected flux from a perfect-reflecting and
perfect-diffusing material located in place of the specimen. Unit: 1
[ISO 5-4]
3.22
2)
reflection density
3)
reflectance factor (optical) density
logarithm to base ten of the reciprocal of the reflectance factor. Unit: 1
3.23
reflectometer
photometer for measuring quantities pertaining to reflection
[International Lighting Vocabulary 845-05-26]
2) ISO 5-4.
3) [International Lighting Vocabulary 845-04-67].
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ISO 13656:2000(E)
3.24
relative density
density from which the density of a reference such as the film base or the unprinted print substrate, has been
subtracted. Unit: 1
[ISO 12647-1]
3.25
sampling aperture
area of the sample that contributes to the measurement
NOTE This is not necessarily the same as the illumination aperture which is the area of the sample illuminated by the
instrument or the mechanical aperture created by an opaque mask used to position the densitometer on the specimen. ISO 5-4
makes very specific requirements on the relationship between each of these.
3.26
screen frequency
screen ruling
number of image elements, such as dots or lines, per unit of length in the direction which produces the highest
-1
value. Unit: cm
[ISO 12647-1]
3.27
screen width
reciprocal of screen ruling. Unit: cm
[ISO 12647-1]
3.28
solid
image of uniform coloration intensity with no half-tone structure
3.29
spectral product
product of the spectral power of the incident flux and the spectral response of the receiver, wavelength by
wavelength
3.30
spectral response (of the receiver)
product of the spectral sensitivity of the photodetector and the transmittance of the optical elements associated with
it
3.31
tone value
dot area (on a print)
A
percentage of the surface which appears to be covered by colorant of a single colour (if light scattering in the print
substrate and other optical phenomena are ignored) calculated from the formula:
��bgDD �bD�Dg
L O L O
ts00
A (%)��100 1 10 1� 10
M P M P
N Q N Q
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ISO 13656:2000(E)
where
D is the reflectance factor density of the unprinted substrate;
0
D is the reflectance factor density of the solid;
s
D is the reflectance factor density of the half-tone.
t
[ISO 12647-1]
or in its equivalent form:
AR(%)��100 R /R�R
bgb g
0t 0 s
where
R is the reflectance factor of the unprinted substrate;
0
R is the reflectance factor of the solid;
s
R is the reflectance factor of the half-tone.
t
NOTE 1 Also known as apparent, equivalent or total dot area.
NOTE 2 The synonym dot area may be applied only to half-tones produced by dot patterns.
NOTE 3 This definition may also be used to provide an approximation of the tone value on certain printing formes.
NOTE 4 In general it is assumed that tone values specified in a digital electronic file are reproduced identically on the film
produced on an imagesetter.
NOTE 5 The equation is known as the Murray-Davies equation.
3.32
tone value
dot area (on a half-tone film of positive polarity)
A
Percentage calculated from the formula:
��DD �D�D
L bgO L b gO
ts00
A (%)��100 1 10 1� 10
M P M P
N Q N Q
where
D is the transmission density of the clear film;
0
D is the transmission density of the solid;
s
D is the transmission density of the half-tone.
t
NOTE Also known as the film printing dot area.
[ISO 12647-1]
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ISO 13656:2000(E)
3.33
tone value
dot area (on a half-tone film of negative polarity)
A
Percentage calculated from the formula:
��bgDD �bD�Dg
F I F I
ts00
A (%)��100 100 1� 10 1� 10
H K H K
where
D is the transmission density of the clear film;
0
D is the transmission density of the solid;
s
D is the transmission density of the half-tone.
t
NOTE Also known as the film printing dot area.
[ISO 12647-1]
3.34
tone value increase
difference between a tone value on the print and the corresponding tone value on the half-tone film or in a digital
electronic file. Unit: percent
NOTE The synonym “dot gain” may be applied only to half-tones produced by dot patterns.
[ISO 12647-1]
4 Requirements
4.1 Introduction
There are a number of variables that must be controlled in order to produce predictable results in colour
reproduction. For a given substrate the most important process variables are the colours of solid patches of the
individual process inks and their overprints, slur/doubling and tone value. For production situations where the
control of the process is not particularly well defined subclause 4.2 defines the control patches required on the
print, the minimum number of patches permitted and their tone value tolerances. Where the printer is reasonably
confident of the process variation anticipated a sub-set of these elements will normally be selected that is most
appropriate for control of this variation.
Subclause 4.2 also defines the requirements of a film control strip, where used, to produce the control patches. The
measurement procedure and data reporting specified in 4.3, 4.4 and 4.5 apply to measurements made on control
patches irrespective of whether these were generated from film, by direct-to-plate or direct-to-press.
4.2 Control strip
4.2.1 Film quality
The quality parameters of a film control strip - such as core density, fringe width, and base density - shall conform
with the pertinent part of ISO 12647.
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ISO 13656:2000(E)
4.2.2 Minimum set of control patches
The control strip shall contain doubling/slur patches as well as single-colour and overprint patches for the process
colour solids, namely K, C, M, Y, (C�M), (C�Y), (M�Y) and (C�M�Y). In addition it shall contain at least
3 well-defined single colour half-tone control patches, labelled with their nominal tone values, for each of the
process colours K, C, M and Y. One shall have a tone value between 20 % and 30 %, another shall have a tone
value between 40 % and 50 % and the third shall be between 70 % and 80 %. There shall also be a well-defined
half-tone control patch for the measurement of mid-tone balance and an area close to, or within, the control strip for
measurement of the unprinted substrate. The complete control strip should be repeated as frequently as possible
across the whole width of the press sheet.
The half-tone dot shape of the image elements used for process control shall be circular and the screen ruling
�1
should lie within 10 cm of the screen frequency of the subject images. All tone values, including those of the mid-
tone control element, shall be within 1 % of the nominal value.
NOTE 1 For positive and negative polarity film control strips, it is desirable to add control patches with several tone values
below 10 % and above 90 %, respectively.
NOTE 2 While it is recognized that the control element is specified as circular and actual work can be circular, square,
elliptical, etc., the use of a specific shape enables process control checks across jobs, presses, and companies.
4.3 Preparations for measurements
Ensure that the measuring head and the sampling aperture of the instrument are clean. If necessary, observe the
manufacturer’s warm-up time recommendations, then calibrate the instrument to read accurately relative to the
perfect reflecting diffuser, according to the manufacturer's instructions
...