ISO/TS 21139-22:2023

TECHNICAL ISO/TS
SPECIFICATION 21139-22
First edition
2023-05
Permanence and durability of
commercial prints —
Part 22:
Backlit display in indoor or shaded
outdoor conditions — Light stability
Permanence et durabilité des impressions commerciales —
Partie 22: Écran rétroéclairé en intérieur ou en extérieur ombragé —
Stabilité de la lumière
Reference number
© ISO 2023
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
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Measures of exposure severity . 2
3.2 Exposure conditions . 2
3.3 Abbreviations . 3
4 Use profile . 4
4.1 General . 4
4.2 Parameters of backlit display . 4
4.3 Frontside exposure and environmental conditions . 6
4.4 Equivalent test conditions . . 6
4.4.1 Severity weighted exposure condition . 6
4.4.2 Equivalent test duration . 8
4.5 Relevance of use . 9
5 Test methods .10
5.1 General . 10
5.2 Sample preparation . 10
5.2.1 Outline . 10
5.2.2 Test target. 10
5.2.3 Sample labelling and marking . 10
5.2.4 Storage conditions between printing and light exposure test . 11
5.2.5 Reference samples . 11
5.2.6 Backing of the specimens . 11
5.3 Light exposure . 11
5.3.1 Test method and filter condition . 11
5.3.2 Light intensity . 11
5.3.3 Temperature and humidity . 11
5.3.4 Duration of the light exposure .12
5.4 Exposure of the frontside of the print .12
5.4.1 Specimen mounting for frontside exposure .12
5.4.2 Duration of the frontside exposure .12
5.5 Exposure of the backside of the print .12
5.5.1 Specimen mounting for backside exposure .12
5.5.2 Duration of the backside exposure .12
6 Measurement .12
6.1 General .12
6.2 Measurement conditions .13
7 Data analysis .14
7.1 General . 14
7.2 Image quality parameter for data analysis . 14
7.3 Equivalent test conditions . 14
7.4 Estimation of time to reach certain change. 14
8 Test report .15
Annex A (normative) Relative severity of the relative spectral irradiance .17
Annex B (normative) Characterization of an example LED light box .21
Annex C (informative) sRGB test target .25
Annex D (informative) Overview of test conditions in backlit context .26
iii
Bibliography .27
iv
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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 ISO/TC 42, Photography.
A list of all parts in the ISO 21139 series can be found on the ISO website.
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.
v
Introduction
Backlit display of prints is a market segment in context of commerce (advertisement, brand shops) and
information (maps, directories). This use profile has specific spectral irradiance and environmental
conditions which are different from e.g. general indoor or in-window display (ISO/TS 21139-21).
Backlit display applies with prints on transparent or translucent foils and/or prints on a textile.
The document focusses on LED-based backlit units and on the other hand provides information on
fluorescent-based backlit units for reference. These backlit displays may be installed indoor or in
shaded outdoor conditions, for examples backlit display units in shelters and patios. Backlit displays
which are subject to solar radiative heating or precipitation, introducing extensive temperature cycling,
are excluded.
Prints on backlit display may fade or otherwise change in appearance due to various environmental
stresses, including light, heat, humidity, atmospheric pollutants, or biological attack, and the
combination of these factors. One of the most critical degradations is light fading caused by intense
irradiation from the backlit unit as well as illumination from the viewing environment, which may
represent various levels of intensity and degrees of spectral irradiance, depending on the installation
site in a building, near to a window or in a shaded outdoor condition. The factors determining the
exposure doses form either frontside or backside are introduced, and the severity of the actual spectral
irradiance is expressed as a ratio to the standardized exposure condition “general indoor” as defined
by ISO 18937-2.
The lighting design of the backlit display unit may cause inhomogeneity of the backside exposure of the
print, which may in turn introduce inhomogeneous patterns of colour fading or discoloration leading
to enhanced visibility of degradation (an example is illustrated in Annex B). The test method described
in this document does not include the assessment of the impact from inhomogeneity of the backside
exposure.
This document provides information about the test conditions for colour fading and discoloration
applicable for the different types of display materials, including transparent or translucent films, fabrics
as well as paper-based reflection prints. Furthermore, the document gives guidance for estimation of
an equivalent exposure dose for the intended time of display, acknowledging the limitations of such
generic extrapolations. The display use profile applies for digital and analogue prints.
This test method does not address the adverse effects of exposure to atmospheric pollutants, including
ozone, and is also limited to the evaluation of colour changes and therefore does not require specific
methods for the evaluation of physical properties, including changes of tensile strength, cockling etc. In
the case that backlit materials are constructed from laminates, the aforementioned factors are of less
importance.
The general concepts for the exposure characterization of prints on a backlit display provided in this
document may also be considered in museum context with details defined by ISO/TS 18950.
vi
TECHNICAL SPECIFICATION ISO/TS 21139-22:2023(E)
Permanence and durability of commercial prints —
Part 22:
Backlit display in indoor or shaded outdoor conditions —
Light stability
1 Scope
This document describes the test methods for light stability measurements of prints on transparent
or translucent foils, sheets and paper or printed on a textile, which are displayed on backlit units
installed in indoor or in shaded outdoor conditions, which are protected against direct precipitation
and radiative heating. Installations of backlit display units in outdoor areas without shading, which are
exposed to direct weathering and/or radiative heating, are excluded.
This document is applicable to the various product classes of “commercial prints” that are suitable for
backlit display. These commercial prints often contain combinations of text, pictorial images and/or
artwork.
This document provides guidelines for colour measurements, data analysis and also provides guidance
for translation of test results into suitable image permanence performance claims considering the
variability of backlit designs and environmental conditions.
This document is applicable to both analogue and digitally printed matter. Methods and principles
apply to both, colour, and monochrome prints.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 18937-1, Imaging materials — Methods for measuring indoor light stability of photographic prints —
Part 1: General guidance and requirements
ISO 18937-2, Imaging materials — Methods for measuring indoor light stability of photographic prints —
Part 2: Xenon‐arc lamp exposure
ISO/PAS 18940-1, Imaging materials — Image permanence specification of reflection photographic prints
for indoor applications — Part 1: Test methods
ISO/TS 21139-1, Permanence and durability of commercial prints — Part 1: Definition of use profiles and
guiding principles for specifications
ISO/TS 21139-21, Permanence and durability of commercial prints — Part 21: In‐window display — Light
and ozone stability
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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 https:// www .electropedia .org/
3.1 Measures of exposure severity
3.1.1
relative severity
ρ
RSI1/RSI2
ratio of density loss due to light fading for exposure under a given RSI1 in comparison to another given
RSI2 with both exposures at the same level of illuminance E []klx , based on the evaluation of average
v
light fading for a set of colorants used in digital prints
Note 1 to entry: For standardized RSI the relative degree of light fading obtained for the same exposure dose
expressed in klx·h has been expressed in relative units to each other based on experimental data and an action
spectrum model obtained for typical CMY colorants used in digital printing. The combined information of
exposure intensity [klx] and ρ is therefore equivalent to the description of a spectral exposure, see
RSI1/RSI2
Annex A.
Note 2 to entry: Standardized RSI include daylight filtered Xenon-arc (see ISO 18930), window-glass filtered
Xenon-arc for simulated in-window display or with additional UV blocking for general indoor display (see
ISO 18937-2), fluorescent light (see ISO 18909) and LED light (see ISO 18937-3).
EXAMPLE In this method, ρ denotes the relative severity of a RSI under question compared to the
RSI/GI
condition of “General Indoor” exposure.
3.1.2
severity weighted exposure

