ISO 18936:2020

INTERNATIONAL ISO
STANDARD 18936
Second edition
2020-08
Imaging materials — Processed
colour photographs — Methods for
measuring thermal stability
Matériaux pour l'image — Photographies couleurs après traitement
— Méthodes de mesure de la stabilité thermique
Reference number
ISO 18936:2020(E)
©
ISO 2020

---------------------- Page: 1 ----------------------
ISO 18936:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 18936:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 2
5 Sample preparation . 2
5.1 Target selection . 2
5.2 Use of replicates and reference samples. 3
6 Holding and measurement conditions . 3
7 Test methods — Thermal stability. 4
7.1 General . 4
7.1.1 “Free hanging” versus “sealed bag” methods . 4
7.1.2 Humidity effects . 5
7.1.3 Testing of low glass transition temperature products . 5
7.1.4 Concerns around the effects of atmospheric pollutants . 5
7.2 Test methods and equipment . 6
7.2.1 Temperature . 6
7.2.2 Relative humidity . 6
7.2.3 Number of specimens . 7
7.2.4 Free hanging method at constant relative humidity . 7
7.2.5 Sealed bag method (constant moisture content). 8
7.3 Computation of dark stability . 8
8 Test report . 8
8.1 General reporting requirements . 8
8.2 Test reporting . 9
Annex A (informative) Illustration of Arrhenius calculation for dark stability .10
Bibliography .13
© ISO 2020 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 18936:2020(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee 42, Photography.
This second edition cancels and replaces the first edition (ISO 18936:2012), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the test methods for environmental stress factors have been changed to align with ISO 18944;
— the calculations and computations section has been removed as they are now contained in ISO 18944;
— Annex A has been removed as the method for interpolation is now contained in ISO 18944:2018,
Annex B;
— the usage and reporting requirements have been updated to ensure consistency within the
documents of the ISO 189## family.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 18936:2020(E)

Introduction
This document covers the methods and procedures for measuring the long-term, dark storage stability
(thermal stability) of colour photographs.
Today, the majority of photographs are made with colour dyes and pigments. The length of time that such
photographs are to be kept can vary from a few days to many hundreds of years, and the importance of
image stability can be correspondingly small or great. Often the ultimate use of a particular photograph
may not be known at the outset. Knowledge of the useful life of colour photographs is important to many
users, especially since stability requirements often vary depending upon the application. For museums,
archives and others responsible for the care of colour photographic materials, an understanding of the
behaviour of these materials under various storage and display conditions is essential if they are to be
preserved in good condition for long periods of time.
Any change in density, contrast or stain, whether due to colourant fading, changes in colourant
morphology or discolouration of residual substances will change the appearance of the photograph.
The most damaging change tends to be contrast balance distortions brought about by differential fading
of the three image colourants. These manifest themselves as shifts in colour balance from highlights to
shadows that are especially noticeable in a scale of neutrals, for example a shift from magenta to green
due to fading of the photograph’s magenta image colourant, or from yellow to blue or cyan to red due to
fading of the yellow or cyan colourant.
The second most consequential change is that caused by an increase in stain. The result may simply be
a discolouration of the D areas (unexposed processed media or unprinted substrate) or a change in
min
the D colour balance.
min
Cyan, magenta, yellow, and sometimes black, red, green and blue colourants that are dispersed in
transparent binder layers, or absorbed onto special receiver layers coated onto transparent or white
opaque supports, form the images of most modern colour photographs. Colour photographic images
typically fade during storage and display; they will usually also change in colour balance because the
image colourants seldom fade at the same rate. In addition, a yellowish (or occasionally another colour)
stain may form and physical degradation may occur, such as embrittlement and cracking of the support
and image layers. The rate of fading and staining is governed principally by the intrinsic stability of
the colour photographic material and by the conditions under which the photograph is stored and
displayed. The quality of chemical processing or post-processing is another important factor. Post-
processing treatments and, in the case of digitally generated photographs, post-production treatments,
such as application of lacquers, plastic laminates and retouching colours, also may affect the stability of
colour materials.
The three main factors that influence storage behaviour or dark stability are the temperature and
relative humidity (RH) of the air that has access to the photograph, as well as atmospheric pollutants
to which the photograph is exposed. High temperature, particularly in combination with high relative
humidity, will accelerate the chemical reactions that can lead to degradation of one or more of the
image colourants. Low-temperature, low-humidity storage, on the other hand, can greatly prolong the
life of photographic colour images for typical materials. Other potential causes of image degradation
are microorganisms and insects.
Most modern photographs degrade too slowly under normal room conditions to permit evaluation of
their dark storage stability within reasonable periods. However, it is possible to assess the probable, long-
term changes of some photographs under low and moderate storage conditions with accelerated, high-
temperature tests, because recognizable losses in image quality under high temperatures are apt to be
generated also under milder temperatures, if at a slower pace. The effects of relative humidity on thermal
degradation can also be evaluated with Arrhenius tests conducted at two or more humidity levels.
Long-term changes in image density, colour balance and stain level can be reasonably estimated only
when good correlation has been confirmed between accelerated tests and actual conditions of use.
© ISO 2020 – All rights reserved v

