ISO 18937:2020

INTERNATIONAL ISO
STANDARD 18937
Second edition
2020-01
Imaging materials — Photographic
reflection prints — Methods for
measuring indoor light stability
Matériaux pour l'image — Tirages photographiques par réflexion —
Méthodes de mesure de la stabilité de la lumière en intérieur
Reference number
ISO 18937:2020(E)
©
ISO 2020

---------------------- Page: 1 ----------------------
ISO 18937:2020(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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 18937:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements and limitations . 1
5 Test methods — General. 2
5.1 Safety cautions . 2
5.2 Light source and filters . 2
5.3 Humidity control . 2
5.4 Temperature control . 2
5.5 Air quality in the test environment . 2
5.6 Duration of exposures. 3
6 Light source conditions . 3
6.1 Light source measurements . 3
6.2 Light exposure equipment . 4
6.3 Specifications for optical filters . 4
6.3.1 General. 4
6.3.2 Filter specifications for simulating general indoor display conditions . 4
6.3.3 Filter specifications for simulating in-window display conditions . 4
6.3.4 Use of an IR-reducing filter . 4
6.3.5 Filter configuration . 4
6.4 Verification of chamber fade uniformity . . 4
7 Light source specifications . 5
7.1 Simulated general indoor display . 5
7.1.1 Application . 5
7.1.2 Filtered xenon-arc configuration to simulate general indoor display conditions . 5
7.1.3 Spectral irradiance . 5
7.1.4 Radiation intensity, temperature, and humidity . 6
7.2 Simulated in-window display . 6
7.2.1 Application . 6
7.2.2 Filtered xenon-arc configuration to simulate in-window display conditions . 7
7.2.3 Spectral irradiance . 7
7.2.4 Radiation intensity, temperature and humidity . . 7
8 Specimen preparation . 8
8.1 Specimens . 8
8.1.1 Use of replicates and reference specimens . 8
8.1.2 Setting of dummy specimens for the open space . 8
8.1.3 Test target design and format . 9
8.2 Conditioning the prints after printing . 9
9 Test report . 9
9.1 General reporting requirements . 9
Annex A (informative) Evaluation of light stability reciprocity behaviour .11
Annex B (informative) Relative spectral transmittance of filters .13
Annex C (informative) Example of chamber uniformity verification method .15
Annex D (informative) Sample temperature measurements based on different parameters.17
Bibliography .20
© ISO 2020 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 18937:2020(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 on 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 the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 18937:2014), which has been technically
revised. The main changes compared to the previous edition are as follows:
— Removal of non-xenon light sources;
— Removal of non-essential verbiage to improve method clarity;
— Inclusion of annex on actual sample temperature measurements during exposure;
— Inclusion of continuous phase test for in-window display conditions.
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 18937:2020(E)

Introduction
This document addresses the methods and procedures for measuring the indoor light stability of
[3]–[5][11]–[16][20]
photographic reflection prints .
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 lightfastness level
of colour photographs is important to manufacturers to improve print materials and to many users,
especially since stability requirements may vary depending upon the application.
The images of most modern analogue and digitally-printed colour photographs are made up of cyan,
magenta, yellow, red, green, blue, orange, black, grey, white or other colourants. Colour photographic
images typically fade during storage and display; they will usually also change in colour balance because
the various image colourants seldom fade at the same rate. In addition, a yellowish (or occasionally
other 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 can vary appreciably and is governed
principally by the intrinsic stability of the colour photographic material and by the conditions under
which the photograph is stored and displayed. For silver halide prints, the quality of any chemical
processing is another important factor. Post processing treatments and post-production treatments,
such as application of lacquers, plastic laminates, and retouching colours, also may affect the stability of
colour materials.
The light stability of colour photographs is influenced primarily by the intensity of the radiation/light
source, the duration of exposure to light, the relative spectral irradiance of the light source, and the
ambient temperature and humidity conditions. However, the normally slower dark fading and staining
reactions also proceed during display periods and will contribute to the total change in image quality.
Ultraviolet radiation is particularly harmful to some types of colour photographs and can cause rapid
fading as well as degradation of the underlying substrate. Information about the light stability of colour
photographs can be obtained from accelerated light stability tests. These require special test units
equipped with high-intensity light sources in which test strips can be exposed for days, weeks, months,
or even years, to produce the required amount of image fading (or staining). The temperature and
moisture content of the specimen prints should be directly or indirectly controlled throughout the test
period, and the types of light sources should be chosen to yield data that can be correlated satisfactorily
with those obtained under conditions of normal use.
Accelerated light stability tests for predicting the behaviour of photographic colour images under
normal display conditions may be complicated by “reciprocity failure." When applied to light-induced
fading and staining of colour images, reciprocity failure refers to the failure of a colourant to fade, or
to form stain, equally when irradiated with high-intensity versus low-intensity light, even though the
total light exposure (intensity × time) is kept constant through appropriate adjustments in exposure
duration. The extent of colourant fading and stain formation can be greater or smaller under accelerated
conditions, depending on the photochemical reactions involved in the colourant degradation, on
the kind of colourant dispersion, on the nature of the binder material, and on other variables. For
example, the supply of oxygen that can diffuse into a photograph’s image-containing layers from the
surrounding atmosphere may be restricted in an accelerated test (dry gelatine, for example, is an
excellent oxygen barrier). This may change the rate of colourant fading relative to the fading that would
occur under normal display conditions. The magnitude of reciprocity failure may also be influenced by
the temperature and moisture content of the test specimen prints. Furthermore, light fading may be
influenced by the pattern of irradiation — continuous versus intermittent — as well as by light/dark
cycling rates (see Annex A).
© ISO 2020 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 18937:2020(E)
Imaging materials — Photographic reflection prints —
Methods for measuring indoor light stability
1 Scope
This document describes test equipment and procedures for measuring the light stability of colour
photographic reflection prints designed for display, when subjected to a filtered xenon-arc light source
simulating daylight through windows at specified temperatures and relative humidity.
This document is applicable to photographic reflection prints, made with analogue and digital
print processes. The recommended test methods can be applied to both colour and black-and-white
photographic prints.
This test method assesses colour and density change.
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 2471, Paper and board — Determination of opacity (paper backing) — Diffuse reflectance method
ISO 4892-1, Plastics — Methods of exposure to laboratory light sources — Part 1: General guidance
ISO 18913, Imaging materials — Permanence — Vocabulary
ISO 18941, Imaging materials — Colour reflection prints — Test method for ozone gas fading stability
ISO 18944, Imaging materials — Reflection colour photographic prints — Test print construction and
measurement
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18913 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/
4 Requirements and limitations
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. Conversion of data obtained from these methods
for the purpose of making public statements regarding product life shall be in accordance with the
applicable documents for specification of print life.
The test methods in this document may be useful as stand-alone test methods for the absolute or
comparative stability of image materials with respect to colour fading or measurement of physical
properties. Caution shall be used when comparing test results for different materials. Comparisons shall
be limited to test cases using test apparatus with matching specifications and matching test conditions.
© ISO 2020 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 18937:2020(E)