E
RSI1/RSI2
measure of the exposure intensity of a given RSI1, where the illuminance E []klx is weighted with the
v
duty cycle τ [%] in application and with the relative severity (3.1.1) in comparison to a reference RSI2
Note 1 to entry: In this test method, the RSI of the general indoor filtered Xenon-arc test method as defined in
ISO 18937-2 is used as the reference RSI, so RSI2 = GI. This spectral irradiance can be achieved using optical
filters such as L-37 (Hoya Co.) and SC-37 (Fujifilm Co.).
Note 2 to entry: The severity weighted exposures of the frontside and the backside of a print display on a backlit
unit are typically different because of different RSIs, duty cycles and/or intensity.
3.1.3
severity weighted exposure dose

H
RSI1/RSI2
measure of the severity weighted exposure that is accumulated during a nominal display duration time,
t
Note 1 to entry: In this test method, the RSI of the general indoor filtered Xenon-arc test method as defined in
ISO 18937-2 is used as the reference RSI, so RSI2 = GI (“General Indoor”).
Note 2 to entry: The severity weighted exposure dose accumulated on the frontside and the backside of a backlit
displayed print, respectively, are typically different and both contribute to colour fading.
3.2 Exposure conditions
3.2.1
UV cut-on [wavelength]
λ
0,05 %
wavelength at which the cumulative intensity of a RSI I(λ) has reached 0,05 % of its total integrated
intensity over the spectral range of 295 nm to 800 nm
»0,05 % 800 nm
Note 1 to entry: I» d»I/,»» d = 005 %
() ()
∫∫
295 nm 295 nm
3.2.2
shaded outdoor conditions
exposure to indirect terrestrial daylight in a shadow zone, that is characterized by the absence of
radiative heating of the prints on backlit display
Note 1 to entry: The UV cut-on (λ ) is in the range of 295 nm to 310 nm.
0,05 %
EXAMPLE Display in outside shelters and patios.
3.2.3
glass-filtered shaded outdoor display
exposure to shaded outdoor conditions (3.2.2) with optical filtering of the irradiance by the front screen
material of the backlit display unit
Note 1 to entry: Backlit display units in shaded outdoor conditions practically always require a front screen
in front of the print for reasons of electrical safety. Such a front screen is most often realized by safety glass
or a similar suitable material. The UV cut-on of PVB laminated safety glass varies between 300 nm to 400 nm
depending on its construction and its material formulation. For the purpose of this standard, 6 mm window glass
is defined as reference for the filter transmission, acknowledging that the UV transmission of different types of
front screens varies.
EXAMPLE Display in backlit units in shaded outdoor conditions with a safety glass front screen.
3.2.4
in-window display
exposure to indirect terrestrial daylight through standard architectural window glass (6 mm)
Note 1 to entry: The UV cut-on (λ ) is around 320 nm.
0,05 %
EXAMPLE Display in store windows or in other glass-enclosed architectural constructions (hallways,
lobbies, verandas), that face toward the outdoors.
3.2.5
general indoor display
exposure to indirect lighting, from due to filtering (through window glass) and shading is often the
principal illumination
Note 1 to entry: The UV cut-on (λ ) is around 350 nm.
0,05 %
EXAMPLE Display in store windows or in other glass-enclosed architectural constructions (hallways,
lobbies, verandas), that face toward the outdoors.
3.3 Abbreviations
CCT correlated colour temperature (IEV ref: 845-23-068)
CIE Commission internationale de l'éclairage (International Commission on Illumination)
[2]
ΔE colour difference defined in ISO/CIE 11664-4
ab
∆E average of the colour differences of the patches of the test target (vs. initial)
ab, ave
∆E maximum of the colour differences of the patches of the test target (vs. initial)
ab, max
[3]
∆E colour difference ΔE2000 as defined in ISO/CIE 11664-6
klx·h kilolux times hour
Mlx·h megalux times hour
RSI relative spectral irradiance in W/(m nm)
GI RSI defined by the test condition “General Indoor” – see ISO 18937-2
E klx
[] illuminance (visually weighted)
v
GI
 illuminance at the test condition of “General Indoor”
E [klx]
v
τ [%] duty cycle

