ISO 12218:1997
(Main)Graphic technology — Process control — Offset platemaking
Graphic technology — Process control — Offset platemaking
Technologie graphique — Maîtrise des procédés — Confection des plaques offset
Grafična tehnologija - Vodenje procesov - Izdelava tiskovne forme za ofsetni tisk
General Information
Standards Content (Sample)
IS0
INTERNATIONAL
12218
STANDARD
First edition
1997-08-01
Graphic technology - Process control -
Offset platemaking
- Martrise des procbdb - Confection des plaques
Technologie graphique
offset
Reference number
IS0 12218:1997(E)
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IS0 12218:1997(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
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 IS0 12218 was prepared by Technical Committee
ISO/TC 130, Graphic technology.
Annex A forms an integral part of International Standard. Annexes B to F
are for information only.
0 IS0 1997
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 l Cl-l-121 1 Geneve 20 l Switzerland
Internet central @ iso.ch
x.400 c=ch; a=400net; p=iso; o=isocs; s=centrat
Printed in Switzerland
ii
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@ IS0 IS0 12218:1997(E)
Introduction
During the production of an offset printing plate, a carrier material is coated
with a thin radiation-sensitive film. The plate is referred to as pre-
sensitized, if it has been produced by a plate manufacturer rather than at
the printing site. The radiation-sensitive coating on the offset printing plate
may be either positive-acting (for positive polarity films) or negative-acting
(for negative polarity films). Some offset printing plates with positive-acting
coatings can be used as convertible plates together with negative polarity
input material.
During preparation of the offset printing forme, analogue information is
transferred from half-tone film to an offset printing plate by means of a
contact exposure step with radiation to which the plate coating has been
sensitized. Alternatively, offset printing plates may be exposed using
optical projection of reflection or transmission copy or by direct writing
techniques. While such alternative processes are not covered by this
International Standard, many of the principles included may be applied by
analogy.
Before the exposure step, the emulsion side of the half-tone film is brought
into intimate contact with the sensitized plate coating, usually by means of
a vacuum contact frame. The radiation used for the exposure step may
contain both diffuse and unidirectional components. The benefit of diffuse
radiation is that artefacts like film edges of positive polarity films, scratches
and dust particles are not rendered on the plate. However, care is required
because diffuse radiation accentuates any artefacts associated with areas
where film and plate are out of contact or where the contact is poor.
During the exposure step, the optical spread function of the printing plate
and the diffusive component of the radiation cause final detail on a positive-
acting printing plate to be reduced in width so that the image elements are
somewhat smaller than the original on the film. With negative-acting plates,
the same physical phenomena result in fine detail on the plate that is
somewhat larger than the original.
After the exposure step but before development, the coating usually shows
a colour difference between exposed and unexposed areas.
The development of an offset printing plate normally consists of the
removal of the coating in the non-printing areas. These are the exposed
areas of a positive-acting plate and the unexposed areas of a negative-
acting plate.
The quality of the printing forme produced by the platemaking operation
depends in particular on the following process parameters:
- exposure step, especially vacuum conditions (including drawdown);
- chemical composition and temperature of the developer fluid;
- condition of rollers and brushes;
. . .
III
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IS0 12218:1997(E)
- processing speed (development time);
- finisher condition.
colour contrast between printing and non-printing
After development, the
areas reater than before.
is usually mu
ch g
Following exposure and processing, baking or post-exposure treatment
may be used prior to correction and gumming. The developed offset
printing plate is thus transformed into a press-ready offset printing forme.
During correction, image elements are deleted (negative correction) or
added (positive correction). At the gumming stage, a thin coating of a
colloidal solution is applied to the image side of the plate in order to protect
the surface and to prevent toning during the printing operation. Baking or
post-exposure treatment is a heat treatment which increases the durability
of the coating with regard to chemical or mechanical wear.
Determination of the optimum exposure for positive-acting offset printing
plates: There are three importa .nt considerations.
The exposure should be strong enough for artefacts like film edges
a)
and dust particles to not n ormally show on the offset printing forme.
b) The exposure should not be so strong that the transfer of fine highlight
half-tone dots is impaired.
c) Since the exposure also determines tone value, which is very
important in process control, the exposure should be controlled such
that the tone value decrease from the half-tone film to the offset
printing forme is constant, irrespective of the offset printing plate type
and processing conditions.
For half-tone screens with screen frequencies of 70 cm-1 or less, it has
been found in practice that it is possible to meet considerations a) and b)
with a single exposure step. The exposure used is appreciably stronger
than one which would result in the best possible resolution (but where
artefacts would also be rendered). Consideration c) can be met in addition
by observing a suitably selected microline reading.
For periodic or non-periodic fine screens which contain image elements of
less than 25 urn size, conditions a) and b) cannot be met with a single
exposure step. Instead, a first exposure step is carried out that yields the
best possible resolution or slightly above. During the second exposure
step, the subject areas are protected by a so-called burnout mask; an
extended exposure removes the artefacts.
For positive-acting printing plates, it has been found that microline targets
can be used to define an exposure range which ensures a reproducible
tone value decrease from the half-tone film to the offset printing forme. In
this range, the tone value decrease from the half-tone film to the printing
forme is a linear function of the microline reading; the function depends on
the platemaking resolution. For a particular plate, under given exposure
and processing conditions, the graph of the positive microline reading over
the logarithm of the exposure thus characterizes the tone value
dependence on exposure. The slope of the graph is a measure of the rate
of tone value change with exposure change. Therefore, a steep slope
indicates less exposure latitude than a less steep one.