No accelerated laboratory exposure test can be specified as a total simulation of actual use conditions.
Results obtained from these laboratory accelerated exposures can be considered as representative of
actual use exposures only when the correlation has been established for the specific materials being
tested and when the type of degradation is the same. The relative durability of materials in actual use
conditions can be very different in different locations because of differences in radiant energy (both in
spectral irradiance and intensity), relative humidity, temperature, pollutants such as ozone, and other
factors.
Print image stability results from this test method, especially in terms of the amount of acceleration
and/or correlation to end-use service life, that are determined for one printer model, software settings,
colourant, and media combination should not be applicable to another printer model, software settings,
colourant, and media combination.
5 Test methods — General
5.1 Safety cautions
In light stability tests, a high irradiance level is used, often with significant UV content. Special care
shall be taken to avoid eye injury or skin erythema. Precautions should be taken to ensure that the light
source cannot inadvertently be viewed without suitable eye and skin protection.
5.2 Light source and filters
This document references the use of a filtered xenon-arc light source (daylight behind window glass)
as described in ISO 4892-2 for accelerated tests with the intention of reproducing as closely as possible
[17][18]
different end-use lighting conditions . In addition, special filtering of the xenon-arc lamp is used
(which is not referenced in ISO 4892-2) to achieve the lighting conditions applicable to this method.
5.3 Humidity control
The relative humidity of the air circulating the test chamber shall be controlled. The location of sensors
used for measuring humidity shall be as specified in ISO 4892-1.
5.4 Temperature control
Uninsulated black metal panels are used to indirectly control specimen temperature. The black panel
shall be constructed in accordance with ISO 4892-1 and mounted at the specimen exposure plane. The
uninsulated black panel shall be controlled.
Chamber (ambient) temperature shall be controlled. The sensor shall be shielded from light and
mounted near the exposure zone.
Most lightfastness apparatus use ambient laboratory air to control chamber air temperature. Therefore,
laboratory conditions shall be maintained such that the apparatus can control temperature effectively.
Air refrigeration units may be required to maintain the chamber air temperature depending on the
laboratory air temperature.
NOTE The ± tolerances given for testing set points are the allowable fluctuations of the parameter about the
given value under equilibrium conditions. This does not mean that the apparatus set point value can be varied by
± the amount indicated from the given value, just that the measured value may. These ± tolerances are also not
intended as requirements for chamber uniformity.
5.5 Air quality in the test environment
Some types of print materials can be highly sensitive to degradation caused by ozone or other airborne
pollutants. Therefore, the test facility where print specimens are made, dried, exposed, and measured,
shall be ozone free (<2 nl/l average over any 24-h period) for ozone sensitive samples, as determined
in accordance with ISO 18941. A material that is not sensitive to ozone shall have demonstrated no
2 © ISO 2020 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 18937:2020(E)