severity weighted exposure dose at which a certain colour change ΔE is observed
H
ab
ΔEab
4 Use profile
4.1 General
This document describes a test method for prints on transparent or translucent foils and/or on textiles
that are displayed on backlit units indoors or in shaded outdoor conditions, where the primary stress
factors are exposure to light from both backside and frontside.
NOTE 1 Heat, humidity and atmospheric pollutants can also be stress factors, however this document focuses
on light stability. Heat can have effects on prints that are displayed for long time periods on backlit units with
elevated temperature, e.g. due to radiative heating by sunlight through window glass or due to dissipative
heating from electrical appliances in poor-ventilated constructions of the backlit unit itself.
The use profile of commercial prints is described in general in ISO/TS 21139-1. It specifically describes
test methods for backlit display indoor and in shaded outdoor conditions, defined as display use profiles
A3 and B1 b) of ISO/TS 21139-1:2019, Table 3, respectively.
NOTE 2 The overall appearance of the displayed prints can also be affected by factors given by the backlit unit
itself, including a non-homogenous distribution of the intensity and/or the correlated colour temperature (CCT)
of the backlit lighting and/or changes of any other element of the backlit unit, e.g. yellowing of the front screen.
4.2 Parameters of backlit display
A backlit display unit is designed to provide a backside illumination of the print, such that the
brightness of the displayed print is comparable to or larger than the light level of the surrounding
viewing environment. Furthermore, the CCT of the lamps in the backlit unit is often selected to match
the viewing environment, which is typically between 5 000 K to 6 500 K for naturally illuminated areas
and 3 000 K or 4 000 K for some indoor installations.
The spectral irradiance, intensity, and homogeneity of the backside exposure of the print depends on
the construction of the backlit unit. These parameters together with the duty cycle of the backside
illumination determine the severity of the exposure of the print from its backside. Table 1 provides an
overview of typical parameters associated with LED or fluorescent lamp illuminated light box designs.
The level of temperature increase of the print on backlit display is driven by the dissipative heating
from the backlit lighting system in operation and the degree of air ventilation of the light box in a
certain environment. The amount of temperature increase is larger in the case of poor air ventilation.
Factors that reduce air ventilation include an airtight design of the housing, its eventual installation
onto or especially into a wall, the use of a front screen and/or the display of a print on a foil (as opposed
to a fabric with an open mesh structure). For heat sensitive materials the temperature increase above
the surrounding temperature may have to be considered.
Table 1 — Parameters of backlit displays
a
Backlit displays Illumination type
Display parameters Bare-bulb Fluores- Glass-filtered Fluo-
LED
cent rescent
Relative spectral irradiance (RSI) see ISO 18937-3, phos-
phor-converted
See Annex A see ISO 18909
‘blue pumped’ LED
(5 000 K CCT)
Irradiance level E
v
7 to 10
at the backside of the print [klx]
a
Typical UV content no RSI below 400 nm, but
b
intense blue emission peak mercury lines at 313 nm and 365 nm
at 450 nm
a a a
relative severity ρ
0,73 0,74 0,64
RSI/GI
Temperature non-ventilated
increase [K] (e.g. front screen +7 +15 +15
over ambient and/or foil)
ventilated (e.g.
open front and
+5 +10 +10
mesh material /
fabric)
Duty cycle τ [%] Between “x %” (‘cyclic’) and 100 % (‘24/7’)
a
See Annex A.
b
The intensity of the UV lines at 313 nm and 365 nm, that are typically emitted from fluorescent lamps, depends on
several factors, including the amount of mercury used in a specific type of lamp and the level of UV attenuation from the
glass envelope of the lamp and the type and thickness of the phosphor layer. During the use time of the fluorescent lamps,
pinholes can be introduced in the phosphor layer, which can increase the intensity of the UV emission lines over time. On