Mcroline targets useful for process control of platemaking contain a
number of subtargets with graduated linewidths ranging from a few
micrometres to several tens of micrometres. As distinct from the targets
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@ IS0 IS0 12218:1997(E)
used for testing the resolving power in photography, microline targets show
line-to-space ratios other than 1 :I. Usual ratios are 1:9, 35 and 1:4. Within
the usual exposure range the microline reading depends very slightly on
the line-to-space ratio. It is important to realize that the microline reading
depends on the density level between the microlines. It must not be
appreciably higher than elsewhere on the control strip film. As an
alternative to a division into subtargets of constant linewidth, a single target
with microlines of continuously variable linewidth may also be used. Since
there can be directional effects during the manufacture of printing plates as
well as during plate processing, it is a good plan to average over readings
taken at right angles or to use targets with circular microlines. It is
important to note that microline readings always refer to the width of the
microlines on the film, not to the (unknown) width on the printing forme.
Determination of exposure negative-acting offset printing plates: There are
three important considerations.
1) The exposure should be strong enough to achieve a sufficient and
reliable run length.
2) The exposure should not be so strong that there is excessive tone
value increase from the half-tone film to the printing forme or that
extreme shadow detail is lost.
3) The tone value increase from the half-tone film to the offset printing
forme should be at a specified level.
Since the first criterion is overriding, negative-acting offset printing plate
exposures usually follow the recommendation of the plate manufacturer,
which is expressed as the reading of a continuous-tone step wedge. Once
the optimum exposure has been established, microline targets may be
used to additionally monitor the consistency of subsequent exposures.
They should not be used as the primary exposure determinant.
Some users employ weaker exposures than the manufacturer
recommends, in an effort to obtain a desired tone reproduction in automatic
processing systems with plates that have long run-length coatings which
require more aggressive processing.
With some plates, the run length may be extended by post-exposure or
heat treatment. In these cases, the manufacturer may recommend a lower
range of allowable exposures and a post-exposure or baking treatment to
extend press life. This may result in a lower tone value increase than
otherwise.
Apart from its use for determining the exposure of negative-acting offset
printing plates, a continuous-tone step wedge may also be used to assess
the development process. A useful quantity in this respect is the
platemaking gradation; it characterizes the reaction of the plate coating to
the amount of radiation under a given processing condition. A change of
the platemaking gradation indicates that the processing conditions or the
coating have changed.
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IS0 12218:1997(E)
INTERNATIONAL STANDARD @ IS0
Graphic technology - Process control - Offset platemaking
II Scope
This International Standard establishes unified terminology, test methods and requirements for the process control
of the preparation of the offset printing forme.
This International Standard
- applies to pre-sensitized metal plates;
- applies to contact exposures whether in a contact frame, step-and-repeat machine or other automated
processor;
- does not apply to optical projection or direct writing techniques, although the principles may be applied by
analogy;
- does not apply to non-periodic half-tone screens, although the tone value specifications may be applied by
analogy.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain
registers of currently valid International Standards.
Pan 2: Geometric conditions for transmission density.
IS0 5-2:1991, Photography - Density measurements -
IS0 5-3:1995, Photography - Density measurements - Part 3: Spectral conditions.
IS0 12647-l :1996, Graphic technology - Process control for the manufacture of half-tone colour separations,
- Part 1: Parameters and measurement methods.
proof and production prints
3 Definitions
For the purposes of this International Standard, the following definitions apply. They are given in alphabetical order.
NOTE - For quantities, the preferred unit is given together with the definition. By definition, the unit of so-called
dimensionless quantities is 1.
3.1 baking: Heat treatment to increase the durability of the coating with regard to chemical attack or mechanical
wear.
3.2 contact exposure step: Process step where an offset printing plate is exposed in intimate contact with a half-
tone film.
NOTE - Usually, a vacuum is used in order to achieve an intimate contact.
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ISOl2218:1997(E)
3.3 continuous-tone step wedge: Linear array of continuous-tone control patches whose transmittance densities
are stepped in increments of 0,15 or 0,30.
control patch: Area produced for control or measurement purposes. [ISO 12647-l]
3.4
3.5 control strip: One-dimensional array of control patches. [ISO 12647-l]
3.6 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-I]
3.7 exposure: Product of the irradiance and duration of the photochemically active radiation during the exposure
process step. Unit: J/m*.
NOTE - Instead of the photochemically active (actinic) radiation, any measure for the total energy deposited may be used
once the correspondence has been ascertained.
3.8 exposure latitude: Maximum ratio of exposures that produce acceptable results. Unit: 1.
3.9 exposure step: Process step where an offset printing plate is exposed to photochemically active radiation.
3.10 film polarity: Positive if clear and solid areas on the half-tone film correspond to unprinted and solid areas
on the print, respectively. Negative if clear and solid areas on the half-tone film correspond to solid and unprinted
areas on the print, respectively.
NOTE - Adapted from IS0 12647-l.
3.11 fringe width (of an isolated opaque half-tone dot): 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: urn.
NOTE - Adapted from IS0 12647-1.
3.12 half-tone film: Film for use with a half-tone printing process showing image elements like dots or lines.
[ISO 12647-l]
3.13 microline reading: Under given conditions, the minimum width of positive or negative microlines on half-
tone film that transfer such that at least 50 % of the length of the microlines is clearly visible on the offset printing
forme. The positive microline reading refers to positive microlines (dark lines on light ground) and the negative
microline reading refers to negative microlines (clear lines in dark solid) as they appear on the offset printing forme.
Unit: urn.
3.14 microline target: Control patch compressing fine positive and negative lines or various but accurately
designated widths.
3.15 negative-acting (offset printing) plate: Offset printing plate for use with negative polarity film.
