ISO 12645:1998

INTERNATIONAL ISO
STANDARD 12645
First edition
1998-11-01
Graphic technology — Process control —
Certified reference material for opaque area
calibration of transmission densitometers
Technologie graphique — Maîtrise de procédé — Matériel de référence
certifié pour la calibration des densitomètres transmission en pourcentage
de superficie opaque
A
Reference number
ISO 12645:1998(E)

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ISO 12645:1998(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.
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.
International Standard ISO 12645 was prepared by Technical Committee ISO/TC 130, Graphic technology.
Annexes A to E of this International Standard for information only.
©  ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
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ISO ISO 12645:1998(E)
Introduction
In the graphic arts, the modulation of transmittance or reflectance is often achieved by printing half-tones rather than
continuous tones, as would be the case in photography.
The process control of the generation and transfer of half-tone patterns is essential to the graphic arts industry. For
the process control of transmission type material one monitors the percentage of the total area that is virtually
opaque. Measurements are carried out on well-defined half-tone control patches which contain regularly-spaced,
circular half-tone dots. The measuring instrument is either a densitometer or a colorimeter of the spectrophotometer
or tristimulus type. The opaque area percentage is usually set equal to the ratio of the relative transmittance factors
of the half-tone and that of the solid. With densitometers one calculates the transmittance factors from the ISO
standard diffuse (opal) densities, the corresponding expression for the ratio has been given by Murray and Davies,
see [E.2] and equations (1) and (2) of clause 3.
In principle, the opaque area percentage for positive-polarity films should be identical to the tone value as defined in
the graphic arts; for negative-polarity films it should be identical to 100 % minus the tone value. In reality, however,
there may be errors which produce small deviations between the two quantities; there is thus a need for an
independent check on the determination of the tone value by densitometers and colorimeters.
In the graphic arts process chain, particularly important steps are "image setting", "film duplication" and
"platemaking". During these steps, there is no highly diffusive medium present which could be compared to the
opal-glass diffuser of densitometers according to ISO 5-2 [E.3]. It would thus appear that the opal-glass instrument
is not the best solution for graphic arts applications. However, the opal-glass design permits a relatively simple
construction; such instruments are widely used in the graphic arts industry.
When calibrating a densitometer with a stepped density reference material (at least two steps), usually both the zero
point and the slope factor are adjusted. The latter is the ratio of the density difference between the steps as read by
the densitometer and the density difference as reported by a national standardizing laboratory. If the ratio deviates
from 1, the base of the logarithm used for converting transmittance factor into transmittance factor density is not 10
as required by definition. As a consequence, the tone values calculated from the densities may deviate from the
opaque area percentages. It is important to note that the (absolute) ISO standard diffuse density calibration of a
densitometer may be dependent on the thickness of the sample. The stepped density reference material used for
the density calibration should therefore have the same thickness as the samples to be measured, otherwise only
relative densities can be measured. For many graphic arts applications, however, absolute density measurements
are not the issue but the control of the opaque area percentage of half-tone film. This quantity determines the size
of image elements, such as half-tone dots or lines, produced by transfer steps such as film duplication and
platemaking. The densitometer is then zeroed on the clear film and the densities of a half-tone and a solid are
measured. The Murray-Davies-formula, [E.2] and equations (1) and (2) of clause 3, is used to determine tone
values from the density values, see also ISO 12647-1.
It is known that transmission densitometers of the opal type are subject to multiple oblique-angle interreflections
between the opal and the surfaces close to it, see ISO 5-2 [E.3]. These include the two surfaces of the sample and
those on the side of the instrument opposing the opal. For samples of low density, reflections from all those
surfaces may contribute to the efflux measured at the back of the opal. At (absolute) ISO standard diffuse densities
near zero, the densities read by an opal instrument are thus slightly lower than those of a diffuse sphere type
instrument; 0,03 is a typical difference. This effect has to be taken into account if transmission densities or
reflectance factors read by instruments of different design are compared. For samples of higher densities, the
reflections from all surfaces but the surface next to the opal are effectively suppressed. During the measurement of
a half-tone pattern, the latter influences the interreflection effect in two ways: The image elements may have a
reflectance different from the film base, they also tend to attenuate reflections from surfaces further removed from
the opal. Therefore, the tone values determined from densities measured with an opal instrument may differ from
the actual opaque area percentage of the sample which can be determined by microscopic measurement or by a
sphere type instrument (which does not show the interreflection effect).
ISO 12647-1 specifies that transmission densities shall be measured by instruments complying to ISO 5-2, thus
there can be no ambiguity when quoting (absolute) ISO standard diffuse densities such as clear film densities.