---------------------- Page: 5 ----------------------
ISO 18936:2020(E)

Density changes induced by the test conditions and measured during and after incubations include
those in the support and in the various auxiliary layers that may be in a particular product. With most
materials, however, the major changes occur in the image-bearing layer. An exception to this is found in
some inkjet papers where the inks are thermally stable and substrate yellowing is the failure mode (see
Reference [9]).
The tests for predicting the stability of colour photographic images in dark storage are based on an
adaptation of the Arrhenius method described by Bard et al. (see References [2] and [3]) and earlier
references by Arrhenius, Steiger and others (see References [4], [5] and [6]). Although this method
is derived from well understood and proven theoretical precepts of chemistry, the validity of its
application to predicting changes of photographic images rests on empirical confirmation. Although
many chromogenic-type colour products yield image fading and staining data in both accelerated and
non-accelerated dark ageing tests that are in agreement with the Arrhenius relationship, some other
types of products do not. For example, integral-type instant colour print materials often exhibit atypical
staining at elevated temperatures. The treatment of some chromogenic materials at temperatures
above 80 °C and 60 % RH may cause the loss of incorporated high-boiling solvents and abnormal
image degradation. The dyes of silver dye-bleach images deaggregate at combinations of very high
temperature and high relative humidity, causing abnormal changes in colour balance and saturation
(see Reference [7]). In general, photographic materials tend to undergo dramatic changes above 60 %
RH (especially at the high temperatures used in accelerated tests) owing to changes in the physical
properties of gelatine and other binder materials. Lower maximum relative humidity may need to
be tested for some of the more humidity-sensitive inkjet materials because of phase changes such as
melting point or glass transition temperature.
vi © ISO 2020 – All rights reserved

---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 18936:2020(E)
Imaging materials — Processed colour photographs —
Methods for measuring thermal stability
1 Scope
This document describes test methods for determining the long-term dark storage stability of colour
photographic images.
It is applicable to colour photographic images made with traditional photographic materials. These
images are generated with systems such as chromogenic, silver dye-bleach, dye transfer, dye-diffusion-
transfer “instant”, and similar systems. The test method specified in this document also covers the
dark-stability of digital colour images produced with dry- and liquid-toner electrophotography, thermal
dye transfer (sometimes called “dye sublimation”), and inkjet printing systems.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 5-3, Photography and graphic technology — Density measurements — Part 3: Spectral conditions
ISO 5-4, Photography and graphic technology — Density measurements — Part 4: Geometric conditions for
reflection density
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic
arts images
ISO 18913, Imaging materials — Permanence — Vocabulary
ISO 18924, Imaging materials — Test method for Arrhenius-type predictions
ISO 18941, Imaging materials — Colour reflection prints — Test method for ozone gas fading stability
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18913 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
operational fluctuations
positive and negative deviations from the setting of the sensor at the operational control set point
during equilibrium conditions in a laboratory-accelerated weathering device
Note 1 to entry: The operational fluctuations are the result of unavoidable machine variables and do not include
measurement uncertainty. The operational fluctuations apply only at the location of the control sensor and do
not imply uniformity of conditions throughout the test chamber.
[SOURCE: ASTM G 113]
© ISO 2020 – All rights reserved 1

---------------------- Page: 7 ----------------------
ISO 18936:2020(E)