[ISO 12647-21
3.16 offset printing forme: Fo rme for planographic printing where the printi ng parts accept and the non-printing
parts do not accept pri nting ink.
NOTE - An offset plate is transformed into an printing forme means of the exposure, development and post-
bY
treatment process steps.
3.17 offset (printing) plate: Plane workpiece whose surface has been coated such that an offset printing forme
can be produced thereof. [ISO 12647-21
NOTE - In current usage also known as an offset printing forme.
3.18 out-of-contact phenomenon: Insufficient transfer of image elements from a half-tone film to an offset
printing plate as a result of inadequate contact between them.
NOTE - This may be caused by a mechanical object or an air bubble (due to insufficient vacuum drawdown).
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3.19 platemaking; preparation of the offset printing forme: Process steps by which an offset printing forme is
prepared from an offset printing plate.
3.20 platemaking gradation: Measure of the dependence of the coating thickness on the offset printing forme on
exposure. Unit: 1.
3.21 platemaking resolution: Minimum microline reading for both positive and negative microlines that is
achievable with a single exposure step. Unit: pm.
NOTES
I Not to be confused with the term “resolving power” in photography.
2 Sometimes referred to as the optimum resolution.
3.22 positive-acting (offset printing) plate: Offset printing plate for use with positive polarity film. [ISO 12647-21
3.23 pre-sensitized offset plate: Offset plate whose surface has been coated by the manufacturer with a
radiation-sensitive coating.
3.24 reflectance factor (I?): Ratio of the measured reflected flux from the specimen to the measured reflected
flux from a perfectly reflecting and perfectly diffusing material located in place of the specimen. Unit: 1. [ISO 5-41
3.25 reflectance factor densityl); reflection density*): The logarithm to the base 10 of the reciprocal of the
reflectance factor, R. Unit: 1.
3.26 screen ruling; screen frequency: Number of image elements, such as dots or lines, per unit of length in the
direction which produces the highest value. Unit: cm-l. [ISO 12647-l]
3.27 screen width: Reciprocal of screen ruling. Unit: cm. [ISO 12647-l]
3.28 step-and-repeat machine: Apparatus for making automated-contact exposure steps onto an offset plate.
3.29 tone value; dot area (on a print or a printing forme), 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:
/ l-1()-~-~)
I[ I
where
DO is the reflectance factor density of the unprinted print substrate, or the non-printing parts of the printing
forme;
D, is the reflectance factor density of the solid;
Dt is the reflectance factor density of the half-tone.
Unit: Percent.
NOTES
1 Adapted from IS0 12647-1.
2 Also known as apparent, equivalent or total dot area.
The synonym “dot area” may be applied only to half-tones produced by dot patterns.
3
1) International Lighting Vocabulary ([4] in annex F).
2) IS0 5-4.
3
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IS0 12218:1997(E)
4 This definition may be used to provide an approximation of the tone value on certain printing formes.
5 In general it is assumed that the tone value (termed “ink value” in IS0 12640) of the data is reproduced identically on the
film produced by an image setter. Final films should reproduce those tone values.
3.30 tone value; dot area (on a half-tone film of positive polarity), A: Percentage calculated from the formula:
/ j-lo- ‘Ds-~)
I[ I
where
DO is the transmittance density of the clear half-tone film;
D, is the transmittance density of the solid;
Dt is the transmittance density of the half-tone.
Unit: Percent. [ISO 12647-I]
NOTES
1 Adapted from lS0 12647-l.
2 Also known as the film-printing dot area.
3.31 tone value; dot area (on a half-tone film of negative polarity), A: Percentage calculated from the formula:
A (%) = 100 - 100”
I
where
DO is the transmittance density of the clear half-tone film;
D, is the transmittance density of the solid;
Dt is the transmittance density of the half-tone.
Unit: Percent.
NOTES
1 Adapted from IS0 12647-1.
2 Also known as the film-printing dot area.
3.32 transmittance (optical) density ’); transmission density*): The logarithm to the base 10 of the reciprocal
of the transmittance factor, T. Unit: 1 l
3.33 transmittance factor (T): Ratio of the luminous flux transmitted through an aperture covered by a specimen
to the luminous flux th rough the aperture without the specimen in place. Unit: 1.
3.34 vacuum frame; ~ contact
frame; exposure frame: Manually operated vacuum apparatus where contact
exposure steps can be carried ou t.
1) International Lighting Vocabulary ([4] in annex F).
2) IS0 5-2.
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4 Requirements
4.1 Colour separation film quality
Unless otherwise specified, the core density shall be at least 2,5 above the transmittance density of the clear film
(film base plus fog). The transmittance density in the centre of a clear half-tone dot shall not be more than 0,l above
the corresponding value of a large clear area. The transmittance density of the clear film shall not be higher than
0,15. Measurements shall be made with a (UV) transmission densitometer whose spectral products conform to IS0
type 1 printing density as defined in IS0 5-3.
The fringe width shall be not greater than one-fortieth of the screen width; the half-tone dot shall not be split up into
distinct parts. The quality of the colour separation film shall be evaluated according to A.1 of annex A.
NOTES
1 The clear film density requirement is based on the understanding that
- the density range of the clear areas of all films that are to be exposed onto an offset printing plate, for consistent work,
should not exceed 0,lO;
- 0,05 represents the lowest commonly found value for IS0 type 1 printing density. In order to minimize the impact of the
use of half-tone films with clear film densities above this range, agreements between the supplier of colour separations
and the recipient are required. Contacting or duplicating can also be used to bring half-tone films with dissimilar clear film
densities into agreement.
2 As a practical guide, a core density of 2,5 above the clear film density will normally be achieved, if the density of large solid
areas is more than 3,5 above the clear film density.