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The interreflection effect can be artificially attenuated if the opal is kept at a distance of, say, 25 mm from the
measured object. Another method is to place a neutral filter with a density of at least 0,3 between the opal and the
sample. The filter tends to attenuate the light coming back from the opal towards the sample and also the
reflections.
The interreflection effect may produce a discrepancy of typically 3 % between the tone value of a half-tone as
determined with a transmission densitometer of the opal type and the opaque area percentage as determined from
independent measurements of the dot pattern. The discrepancy cannot be circumvented by proper zeroing or
adjustment of the slope factor of the instrument, it depends on the magnitude of the opaque area percentage. Thus
there is a need for an independent check on the tone value determination by a transmission densitometer and, if
necessary, the establishment of a table that converts tone values into opaque area percentages. This International
Standard specifies requirements for two types of reference materials that provide half-tone control patches with dot
patterns of accurately defined dimensions that may serve as "Certified reference materials" (CRM) if the
1)
requirements of ISO 15790 are also fulfilled.
Reference materials may be produced from half-tone film material or a thin glass slab with a chromium deposit. The
latter's advantages are a very small fringe width, a wavelength-independent attenuation and a good uniformity of
opaque area percentage over the control patch. The thickness, the scattering properties and the wavelength
dependence of the transmission density differ from those of graphic arts film material. It might thus be suspected
that this invalidates the use of such a reference material as a standard. It has been shown, however, that thickness
changes of less than 1 mm shift only the zero point of a densitometer, thus the densitometric opaque area
percentage calculation is not affected. Other test series showed that interposing films of varying scattering
properties between the chromium/glass reference material and the opal did not affect the opaque area percentage
determination, within the measurement accuracy. The absence of a wavelength dependence can be regarded as an
asset: Since the wavelength dependencies of graphic arts films differ much from each other, there is no established
dependence that could be used for such a reference material.
___________
1)  See informative annex E.
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INTERNATIONAL STANDARD  ISO ISO 12645:1998(E)
Graphic technology — Process control — Certified reference
material for opaque area calibration of transmission densitometers
1 Scope
This International Standard defines requirements for a half-tone certified reference material which may be used for
the opaque area percentage calibration of transmission densitometers or colorimeters for use in the graphic arts.
This International Standard is not applicable to the calibration of transmission densitometers in terms of ISO
standard diffuse density.
NOTE Test methods for determining the opaque area percentage are given in the informative annex A.
2 Normative reference
The following normative document contains 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 edition of the normative document 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 12647-1:1996, Graphic technology — Process control for the manufacture of half-tone colour separations,
proof and production prints — Part 1: Parameters and measurement methods.
3 Definitions
For the purposes of this International Standard, the definitions given in CIE 17.4 and the following apply. They are
given in alphabetical order.
NOTE For quantities, the preferred unit is given together with the definition. By definition, the unit of formerly so-called
“dimensionless quantities” is 1.
3.1
control patch
area produced for control and measurement purposes
[ISO 12647-1]
3.2
core density
transmission density in the centre of an isolated opaque image element such as a half-tone dot or line
Unit: 1
[ISO 12647-1]
3.3
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
Unit: μm
[ISO 12647-1]
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ISO 12645:1998(E) ISO
3.4
opaque area percentage
P
percentage of total area that is nearly opaque to radiation in the wavelength range of interest
3.5
polarity (of a transmission object)
positive if clear and solid areas on the object correspond to unprinted and solid areas on the print, respectively.
Negative if clear and solid areas on the object correspond to solid and unprinted areas on the print, respectively
3.6
screen ruling
screen frequency
f
number of image elements, such as dots or lines, per length in the direction which produces the largest number
-1
Unit: cm
[ISO 12647-1]
3.7
screen width
reciprocal of screen frequency
Unit: cm
[ISO 12647-1]
3.8
tone value
dot area (on a transmission object of positive polarity)
A
percentage calculated from the formula:
−−()DD
t0
11− 0
=−100%%⋅100 . . . (1)
A
−−()DD
s0
11− 0
where
D is the transmission density of the clear substrate;
0
D is the transmission density of the solid;
s
D is the transmission density of the half-tone.
t
[ISO 12647-1]
NOTE Also known as film printing dot area.
3.9
tone value
dot area (on a half-tone object of negative polarity)
A
percentage calculated from the formula:
−−()DD
t0
11− 0
A=−100%%⋅100 . . . (2)
−−()DD
s0
11− 0
where
D is the transmission density of the clear substrate;
0
D is the transmission density of the solid;
s
D is the transmission density of the half-tone.
t
[ISO 12647-1]
NOTE Also known as film printing dot area.
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ISO ISO 12645:1998(E)
4 Requirements
4.1 Materials
The substrate shall consist of a clear material with a pattern of opaque half-tone dots on one side. At normal
incid
...