the other hand, the UV lines will be largely attenuated when an UV absorbing (diffusor) screen is present in between the
fluorescent lamps and the print on display. The glass-filtered fluorescent condition is realized most often, whereas the
bare-bulb condition can be regarded as worst case.
For print materials with limited light stability a certain level of inhomogeneity of the backlit illumination
(see example in Annex B) may be sufficient to introduce visible patterns of discoloration. The level of
inhomogeneity of the light intensity, expressed as 2 × (I − I )/(I + I ), may typically range
max min max min
from 10 % to 50 % and stems from the light box design, including:
a) the position, geometry, and type of the lighting elements, such as e.g.
1) array of linear lamps, e.g. LED lines, or fluorescent tubes,
2) array of spot lamps, e.g. grid or matrix of individual LED spots, and
3) continuous area illumination, e.g. edge-lit backside diffusor screen;
b) the efficiency of the light distribution by the combination of all optical elements:
1) angular emission of the lighting elements, also considering lenses;
2) diffusor screens;
3) reflectivity of the inner walls.
Also, the reflectivity of the backside of the print itself, when mounted on the backlit unit, may contribute
to the overall system illumination homogeneity.
4.3 Frontside exposure and environmental conditions
The frontside of the print is exposed by the ambient illumination that is present at the installation site
of the backlit display unit. The corresponding environmental parameters may range between those
typical for general indoor display [A2 of ISO/TS 21139-1:2019, Table 3], for in-window display [A1 of
ISO/TS 21139-1:2019, Table 3] or for protected outdoor display [B1 b) ISO/TS 21139-1:2019, Table 3].
Users shall identify the most severe test condition anticipated for their display application and based
on that condition estimate a typical amount of total light exposure during the defined display period.
Guidelines are provided in ISO/TS 21139-1 and examples are given further below.
Table 2 — Characterization of standardized environmental display conditions
Environmental display pa- General indoor In-window Glass-filtered shaded
rameters display display outdoor display
Relative spectral irradiance See ISO 18937-2 See ISO 18937-2 See ISO 18937-2
a a
(RSI) (General indoor display) (In-window display) (In-window display) *
Depending on their UV filtering characteristics some screen materials be-
tween light source and print may reduce the UV fraction, to which the print is
UV filter function in the front
exposed from the frontside or backside, respectively. Also, the supporting sub-
screen of the backlit unit
strate (film) of the print may act as UV-filter for the corresponding direction of
exposure.
b
Informative: UV fraction 4 % 6 % 6 %
Informative: λ [nm] 370 to 375 340 to 345 340 to 345
50 % T
Informative: λ [nm] 350 nm 320 nm 320 nm
0,05 %
c c c c
Relative severity ρ
1,0 1,2 1,2
RSI/GI
12/24 (= 50 %) to Typically, Typically,
Duty cycle τ [%]
24/24 (= 100 %) 12/24 (= 50 %) 12/24 (= 50 %)
a
The test method ‘in-window display’ of ISO 18937-2 with continuous light exposure is equivalent to the light stability
test method stipulated in ISO/TS 21139-21.
b
UV fraction is indicated as ratio of cumulative radiant energy in the range of 300 nm to 400 nm versus the cumulative
radiant energy in the range of 300 nm to 800 nm (see ISO/TS 21139-1:2019, Annex D). For comparison: natural daylight has
~8 % UV fraction.
c
Reference values from Annex A.
4.4 Equivalent test conditions
In the practical application, any of the combinations of Table 1 for backside exposure and Table 2 for
frontside exposure could be observed. To reduce the variability of testing the concept of equivalent test
conditions is applied in this test method and the equivalent exposure dose is determined.
In this method the concept of “severity weighted exposure” is applied, which allows to characterize the
exposure intensity in terms of illuminance (lux), still considering the different UV content of a given
RSI. More background on this approach is given in Annex A.
4.4.1 Severity weighted exposure condition