If a user wishes to use a blue filter to transmittance density measurements, it is necessary to determine, for the particular
3
film type and processing conditions, the correlation between densities obtained with the blue filter and those obtained with an
IS0 type 1 printing density instrument.
4.2 Control patches
The platemaking process steps shall be monitored by exposing at least one control strip along with the work on the
offset printing plate. The control strip shall contain well-defined half-tone control patches with tone value
designations accurate to + 1 %. The shape of the half-tone dot structure shall be circular. The screen ruling shall be
constant, it shall be selected from the range 50 cm-1 to 70 cm-l. The core density shall be not less than 3,0 above
the transmittance density of the clear film (film base plus fog). The fringe width shall be not greater than 2 urn as
determined by the apparatus described in A.2 of annex A.
For positive-acting plates, a microline target is the primary tool for exposure control. It shall be exposed with the
work, a continuous-tone step wedge should be exposed at the same time. For negative-acting plates, a continuous-
tone step wedge is the primary tool for exposure control. It shall be exposed with the work; a microline target may
be exposed at the same time. The line-to-space ratio of the microline targets shall be selected from the range 1:9 to
1:2; it shall be kept constant throughout the target(s). The continuous-tone step wedge should possess a step
increment of 0,15 or smaller.
4.3 Selection of the exposure
4.3.1 Positive-acting plates
The platemaking process shall be controlled such that the tone values of the 40 % or 50 % control patches on the
ready-to-print but not yet inked offset printing forme are smaller than those of the corresponding control patches on
the control strip film by the amounts given in table 1. The tone values on the offset printing forme and the control
strip film shall be determined by the methods given in 5.1 and 5.2, respectively. Instead of the reference method
given in 5.1, a secondary method may be used if the results can be directly related to those of the reference
method.
NOTE - Secondary methods are described in annexes B and C.
5
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IS0 12218:1997(E)
Table 1 - Tone value decrease from the half-tone film to the printing forme
Tone value decrease
Screen rulingl)
40 % control patch 50 % control patch
cm-1 % %
50 2,5 to 3,5 3,0 to 4,0
3,5 to 5,0
60 3,0 to 4,0
3,5 to 4,5 4,5 to 6,0
70
1 In this range the tone value change is proportional to screen ruling.
I 1
4.3.2 Negative-acting plates
No requirement is given. The exposure shall be selected according to the plate manufacturer ’s recommendation.
Compared to the manufacturer ’s recommendation for optimum exposure, the exposure actually used shall not be
weaker than one 0,15 density step, as shown by a continuous-step wedge. If a manufacturer recommends a range
of exposures for plates that can be treated after development by either baking or additional exposure, the actual
exposure shall not be weaker than the lower limit of the range specified.
NOTE - With these conditions, the tone value increase from the half-tone film to the offset printing forme is usually within the
ranges shown in table 2 for information only. Under the conditions mentioned in the last but one paragraph of the introduction,
the tone value increase may be slightly lower.
Table 2 - Tone value increase from the half-tone film to the printing forme
(for information only)
Tone value increase
Screen rulingl)
40 % control patch 50 % control patch
cm-1 % %
3,0 to 4,0
50 2,5 to 3,5
3,0 to 4,0 3,5 to 5,0
60
70 3,5 to 4,5 4,5 to 6,0
1) In this range the tone value change is proportional to screen ruling.
4.4 Reproduction limits
Isolated opaque half-tone dots of more than 25 urn in diameter on the control strip film shall transfer onto the offset
printing forme in a uniform and consistent manner. Likewise, negative half-tone dots (clear circular holes) with
diameters more than 25 urn on the control strip film shall transfer onto the offset printing forme in a uniform and
consistent manner.
5 Test methods
5.1 Tone value on an offset printing forme - Reference method
With the aid of a standard test object (with resolution in the micrometre range) calibrate for either area or diameter
determination:
- an electron probe microanalyser, or
- a scanning electron microscope, or
a microscope-videocamera image analyser combination.
-
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IS0 12218:1997(E)
Determine the area taken up by a single circular half-tone dot on the offset plate. Calculate the tone value by
dividing the average area covered by a single dot by the square of the screen width. Measure and average over as
many half-tone dots as needed for a tone value accuracy of & 05 %.
5.2 Tone value of a half-tone control patch on a control strip
Using a transmission densitometer conforming to IS0 5-2, determine the transmittance densities of the film base
material, DO, the solid tone, D,, and a well-defined half-tone area, Dt. calculate the tone value from the pertinent
definition, that is 3.30 for positive polarity and 3.31 for negative polarity half-tone films.
In order to assure sufficient accuracy, the sampling aperture of the instrument should have a diameter not less than
15 times the screen width, it shall be not less than ten times the screen width. This requirement also applies, by
analogy, to the area of non-circular sampling apertures.
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IS0 12218:1997(E)
Annex A
(normative)
Determination of quality parameters of half-tone dots on a half-tone film
A.1 This method shall be applied only to those half-tone films whose clear film IS0 Status T (blue) transmittance
density is 096 or less. Other films shall be tested by the method described in A.2 or any other method whose
equivalence to A.2 has been proven. Place a film control strip with a microline target, film emulsion orientation up,
on a light table and cover it with the film to be evaluated, film emulsion orientation down. With a hand-held
microscope of between 60. and IOO-fold magnification, observe the isolated opaque half-tone dots. If the microlines
are distinctly visible below the half-tone dots then the core density is too low.
The fringe width shall be estimated by comparing it to the width of the microlines which is stated on the microline
target. For the fringe width evaluation, the colour separation film should be illuminated from below by light of oblique
angles of incidence, a condition known as dark field illumination.
width
NOTE - With some experience, the compliance of half-tone dots to a specified maximum fringe can be predicted with
near certainty.
