ISO 18941:2020

INTERNATIONAL ISO
STANDARD 18941
Third edition
2020-07
Imaging materials — Colour reflection
prints — Test method for ozone gas
fading stability
Matériaux pour l'image — Tirages par réflexion en couleurs —
Méthode d'essai de la stabilité de la décoloration à l'ozone
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 3
5 Sample preparation . 3
5.1 Target selection . 3
5.2 Use of replicates and reference samples. 3
6 Holding and measurement conditions . 3
7 Test methods — Gas fading (ozone) . 5
7.1 General . 5
7.2 Apparatus . 5
7.2.1 Ozone test device . 5
7.2.2 Source of ozonized air . 8
7.2.3 Means for adjusting, controlling and maintaining ozone concentration . 9
7.2.4 Means of determining the ozone concentration .10
7.2.5 Means of controlling gas flow .10
7.2.6 Test piece carrier .11
7.3 Test procedure .12
8 Test environment conditions .13
8.1 Humidity control calibration .13
8.2 Relative humidity .13
8.3 Temperature .13
8.4 Ozone concentration .13
9 Test report .14
9.1 General reporting requirements .14
9.2 Ozone test reporting .14
Annex A (normative) Reciprocity considerations .16
Bibliography .18
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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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).
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iso/ foreword .html.
This document was prepared by Technical Committee 42, Photography.
This third edition cancels and replaces the second edition (ISO 18941:2017), 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 18944:2018;
— the calculations and computations section has been removed as they are now contained in
ISO 18944:2018;
— 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
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

Introduction
In image permanence testing, there are four environmental variables known to affect the stability of a
[13][14][15][16][17][18][19][20][21]
photographic image: heat, light, moisture and air pollution, such as ozone
[22][23][24][25][26]
. Although natural ageing under “real-world” environmental levels of these variables is
considered the only certain test for image permanence, the high stability of most modern photographic
products makes testing under ambient conditions too lengthy a process to be of practical use. Thus, a
widely used alternative to natural ageing is accelerated ageing, whereby a sample specimen is exposed
to each environmental variable individually and at levels considerably greater than ambient, forcing
degradation of the image by that single factor in a far shorter length of time.
This document covers the equipment, methods and procedures for generating a known ozone exposure
and the subsequent measurement and quantification of the amount of change produced within a
photographic image due to that exposure. It is important to note that if predictions of absolute product
longevity are of concern to the experimenter, then further knowledge shall be gained regarding the
reciprocal behaviour of the test product under the experimental accelerated ozone conditions. See
Annex A for more information on reciprocity.
Additionally, there are other known variables in an ozone test setup that can affect the rate at which
an image will degrade in the presence of ozone. These include air flow over the sample, the nature of
the chemical reaction that is occurring, the relative quantities of the reactants (ozone and colorant
molecules) and the humidity content and the pH of the image recording layer. Each of these variables can
affect the reciprocal response and needs to be understood for a clear analysis of the accelerated data.
In some products, such as most dyes on swellable inkjet media and in silver halide products in
gelatine, the ozone reaction can be considered to be “diffusion-controlled,” whereby ozone first needs
to permeate a protective surrounding matrix before coming in contact with a colorant molecule and
reacting. Further, the reacted components then need to be desorbed and removed from the surface
before fresh, unreacted molecules can again diffuse, adsorb and react. In this type of process, a simple
increase in ozone concentration might or might not yield a proportional increase in reaction rate as
diffusion, adsorption and, in some cases, desorption may be the dominant factor controlling the rate of
reaction.
The relative quantities of the reactants (ozone and colorant) will also affect the rate of reaction and
reciprocal behaviour. Under the assumed ambient conditions, a photographic image would undoubtedly
contain a vast excess of colorant molecules relative to the local concentration of ozone molecules in the
air. Here, ozone would likely be the limiting factor controlling the rate of reaction and, in the absence
of other controlling factors, an increase in ozone concentration will produce a proportional increase
in the rate of reaction. At some precise ozone concentration, the quantity of reactants would be equal
and the reaction would proceed at a maximum rate. At this point, however, a further increase in ozone
concentration would not accelerate the reaction rate, causing a failure in the reciprocal relationship that
is required for converting accelerated data into predictions of ambient performance. For this reason, if
product longevity predictions are to be made, this ozone concentration needs to be determined and
never exceeded during testing.
This document has been primarily developed via testing with inkjet images on porous “instant-dry”
photographic media, which have been shown to be susceptible to fading by oxidative gases present in
[13][14][19][20][21]
polluted ambient air . While many chemical species may be present in polluted air, it has
been shown that most of the fade observed for current inkjet systems can be explained by oxidation
[21][27][28]
by ozone . Additionally, this method may reasonably be used for colour photographic images
made with other digital and traditional “continuous-tone” photographic materials such as chromogenic
[26]
silver halide, silver dye-bleach, dye transfer , dye-diffusion-transfer “instant” and other similar
systems. However, since these systems have, in general, been shown to be much less sensitive to
oxidative degradation by ozone, relatively small levels of image degradation with this accelerated test
method may not be realized within the typical duration of such a test for these imaging systems.
High levels of ozone, often found outside major metropolitan areas in summer months, together with
high levels of humidity, will greatly accelerate the fade. Since ozone is a highly reactive gas, storage of
photographs in any kind of gas-impermeable enclosure, such as framed behind gl
...