In a first step, the user of this method needs to determine the “severity weighted exposure” E of
RSI/GI

frontside and backside exposure, respectively, as given in Formulae (1) and (2). E : represents a
RSI/GI
measure of light intensity E [klx], that is weighted with the duty cycle τ of the exposure and the
v
relative severity ρ of the RSI of either side of the print, respectively, i.e. ρ or ρ . The
RSI/GI frontG/ I back/GI
relative severity ρ provides a ratio of degradation due to photolytic action of exposure under a
RSI/GI
given RSI in comparison to that of the “general indoor” condition as defined in ISO 18937-2. Annex A
provides reference values for the relative severity of typical colorants based on a general action factor
model. As alternative, the relative severity can be evaluated based on the measurement of the spectral
[13]
action factor of a colorant set under investigation.

EE klxk =  lx ⋅τρ% ⋅ (1)
     
RSI at frontG/ IavRSI t ffrontG/ I

EE klxk =  lx ⋅τρ% ⋅ (2)
RSIv atback/GI       RSI atbaackG/ I
with

is the severity weighted exposure;
E
RSI/GI
E []klx
is the illuminance (visually weighted exposure);
v
τ [%] is the duty cycle;
is the relative severity of the RSI incident on the frontside or the backside of the print
ρ
RSI/GI
compared to general indoor (see Annex A).

Table 3 provides examples of severity weighted exposure values E in standardized exposure
RSI/GI
conditions.