A.2 A scanning microdensitometer shall be used, that is a transmission microscope where a diaphragm has been
introduced either into the illumination stage or the image plane with the effect that an aperture is form
...
SLOVENSKI STANDARD
SIST ISO 12218:2002
01-november-2002
*UDILþQDWHKQRORJLMD9RGHQMHSURFHVRY,]GHODYDWLVNRYQHIRUPH]DRIVHWQLWLVN
Graphic technology -- Process control -- Offset platemaking
Technologie graphique -- Maîtrise des procédés -- Confection des plaques offset
Ta slovenski standard je istoveten z: ISO 12218:1997
ICS:
37.100.01 *UDILþQDWHKQRORJLMDQD Graphic technology in
VSORãQR general
SIST ISO 12218:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 12218:2002
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SIST ISO 12218:2002
IS0
INTERNATIONAL
12218
STANDARD
First edition
1997-08-01
Graphic technology - Process control -
Offset platemaking
- Martrise des procbdb - Confection des plaques
Technologie graphique
offset
Reference number
IS0 12218:1997(E)
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SIST ISO 12218:2002
IS0 12218:1997(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
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 IS0 12218 was prepared by Technical Committee
ISO/TC 130, Graphic technology.
Annex A forms an integral part of International Standard. Annexes B to F
are for information only.
0 IS0 1997
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 l Cl-l-121 1 Geneve 20 l Switzerland
Internet central @ iso.ch
x.400 c=ch; a=400net; p=iso; o=isocs; s=centrat
Printed in Switzerland
ii
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SIST ISO 12218:2002
@ IS0 IS0 12218:1997(E)
Introduction
During the production of an offset printing plate, a carrier material is coated
with a thin radiation-sensitive film. The plate is referred to as pre-
sensitized, if it has been produced by a plate manufacturer rather than at
the printing site. The radiation-sensitive coating on the offset printing plate
may be either positive-acting (for positive polarity films) or negative-acting
(for negative polarity films). Some offset printing plates with positive-acting
coatings can be used as convertible plates together with negative polarity
input material.
During preparation of the offset printing forme, analogue information is
transferred from half-tone film to an offset printing plate by means of a
contact exposure step with radiation to which the plate coating has been
sensitized. Alternatively, offset printing plates may be exposed using
optical projection of reflection or transmission copy or by direct writing
techniques. While such alternative processes are not covered by this
International Standard, many of the principles included may be applied by
analogy.
Before the exposure step, the emulsion side of the half-tone film is brought
into intimate contact with the sensitized plate coating, usually by means of
a vacuum contact frame. The radiation used for the exposure step may
contain both diffuse and unidirectional components. The benefit of diffuse
radiation is that artefacts like film edges of positive polarity films, scratches
and dust particles are not rendered on the plate. However, care is required
because diffuse radiation accentuates any artefacts associated with areas
where film and plate are out of contact or where the contact is poor.
During the exposure step, the optical spread function of the printing plate
and the diffusive component of the radiation cause final detail on a positive-
acting printing plate to be reduced in width so that the image elements are
somewhat smaller than the original on the film. With negative-acting plates,
the same physical phenomena result in fine detail on the plate that is
somewhat larger than the original.
After the exposure step but before development, the coating usually shows
a colour difference between exposed and unexposed areas.
The development of an offset printing plate normally consists of the
removal of the coating in the non-printing areas. These are the exposed
areas of a positive-acting plate and the unexposed areas of a negative-
acting plate.
The quality of the printing forme produced by the platemaking operation
depends in particular on the following process parameters:
- exposure step, especially vacuum conditions (including drawdown);
- chemical composition and temperature of the developer fluid;
- condition of rollers and brushes;
. . .
III
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- processing speed (development time);
- finisher condition.
colour contrast between printing and non-printing
After development, the
areas reater than before.
is usually mu
ch g
Following exposure and processing, baking or post-exposure treatment
may be used prior to correction and gumming. The developed offset
printing plate is thus transformed into a press-ready offset printing forme.
During correction, image elements are deleted (negative correction) or
added (positive correction). At the gumming stage, a thin coating of a
colloidal solution is applied to the image side of the plate in order to protect
the surface and to prevent toning during the printing operation. Baking or
post-exposure treatment is a heat treatment which increases the durability
of the coating with regard to chemical or mechanical wear.
Determination of the optimum exposure for positive-acting offset printing
plates: There are three importa .nt considerations.
The exposure should be strong enough for artefacts like film edges
a)
and dust particles to not n ormally show on the offset printing forme.
b) The exposure should not be so strong that the transfer of fine highlight
half-tone dots is impaired.
c) Since the exposure also determines tone value, which is very
important in process control, the exposure should be controlled such
that the tone value decrease from the half-tone film to the offset
printing forme is constant, irrespective of the offset printing plate type
and processing conditions.
For half-tone screens with screen frequencies of 70 cm-1 or less, it has
been found in practice that it is possible to meet considerations a) and b)
with a single exposure step. The exposure used is appreciably stronger
than one which would result in the best possible resolution (but where
artefacts would also be rendered). Consideration c) can be met in addition
by observing a suitably selected microline reading.
For periodic or non-periodic fine screens which contain image elements of
less than 25 urn size, conditions a) and b) cannot be met with a single
exposure step. Instead, a first exposure step is carried out that yields the
best possible resolution or slightly above. During the second exposure
step, the subject areas are protected by a so-called burnout mask; an
extended exposure removes the artefacts.