3.2
operational uniformity
range around the operational control point for measured parameters within the intended exposure
area within the limits of intended operational range
[SOURCE: ASTM G 113]
3.3
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values
which could be reasonably attributed to the measurement
Note 1 to entry: The parameter may be, for example, a standard deviation (or a given multiple of it), or the half-
width of an interval having a stated confidence level.
Note 2 to entry: Uncertainty of measurement comprises, in general, many components. Some of these components
may be evaluated from statistical distribution of the results of series of measurements and can be characterized
by experimental standard deviations. The other components, which can also be characterized by standard
deviations, are evaluated from assumed probability distributions based on experience or other information.
Note 3 to entry: It is understood that the result of the measurement is the best estimate of the value of the
measurement, and that all components of uncertainty, including those arising from systematic effects, such as
components associated with corrections and reference standards, contribute to the dispersion.
[SOURCE: ASTM G 113]
4 Requirements
This document specifies a set of recommended test methods with associated requirements for
permitted reporting. Data from these tests shall not be used to make life expectancy claims, such
as time-based print lifetime claims, either comparative or absolute. The conversion of data obtained
from these methods for the purpose of making public statements regarding product life shall be in
accordance with the applicable document(s) for specification of print life.
The test methods in this document may be useful as stand-alone test methods for comparison of the
stability of image materials with respect to one specific failure mode. The data from the test methods
of this document may be used in stand-alone reporting of the absolute or comparative stability of image
materials with respect to the specific failure mode dealt with in this document, when reported in
accordance with the reporting requirements of this document. Caution shall be used when comparing
test results for different materials. Comparisons shall only be made when using equipment with
matching specifications, under matching test conditions.
Further clarification is required for this method because it makes use of the Arrhenius methodology
which may directly lead to a predicted product lifetime. Fundamentally, the Arrhenius method defines
a rate of change (see ISO 18924). This document shall use the ISO 18924 Arrhenius method only as a
predictor of rate of change for chemical reactions, not as a predictor of print longevity.
5 Sample preparation
5.1 Target selection
For general testing purposes, users of this document are free to choose whatever target, patches
and starting densities they feel are appropriate for their testing needs. An example of such a target
[26]
is included in ISO 18944 , along with requirements and recommendations for sample preparation.
Applicable documents(s) for specification of print life may require the use of specific targets. Other
[20]
recommendations for sample preparation are contained in ISO 18909 . Image prints may also be
used. When specific starting densities are desired or required, there may not be a step on a printed test
target that corresponds to the exact desired density. Interpolation between two neighbouring density
2 © ISO 2020 – All rights reserved

---------------------- Page: 8 ----------------------
ISO 18936:2020(E)

patches can be used to predict the values for the exact desired starting density. See ISO 18944:2018,
Annex B for details on interpolation between two neighbouring density patches.
5.2 Use of replicates and reference samples
At least two replicate prints are required. Replicates shall be located for testing in different regions of
the test chamber volume.
It is recommended that reference samples be included in every exposure test to track consistency of the
test procedures as well as unintended changes of test conditions (see Reference [21]).
6 Holding and measurement conditions
Measurements and sample holding for measurement and next test phase preparation shall be conducted
in a controlled environment with no time constraint, or in a less controlled environment with a time
constraint. The measurement environment and sample holding environment can influence measured
densities.
NOTE 1 "Sample holding environment" refers to the environment in which samples are held between test
phases, such as before and after measurement, while the samples are not in the active test environment.
The controlled sample holding environment with no time constraint shall meet the following set of
conditions: samples shall be kept in the dark at 23 °C ± 2 °C and at a relative humidity of (50 ± 10) %
while waiting for measurement and while holding between test stages.
The sample holding environment shall be ozone-free (<2 nl/l average ozone concentration over any
24 h period) for ozone-sensitive samples.
−9
NOTE 2 1 nl/l = 1 ppb (1 × 10 ). Although the notation "ppb" (parts per billion) is widely used in the
measurement and reporting of trace amounts of pollutants in the atmosphere, it is not used in this document
because it is language-dependent.
Ozone sensitivity shall be determined in accordance with ISO 18941; ISO 18944 may also be used.
A material that is not sensitive to ozone shall have demonstrated no measurable D (unexposed
min
processed media or unprinted substrate) or printed patch colour change at ambient ozone exposure
levels and measurement condition temperature and humidity, over time periods consistent with
measurement and test staging time periods.
The controlled measurement environment with no measurement process time constraint shall meet
the following set of conditions: ambient illuminance on the sample surface no greater than 200 lx,
temperature of 23 °C ± 2 °C and a relative humidity of (50 ± 10) %, and ozone-free <2 nl/l average ozone
concentration over any 24 h period) for ozone-sensitive samples.
When sample holding and/or measurement are conducted in a less controlled environment, samples
shall be held or measured in the less controlled environment for a maximum of 2 h for each test stage.
The less controlled environment may be unfiltered for ozone, and shall have a maximum relative
humidity of 75 % and maximum temperature of 30 °C, with ambient illuminance on the sample surface
less than or equal to 1 000 lx.
NOTE 3 Stray light decreases the accuracy of measurements taken in less controlled lighting environments.
Shielding the measurement instrument from direct lighting so that the actual measurement surface lighting is no
more than 200 lx can improve measurement accuracy and repeatability.
The temperature and humidity tolerances for the sample holding and measurement environments apply
specifically to the vicinities in which the samples are held and measured. Operational fluctuations,
operational uniformity and uncertainty of measurement shall be contained within the stated tolerances
in those vicinities.
© ISO 2020 – All rights reserved 3