measurable D or printed patch colour change at ambient ozone exposure levels and measurement
min
condition temperature and humidity.
−9
NOTE 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 International
Standards because it is language-dependent.
Either active or passive ozone monitoring can be used. Active monitoring includes real-time measuring
and logging of ozone levels in the test facility. Passive monitoring measures long-term cumulative
ozone levels yielding a final verification that pollutant levels were at or below acceptable levels during
the test. Active monitoring is preferred as passive monitoring cannot indicate whether test conditions
were valid until the test is completed.
If necessary, the apparatus can be fitted with an appropriate filter in the incoming chamber air stream
to reduce the ozone concentration levels, or ozone can be scrubbed in the laboratory air by appropriate
filters.
NOTE 2 The susceptibility of specimens to a given level of airborne pollutants in the air of the test environment
(laboratory) can be qualitatively assessed by exposure of replicate specimens to the condition of high air flow in
a darkened section of the test environment (with the same air quality), running parallel to the intended test
described in this document. If "no measurable change" is obtained as a result of this additional exposure test, the
material is regarded as "not susceptible to airborne pollutants" for the duration of the test and for the given test
environment. This approach represents a fail-safe test for each imaging material of interest that integrates the
effects of ozone and all other potentially harmful pollutants that could be present in the laboratory atmosphere.
5.6 Duration of exposures
The duration of exposures shall be determined with the following considerations:
a) Total exposure required, for example:
1) Total exposure expected in the usage;
2) Total exposure required for the warranty;
3) Total exposure stipulated as endpoint criteria in the applicable International Standards for
specification of print life, when such a specification document is available.
b) Total exposure that will cause an aim change, for example:
1) Total exposure that will cause expected change;
2) Total exposure that will reach endpoint criteria specified in the applicable International
Standards for specification of print life.
c) Total exposure required to cause a change to be reliably detected beyond the noise of the system,
particularly for highly stable systems. A reliable change is considered detected when the test result
has progressed beyond the noise of the test system.
This test method does not include test endpoints to establish test duration.
6 Light source conditions
6.1 Light source measurements
The filtered xenon-arc light source shall be measured at the specimen plane in terms of radiation source
intensity as described in ISO 4892-1.
NOTE Measurement of illuminance or irradiance is used as an integral part of the control system in light test
equipment. The control system can then compensate for reduced UV transmission due to solarisation of the lamp
and filters.
© ISO 2020 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 18937:2020(E)

6.2 Light exposure equipment
Xenon-arc radiant exposure apparatus that can achieve the test conditions and the tolerances
stipulated in Clause 5 shall be used. Any configuration of xenon-arc lamp exposure apparatus can be
used, provided it can achieve the required test conditions and the tolerances.
6.3 Specifications for optical filters
6.3.1 General
The spectral irradiance of the test light source can be modified by the use of optical filters in order to
simulate a particular usage condition. To control the specimen surface temperature at the desired aim
value, IR filters may be employed to reduce infrared energy above 800 nm.
NOTE Ultraviolet radiation is considerably more harmful to some types of printed matter than it is to others
and, therefore, variations in the level (and spectral irradiance) of the ultraviolet radiation in the light source will
affect some materials more than others.
6.3.2 Filter specifications for simulating general indoor display conditions
This filter system shall consist of window-glass filters with spectral irradiance in accordance with
Table 3 and an additional UV cut-off filter with a half-cut wavelength (λ at T = 50 %) of 370 nm to 375 nm.
The resulting spectral irradiance shall be in accordance with Table 1. The intent of this filter is to
simulate an indoor exposure further away from a window, when a large part of the window-glass
filtered daylight has undergone reflection off the interior of the room before hitting the displayed print.
Examples of the standard UV cut-off filter and their corresponding spectral transmission characteristics
are shown in Table B.1 and Table B.2.
NOTE Examples of an acceptable UV cut-off filter are L-37 (Hoya Co.) and SC-37 (Fujifilm Co.).
In order to maintain conformance, the filter shall be cleaned or replaced per OEM instructions.
6.3.3 Filter specifications for simulating in-window display conditions
This filter system shall consist of window glass filters with spectral irradiance in accordance with
Table 3. The intent of this filter is to simulate terrestrial daylight transmitted through standard
architectural window glass.
In order to maintain conformity, the filter shall be cleaned or replaced per OEM instructions.
6.3.4 Use of an IR-reducing filter
An IR-reducing filter can be used with the filter systems specified in 6.3.2 or 6.3.3 as needed to meet
the black panel temperature and chamber air temperature requirements.
6.3.5 Filter configuration
The optical filters shall be placed at any position between the light source and the specimens to achieve
the required spectral irradiance conditions. The filters can be placed near the light source or near
the specimens, but the air gap between the specimens and the filter shall be at least 2 mm with an
unobstructed airflow between the filter and the specimens.
6.4 Verification of chamber fade uniformity
Chamber fade uniformity assessment is required to qualify the initial illuminance or irradiance,
specimen mounting, air flow configuration, and filter placement configuration with the light stability
test conditions (including lamp and filter spectral irradiance, light intensity, air quality, tem
...