For positive-acting printing plates, it has been found that microline targets
can be used to define an exposure range which ensures a reproducible
tone value decrease from the half-tone film to the offset printing forme. In
this range, the tone value decrease from the half-tone film to the printing
forme is a linear function of the microline reading; the function depends on
the platemaking resolution. For a particular plate, under given exposure
and processing conditions, the graph of the positive microline reading over
the logarithm of the exposure thus characterizes the tone value
dependence on exposure. The slope of the graph is a measure of the rate
of tone value change with exposure change. Therefore, a steep slope
indicates less exposure latitude than a less steep one.
Mcroline targets useful for process control of platemaking contain a
number of subtargets with graduated linewidths ranging from a few
micrometres to several tens of micrometres. As distinct from the targets
iv
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used for testing the resolving power in photography, microline targets show
line-to-space ratios other than 1 :I. Usual ratios are 1:9, 35 and 1:4. Within
the usual exposure range the microline reading depends very slightly on
the line-to-space ratio. It is important to realize that the microline reading
depends on the density level between the microlines. It must not be
appreciably higher than elsewhere on the control strip film. As an
alternative to a division into subtargets of constant linewidth, a single target
with microlines of continuously variable linewidth may also be used. Since
there can be directional effects during the manufacture of printing plates as
well as during plate processing, it is a good plan to average over readings
taken at right angles or to use targets with circular microlines. It is
important to note that microline readings always refer to the width of the
microlines on the film, not to the (unknown) width on the printing forme.
Determination of exposure negative-acting offset printing plates: There are
three important considerations.
1) The exposure should be strong enough to achieve a sufficient and
reliable run length.
2) The exposure should not be so strong that there is excessive tone
value increase from the half-tone film to the printing forme or that
extreme shadow detail is lost.
3) The tone value increase from the half-tone film to the offset printing
forme should be at a specified level.
Since the first criterion is overriding, negative-acting offset printing plate
exposures usually follow the recommendation of the plate manufacturer,
which is expressed as the reading of a continuous-tone step wedge. Once
the optimum exposure has been established, microline targets may be
used to additionally monitor the consistency of subsequent exposures.
They should not be used as the primary exposure determinant.
Some users employ weaker exposures than the manufacturer
recommends, in an effort to obtain a desired tone reproduction in automatic
processing systems with plates that have long run-length coatings which
require more aggressive processing.
With some plates, the run length may be extended by post-exposure or
heat treatment. In these cases, the manufacturer may recommend a lower
range of allowable exposures and a post-exposure or baking treatment to
extend press life. This may result in a lower tone value increase than
otherwise.
Apart from its use for determining the exposure of negative-acting offset
printing plates, a continuous-tone step wedge may also be used to assess
the development process. A useful quantity in this respect is the
platemaking gradation; it characterizes the reaction of the plate coating to
the amount of radiation under a given processing condition. A change of
the platemaking gradation indicates that the processing conditions or the
coating have changed.
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SIST ISO 12218:2002
IS0 12218:1997(E)
INTERNATIONAL STANDARD @ IS0
Graphic technology - Process control - Offset platemaking
II Scope
This International Standard establishes unified terminology, test methods and requirements for the process control
of the preparation of the offset printing forme.
This International Standard
- applies to pre-sensitized metal plates;
- applies to contact exposures whether in a contact frame, step-and-repeat machine or other automated
processor;
- does not apply to optical projection or direct writing techniques, although the principles may be applied by
analogy;
- does not apply to non-periodic half-tone screens, although the tone value specifications may be applied by
analogy.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain
registers of currently valid International Standards.
Pan 2: Geometric conditions for transmission density.
IS0 5-2:1991, Photography - Density measurements -
IS0 5-3:1995, Photography - Density measurements - Part 3: Spectral conditions.
IS0 12647-l :1996, Graphic technology - Process control for the manufacture of half-tone colour separations,
- Part 1: Parameters and measurement methods.
proof and production prints
3 Definitions
For the purposes of this International Standard, the following definitions apply. They are given in alphabetical order.
NOTE - For quantities, the preferred unit is given together with the definition. By definition, the unit of so-called
dimensionless quantities is 1.
3.1 baking: Heat treatment to increase the durability of the coating with regard to chemical attack or mechanical
wear.
3.2 contact exposure step: Process step where an offset printing plate is exposed in intimate contact with a half-
tone film.
NOTE - Usually, a vacuum is used in order to achieve an intimate contact.
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3.3 continuous-tone step wedge: Linear array of continuous-tone control patches whose transmittance densities
are stepped in increments of 0,15 or 0,30.
control patch: Area produced for control or measurement purposes. [ISO 12647-l]
3.4
3.5 control strip: One-dimensional array of control patches. [ISO 12647-l]
3.6 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-I]
3.7 exposure: Product of the irradiance and duration of the photochemically active radiation during the exposure
process step. Unit: J/m*.
NOTE - Instead of the photochemically active (actinic) radiation, any measure for the total energy deposited may be used
once the correspondence has been ascertained.
3.8 exposure latitude: Maximum ratio of exposures that produce acceptable results. Unit: 1.
3.9 exposure step: Process step where an offset printing plate is exposed to photochemically active radiation.
3.10 film polarity: Positive if clear and solid areas on the half-tone film correspond to unprinted and solid areas
on the print, respectively. Negative if clear and solid areas on the half-tone film correspond to solid and unprinted
areas on the print, respectively.
NOTE - Adapted from IS0 12647-l.
3.11 fringe width (of an isolated opaque half-tone dot): 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: urn.
NOTE - Adapted from IS0 12647-1.
3.12 half-tone film: Film for use with a half-tone printing process showing image elements like dots or lines.