Table 3 — Examples of the evaluation of severity weighted exposure values E
RSI/GI
backside exposure frontside exposure
(from the backlit unit) (from the environment)
#1 #2 #3 #4 #5
LED backlit glass-filtered general indoor glass-filtered
Exposure conditions
(5 000 K fluorescent indoor in-window shaded out-
CCT) backlit display door display
(CIE F6)
c
a a a a a
Relative severity ρ
0,72 0,58 0,87 0,87 0,87
RSI/GI
Example illuminance E
v
10 10 0,5 3 20
d
in use profile (klx)
Example duty cycle τ (MIN) 50 % 50 % 50 % 50 % 50 %
 a a a
E [klx]
3,6 2,9 0,22 1,3 8,7
RSI/GI
c
b b b b b
relative severity ρ 0,73 0,64 1,0 1,2 1,2
RSI/GI
Example illuminance E
v
10 10 0,5 3 20
d
in use profile (klx)
Example duty cycle τ (MAX) 100 % 100 % 100 % 50 % 50 %
 b b b b b
E [klx]
7,3 6,4 0,5 1,8 12
RSI/GI
a
Evaluated with a screen between light source and print, that acts as extended UV filter, e.g. λ %T= 400 nm, typical for
UV stabilized polycarbonate or PMMA.
b
Evaluated with RSI typical for use profile, see Table 2.
c
See Annex A.
d
Illustrative set-points estimated based on ISO/TS 21139-1:2019, Clause 4 and Table 3.
In Table 3, two main cases are identified:
— For backlit display in glass-filtered, shaded outdoor conditions, the severity weighted exposure of
the frontside from the environment is typically equal to or up to 4x larger compared to the severity
weighted exposure from the backlit unit.
MAX MIN
— For backlit display in indoor conditions, the severity weighted backside exposure from the backlit
unit is typically considerably larger (up to ~35 times) than the severity weighted frontside exposure
from the ambient environment. The relative severity of a backside illumination by LED and/or by
fluorescent lamps is rather comparable (see Annex A).
NOTE 1 Backlit display units for indoor display of soft signage are often designed without a front screen.
Annex D provides an overview of accelerated laboratory test conditions that would potentially
correspond to the exposure conditions in Table 3: each test condition is associated to a defined RSI with
a characteristic effective UV cut-on wavelength, for which its relative severity ρ has been
RSI/GI
determined (see Annex A). For reasons of practicality and comparability, the test method “general
indoor” (GI) display of ISO 18937-2 (see entry #3 in Table D.1), is chosen as the unique standard method
for the exposure of the frontside and the backside of the prints in this test method, respectively, see
GI

5.3.1. The test is conducted at a set-point of the illuminance E between (50 ± 2) klx to (80 ± 3) klx in
v
the specimen plane.
NOTE 2 The use of a lower level of the set-point value results in proportionally longer durations of the
accelerated test, see 4.4.2.
4.4.2 Equivalent test duration
To define the test duration T of the frontside and backside exposures, a nominal display duration time t
is defined. Criteria for the selection of such representative display durations include worst case
scenarios for the anticipated use profile or other criteria agreed upon between parties. From the

nominal display time, t, the equivalent test dose H is calculated for frontside and backside
RSI/GI
exposure separately as given in Formulae (3) and (4):

Ht klxh··  = h E klx (3)
     
RSIR at frontG//IaSI tfront GI

Ht klxh··  = hk E  llx (4)
RSIR at back//GI     SI at back GI  
with

is the severity weighted exposure dose for the RSI incident on the frontside or the back-
H
RSI/GI
side, respectively;
t[]h
is the exposure duration.
The corresponding durations T of the frontside and backside test exposures, respectively, are

determined as the exposure time needed to provide the equivalent dose H under given test
RSI/GI
GI

conditions E [klx] as givne in Formulae (5) and (6):
v
GI
 
T [h] = H klx · h / E [klx] (5)
front
 
RSI at front/GI v
GI
 
T [h] = H klx · h / E [klx] (6)
back
RSI at backG/ I   v
Table 4 — Examples of test durations T for typical backlit display use profiles
Example 1 Example 2 Example 3
Display configuration Backside Frontside Backside Frontside Backside Frontside
LED backlit general in- LED backlit indoor LED backlit Glass-filtered
(5 000 K door, with no (5 000 K in-window (5 000 K shaded out-
CCT) front screen CCT) display, with CCT) door display
a UV filtering
front screen
In the application
Relative severity
0,73 1 0,73 0,87 0,73 1,2
[1]
ρ
RSI/GI
Illuminance in
10 0,5 10 3 10 20
application E[klx]
v
duty cycle
50 % 100 % 100 % 50 % 50 % 50 %
[1]
τ