---------------------- Page: 9 ----------------------
ISO 18936:2020(E)

The measurement environment and sample holding environment, with respect to temperature,
percentage relative humidity, ozone and light levels, and with respect to fluctuations and uniformity
shall be reported in the test report.
The CIE colour coordinates of the D patch (unprinted paper) shall be measured using ISO 13655
min
measurement conditions for the relative spectral power distribution of the flux incident on the
specimen surface. The conditions are chosen to be relevant to the characteristics being investigated.
White backing is recommended in accordance with ISO 13655. Report the backing used or report
the material opacity, according to ISO 2471, such that backing has no influence on the measurement.
Measurement conditions shall be consistent throughout the test process. Conforming to ISO 13655,
calculated tristimulus values and corresponding CIELAB values shall be computed using the illuminant
and standard observer conditions applicable to the material under test.
NOTE 4 With completely opaque materials such as the aluminium substrate used in outdoor testing, the
backing has no relevance.
Optical densities shall be measured according to ISO 5-3, with the relative spectral power distribution
of the flux incident on the specimen surface conforming to CIE illuminant A, ISO 13655 measurement
condition applicable to the characteristics being investigated, and spectral products conforming to
Status A or Status T density, as appropriate for the material under test.
White backing is recommended in accordance with ISO 5. The ISO 5 standard reflection density shall
be used as defined in ISO 5-4, allowing either annular influx mode or annular efflux mode. Either white
or black backing is allowed. Report the backing used. Measurement conditions shall be consistent
throughout the test process.
NOTE 5 When testing in accordance with an image life specification standard, then either standard status A
or status T density is selected according to that specification standard.
A single measurement instrument shall be used for all of the measurements taken pertaining to a
particular test. For example, initial patch values of a test target print and subsequent degraded patch
values of that particular test target print shall be measured using the same measurement instrument.
Replicate prints may be measured on separate measurement instruments as long as each is consistently
measured on the same instrument used for its initial readings. According to best practice, in the case
of equipment failure the test should be invalidated. A replacement instrument with a known offset,
determined for the test measurement conditions and materials such as those being measured, may be
used when the original instrument is not available. In this case, all measurements shall be corrected
with the known offset.
NOTE 6 It is useful to retain an identical set of print samples for comparison so that the instrument offsets can
be measured later if needed. These print samples are measured together with the test samples prior to ageing
and then stored in a sealed bag in a freezer while the test samples are aged. Offset measurements from materials
[25]
matched to those under test are preferred to measurements using BCRA tiles. See ISO 18920 for print storage
methods.
7 Test methods — Thermal stability
7.1 General
7.1.1 “Free hanging” versus “sealed bag” methods
Long-term dark stability is evaluated by a series of tests carried out at several elevated temperatures
at a particular relative humidity. Two test techniques, known as the “sealed bag” and the “free
hanging” methods, are available for accelerated dark stability testing. These test methods, which can
simulate two kinds of storage conditions, tend to give somewhat di
...