[ISO 12647-l]
3.13 microline reading: Under given conditions, the minimum width of positive or negative microlines on half-
tone film that transfer such that at least 50 % of the length of the microlines is clearly visible on the offset printing
forme. The positive microline reading refers to positive microlines (dark lines on light ground) and the negative
microline reading refers to negative microlines (clear lines in dark solid) as they appear on the offset printing forme.
Unit: urn.
3.14 microline target: Control patch compressing fine positive and negative lines or various but accurately
designated widths.
3.15 negative-acting (offset printing) plate: Offset printing plate for use with negative polarity film.
[ISO 12647-21
3.16 offset printing forme: Fo rme for planographic printing where the printi ng parts accept and the non-printing
parts do not accept pri nting ink.
NOTE - An offset plate is transformed into an printing forme means of the exposure, development and post-
bY
treatment process steps.
3.17 offset (printing) plate: Plane workpiece whose surface has been coated such that an offset printing forme
can be produced thereof. [ISO 12647-21
NOTE - In current usage also known as an offset printing forme.
3.18 out-of-contact phenomenon: Insufficient transfer of image elements from a half-tone film to an offset
printing plate as a result of inadequate contact between them.
NOTE - This may be caused by a mechanical object or an air bubble (due to insufficient vacuum drawdown).
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3.19 platemaking; preparation of the offset printing forme: Process steps by which an offset printing forme is
prepared from an offset printing plate.
3.20 platemaking gradation: Measure of the dependence of the coating thickness on the offset printing forme on
exposure. Unit: 1.
3.21 platemaking resolution: Minimum microline reading for both positive and negative microlines that is
achievable with a single exposure step. Unit: pm.
NOTES
I Not to be confused with the term “resolving power” in photography.
2 Sometimes referred to as the optimum resolution.
3.22 positive-acting (offset printing) plate: Offset printing plate for use with positive polarity film. [ISO 12647-21
3.23 pre-sensitized offset plate: Offset plate whose surface has been coated by the manufacturer with a
radiation-sensitive coating.
3.24 reflectance factor (I?): Ratio of the measured reflected flux from the specimen to the measured reflected
flux from a perfectly reflecting and perfectly diffusing material located in place of the specimen. Unit: 1. [ISO 5-41
3.25 reflectance factor densityl); reflection density*): The logarithm to the base 10 of the reciprocal of the
reflectance factor, R. Unit: 1.
3.26 screen ruling; screen frequency: Number of image elements, such as dots or lines, per unit of length in the
direction which produces the highest value. Unit: cm-l. [ISO 12647-l]
3.27 screen width: Reciprocal of screen ruling. Unit: cm. [ISO 12647-l]
3.28 step-and-repeat machine: Apparatus for making automated-contact exposure steps onto an offset plate.
3.29 tone value; dot area (on a print or a printing forme), 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:
/ l-1()-~-~)
I[ I
where
DO is the reflectance factor density of the unprinted print substrate, or the non-printing parts of the printing
forme;
D, is the reflectance factor density of the solid;
Dt is the reflectance factor density of the half-tone.
Unit: Percent.
NOTES
1 Adapted from IS0 12647-1.
2 Also known as apparent, equivalent or total dot area.
The synonym “dot area” may be applied only to half-tones produced by dot patterns.
3
1) International Lighting Vocabulary ([4] in annex F).
2) IS0 5-4.
3
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4 This definition may be used to provide an approximation of the tone value on certain printing formes.
5 In general it is assumed that the tone value (termed “ink value” in IS0 12640) of the data is reproduced identically on the
film produced by an image setter. Final films should reproduce those tone values.
3.30 tone value; dot area (on a half-tone film of positive polarity), A: Percentage calculated from the formula:
/ j-lo- ‘Ds-~)
I[ I
where
DO is the transmittance density of the clear half-tone film;
D, is the transmittance density of the solid;
Dt is the transmittance density of the half-tone.
Unit: Percent. [ISO 12647-I]
NOTES
1 Adapted from lS0 12647-l.
2 Also known as the film-printing dot area.
3.31 tone value; dot area (on a half-tone film of negative polarity), A: Percentage calculated from the formula:
A (%) = 100 - 100”
I
where
DO is the transmittance density of the clear half-tone film;
D, is the transmittance density of the solid;
Dt is the transmittance density of the half-tone.
Unit: Percent.
NOTES
1 Adapted from IS0 12647-1.
2 Also known as the film-printing dot area.
3.32 transmittance (optical) density ’); transmission density*): The logarithm to the base 10 of the reciprocal
of the transmittance factor, T. Unit: 1 l
3.33 transmittance factor (T): Ratio of the luminous flux transmitted through an aperture covered by a specimen
to the luminous flux th rough the aperture without the specimen in place. Unit: 1.
3.34 vacuum frame; ~ contact
frame; exposure frame: Manually operated vacuum apparatus where contact
exposure steps can be carried ou t.
1) International Lighting Vocabulary ([4] in annex F).
2) IS0 5-2.
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4 Requirements
4.1 Colour separation film quality
Unless otherwise specified, the core density shall be at least 2,5 above the transmittance density of the clear film
(film base plus fog). The transmittance density in the centre of a clear half-tone dot shall not be more than 0,l above
the corresponding value of a large clear area. The transmittance density of the clear film shall not be higher than
0,15. Measurements shall be made with a (UV) transmission densitometer whose spectral products conform to IS0
type 1 printing density as defined in IS0 5-3.
The fringe width shall be not greater than one-fortieth of the screen width; the half-tone dot shall not be split up into
distinct parts. The quality of the colour separation film shall be evaluated according to A.1 of annex A.