E [klx]
RSI/GI
3,65 0,5 7,3 1,305 3,65 12
see Formulae (1) and
(2)
Nominal display du- 4 320 4 320 2 160 2 160 8 640 8 640
ration t [h] (180 d) (180 d) (90 d) (90 d) (360 d) (360 d)

H [Mlx·h]
RSI/GI
15,8 2,2 15,8 2,8 31,5 103,7
‐ see Formulae (3)
and (4)
GI

with the test conditions: RSI = GI, duty cycle τ = 100 % and set point E = 80 klx)
v
Exposure side front Back front back front back
T [h]
– see Formulae (5) 197 27 197 35 394 1 296
and (6)
Table 4 provides illustrative examples of the calculation of the test durations based on Formulae (1) to
(6) for three display configurations, which are combinations of frontside, and backside exposure given
in Table 3.
4.5 Relevance of use
Based on the concept of an equivalent exposure dose, this test method defines the light fading test
procedure that is used to obtain a table or plot of colour changes versus cumulative exposure dose
under standardized equivalent test conditions.
Results obtained via this approach are subject to limitations as explained in ISO 18937-1, including
the assumption of reciprocity. This method applies the equivalent exposure dose for frontside and
backside on individual replicates of the prints under test. Without an actual correlation study, it cannot
be assumed that the colour changes observed separately for the frontside, and the backside exposure
would simply add up for simultaneous exposure typical for actual use. On the other hand, the larger of
the colour changes observed for frontside and backside exposure, respectively, can only be regarded
as the lower estimate of the combined, overall colour change. The main use of this method is the
benchmark of print materials and/or backlit display units under standardized test conditions. The
estimated display duration time until a certain colour change is observed in actual use may differ from
the standardized display duration obtained via this method.
To assess the relative severity of the exposure of a specific backlight display unit for a range of different
print materials, that are intended for display on this unit, the relative severity of the exposure of the
print as displayed on the backlit unit is determined according to Table 3.
When comparing results of light fading obtained on print material exposed according to this test
method versus display on an actual backlit unit, that is installed in ambient conditions representative
for intended use (“live-test”), the following shall be considered:
— The homogeneity of the exposure level on the display box is typically limited, i.e. the actual test
conditions depend on the position of the test material (target) on the display box – see example in
Annex B.
— In a “live test”, other stress factors including air pollution and humidity are typically not controlled
and the actual spectral conditions and duty cycles may differ from the standardized conditions.
— Provided additional spectro-radiometric characterization of the display and the ambient
illumination as well as control of the environmental factors and of the duty cycles of frontside and
backside exposure, a “live test” could be useful to set the generalizations inherent in this method
into perspective. However, this document does not provide the details for such a characterization
that would be required to qualify a “live-test” on a display box as an alternate test method for light
exposure.
5 Test methods
5.1 General
In this test method the exposure of the frontside and the backside of the print are applied separately
on individual replicates of the print materials under test, which means separate testing and individual
reporting of test results from frontside and backside exposure.
The overall durations of the frontside and backside exposure is determined based on the total
equivalent dose in intended use – see Formulae (3) and (4) - under consideration of the test conditions -
see Formulae (5) and (6) in 4.4.
In backlit units without a screen front (e.g. soft signage), atmospheric pollutants may be effective
depending on the display environment, where a high rate of exchange of non-filtered outdoor air can
be expected. For such applications, the test of resistance to fading under ozone exposure should be
considered in addition to this method, following the stipulations of ISO/TS 21139-21. However, many
prints for this application are laminated, which is very effective against pollutants. Or else they are
printed with UV curing technology
...