NOTES
1 The clear film density requirement is based on the understanding that
- the density range of the clear areas of all films that are to be exposed onto an offset printing plate, for consistent work,
should not exceed 0,lO;
- 0,05 represents the lowest commonly found value for IS0 type 1 printing density. In order to minimize the impact of the
use of half-tone films with clear film densities above this range, agreements between the supplier of colour separations
and the recipient are required. Contacting or duplicating can also be used to bring half-tone films with dissimilar clear film
densities into agreement.
2 As a practical guide, a core density of 2,5 above the clear film density will normally be achieved, if the density of large solid
areas is more than 3,5 above the clear film density.
If a user wishes to use a blue filter to transmittance density measurements, it is necessary to determine, for the particular
3
film type and processing conditions, the correlation between densities obtained with the blue filter and those obtained with an
IS0 type 1 printing density instrument.
4.2 Control patches
The platemaking process steps shall be monitored by exposing at least one control strip along with the work on the
offset printing plate. The control strip shall contain well-defined half-tone control patches with tone value
designations accurate to + 1 %. The shape of the half-tone dot structure shall be circular. The screen ruling shall be
constant, it shall be selected from the range 50 cm-1 to 70 cm-l. The core density shall be not less than 3,0 above
the transmittance density of the clear film (film base plus fog). The fringe width shall be not greater than 2 urn as
determined by the apparatus described in A.2 of annex A.
For positive-acting plates, a microline target is the primary tool for exposure control. It shall be exposed with the
work, a continuous-tone step wedge should be exposed at the same time. For negative-acting plates, a continuous-
tone step wedge is the primary tool for exposure control. It shall be exposed with the work; a microline target may
be exposed at the same time. The line-to-space ratio of the microline targets shall be selected from the range 1:9 to
1:2; it shall be kept constant throughout the target(s). The continuous-tone step wedge should possess a step
increment of 0,15 or smaller.
4.3 Selection of the exposure
4.3.1 Positive-acting plates
The platemaking process shall be controlled such that the tone values of the 40 % or 50 % control patches on the
ready-to-print but not yet inked offset printing forme are smaller than those of the corresponding control patches on
the control strip film by the amounts given in table 1. The tone values on the offset printing forme and the control
strip film shall be determined by the methods given in 5.1 and 5.2, respectively. Instead of the reference method
given in 5.1, a secondary method may be used if the results can be directly related to those of the reference
method.
NOTE - Secondary methods are described in annexes B and C.
5
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Table 1 - Tone value decrease from the half-tone film to the printing forme
Tone value decrease
Screen rulingl)
40 % control patch 50 % control patch
cm-1 % %
50 2,5 to 3,5 3,0 to 4,0
3,5 to 5,0
60 3,0 to 4,0
3,5 to 4,5 4,5 to 6,0
70
1 In this range the tone value change is proportional to screen ruling.
I 1
4.3.2 Negative-acting plates
No requirement is given. The exposure shall be selected according to the plate manufacturer ’s recommendation.
Compared to the manufacturer ’s recommendation for optimum exposure, the exposure actually used shall not be
weaker than one 0,15 density step, as shown by a continuous-step wedge. If a manufacturer recommends a range
of exposures for plates that can be treated after development by either baking or additional exposure, the actual
exposure shall not be weaker than the lower limit of the range specified.
NOTE - With these conditions, the tone value increase from the half-tone film to the offset printing forme is usually within the
ranges shown in table 2 for information only. Under the conditions mentioned in the last but one paragraph of the introduction,
the tone value increase may be slightly lower.
Table 2 - Tone value increase from the half-tone film to the printing forme
(for information only)
Tone value increase
Screen rulingl)
40 % control patch 50 % control patch
cm-1 % %
3,0 to 4,0
50 2,5 to 3,5
3,0 to 4,0 3,5 to 5,0
60
70 3,5 to 4,5 4,5 to 6,0
1) In this range the tone value change is proportional to screen ruling.
4.4 Reproduction limits
Isolated opaque half-tone dots of more than 25 urn in diameter on the control strip film shall transfer onto the offset
printing forme in a uniform and consistent manner. Likewise, negative half-tone dots (clear circular holes) with
diameters more than 25 urn on the control strip film shall transfer onto the offset printing forme in a uniform and
consistent manner.
5 Test methods
5.1 Tone value on an offset printing forme - Reference method
With the aid of a standard test object (with resolution in the micrometre range) calibrate for either area or diameter
determination:
- an electron probe microanalyser, or
- a scanning electron microscope, or
a microscope-videocamera image analyser combination.
-
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Determine the area taken up by a single circular half-tone dot on the offset plate. Calculate the tone value by
dividing the average area covered by a single dot by the square of the screen width. Measure and average over as
many half-tone dots as needed for a tone value accuracy of & 05 %.
5.2 Tone value of a half-tone control patch on a control strip
Using a transmission densitometer conforming to IS0 5-2, determine the transmittance densities of the film base
material, DO, the solid tone, D,, and a well-defined half-tone area, Dt. calculate the tone value from the pertinent
definition, that is 3.30 for positive polarity and 3.31 for negative polarity half-tone films.
In order to assure sufficient accuracy, the sampling aperture of the instrument should have a diameter not less than
15 times the screen width, it shall be not less than ten times the screen width. This requirement also applies, by
analogy, to the area of non-circular sampling apertures.
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Annex A
(normative)
Determination of quality parameters of half-tone dots on a half-tone film
A.1 This method shall be applied only to those half-tone films whose clear film IS0 Status T (blue) transmittance
density is 096 or less. Other films shall be tested by the method described in A.2 or any other method whose
equivalence to A.2 has been proven. Place a film control strip with a microline target, film emulsion o
...
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