ISO 13655:2017

INTERNATIONAL ISO
STANDARD 13655
Third edition
2017-07
Graphic technology — Spectral
measurement and colorimetric
computation for graphic arts images
Technologie graphique — Mesurage spectral et calcul colorimétrique
relatifs aux images dans les arts graphiques
Reference number
ISO 13655:2017(E)
©
ISO 2017

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ISO 13655:2017(E)

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ISO 13655:2017(E)

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Spectral measurement requirements . 4
4.1 Instrument standardization and adjustment . 4
4.2 Reflectance factor measurement . 4
4.2.1 Wavelength range, wavelength interval and bandwidth . 4
4.2.2 Illumination requirements and measurement conditions . 5
4.2.3 Sample backing material . 7
4.2.4 Measurement geometry . 7
4.2.5 Data reporting . 7
4.3 Transmittance factor measurement . 8
4.3.1 Wavelength range, wavelength interval and bandwidth . 8
4.3.2 Measurement geometry . 8
4.3.3 Illumination requirements and measurement conditions . 8
4.3.4 Resolution and data reporting . 8
4.4 Self-luminous displays (spectral radiance) measurement . 9
4.4.1 Wavelength range, wavelength interval and bandwidth . 9
4.4.2 Measurement geometry . 9
4.4.3 Polarization . 9
4.4.4 Resolution and data reporting .10
5 Colorimetric computation requirements .10
5.1 Calculation of tristimulus values for reflecting and transmitting samples .10
5.1.1 General.10
5.1.2 Calculations with data having 5 nm interval and bandwidth .11
5.1.3 Calculations with data having 10 nm interval and bandwidth .11
5.1.4 Calculations with data having other intervals and bandwidth .11
5.2 Calculation of tristimulus values for self-luminous displays .11
5.3 CIE 1976 (L*a*b*) colour space; CIELAB colour space .13
5.3.1 General.13
5.3.2 CIELAB colour space formulae .13
5.3.3 CIE 1976 colour difference formulae .14
5.3.4 CIEDE2000 colour difference formulae .14
6 Measurement data reporting requirements .15
6.1 Required information .15
6.2 Recommended information .15
6.3 Electronic data reporting .15
Annex A (normative) Sample backing .16
Annex B (informative) Geometry .21
Annex C (informative) Improving inter-instrument agreement .24
Annex D (informative) Certified reference materials (CRMs) .26
Annex E (informative) Procedures for widening the bandwidth .28
Annex F (informative) Fluorescent specimens .30
Annex G (normative) Test method for UV-cut conformance .32
Annex H (informative) Special cases: Use of polarization .34
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ISO 13655:2017(E)

Annex I (informative) Example computations for converting spectral measurements to
tristimulus values .35
Annex J (normative) Computation of the CIELAB total colour difference (∆E*ab) .41
Annex K (normative) Computation of the CIEDE2000 total colour difference (ΔE ) .42
00
Annex L (informative) Impact of measurement band pass on spectral quantities .46
Bibliography .48
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ISO 13655:2017(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: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 130, Graphic technology, in collaboration
with Technical Committee ISO/TC 42, Photography.
This third edition cancels and replaces the second edition (ISO 13655:2009), which has been technically
revised to:
— clarify the requirements of measurement mode M1;
— restrict the use of unnecessarily wide bandpass and sampling intervals;
— provide more realistic specification for the optical properties of a white backing material;
— restrict the adjustment method of predicting the fluorescent reflectance factor to UV activated
substrates.
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ISO 13655:2017(E)

Introduction
There are many choices allowed when making spectral measurements and performing colorimetric
computations. The specific choices made can result in different numerical values for the same property
for the same sample. Thus, it might not be possible to make valid comparisons unless the data being
compared are all based on the same set of measurement and computational choices. The purpose of
this document is to specify a limited number of such choices for the measurement and computation
of the colorimetric characteristics of graphic arts images and specimens, such as test charts, to allow
valid and comparable data to be obtained. While this document references ISO 3664, the International
Standard established for viewing conditions in graphic arts and photography, it is not expected that
measured colorimetric data will provide an absolute correlation with visual colour appearance.
When the prior revision of this document was started, it was observed that almost all graphic arts
specimens exhibited fluorescence. In most cases, this was due to optical brightening agents (OBA)
contained in the paper substrates. In rare cases, the printing inks were fluorescent. According to the
recommendations of the 1996 version of this document, this would have meant that the source used for
the measurements (i.e. the spectral power distribution of the specimen illumination) was required to
closely match CIE illuminant D50. Yet when the 2009 revision was started, not a single colour-measuring
instrument sold for the graphic arts market provided an illumination system that closely matched CIE
illuminant D50. Instead, most instruments used incandescent lamps for light sources. The spectral
power distributions of such lamps have varying amounts of UV content. The variation in UV content
between instruments could easily amount to a colour difference of 5 Δb* when measuring substrates
with a high level of optical brightening agents. Consequently, the measurement results for unprinted
paper substrates and lighter colours differed appreciably between different instrument models. For a
thorough study of fluorescence effects, see CIE Publication 163.
It had also been observed that graphic arts viewing booths vary with respect to UV content, even those
that comply with the 1996 version of ISO 3664. The practical result was that specimens that have nearly
identical measured colorimetric properties, at times will not visually match when viewed in the viewing
booth, and vice versa. Only part of such discrepancies can be attributed to fluorescence. There can also
be metameric effects due to “non-standard” observers and to instrument wavelength errors, in addition
to deviations in the measurement source away from CIE D50. Despite these other potential influences,
it was deemed important to provide measurement solutions that would minimize the systematic errors
introduced by the interaction of paper fluorescence and variations in the spectral power distribution
of the sample illumination. Methods for the correction of instrument errors and procedures for reliable
visual evaluation of colour images are outside of the scope of this document.
In the 2009 revision, four measurement choices were defined for reflective measurements.
Measurement condition M0 requires the source illumination to closely match that of illuminant A; this
provides consistency with existing instrumentation and ISO 5-3. Measurement condition M1 requires
the colorimetry of the specimen illumination to closely match CIE illuminant D50. Measurement
condition M2 only requires that the spectral power distribution of the specimen illumination be
provided in the wavelength range from 400 nm to at least 700 nm and have no substantial radiation
power in the wavelength range below 400 nm (often referred to as “UVCut”). Measurement condition
M3 has the same sample illumination requirements as M2 and includes a linear polarizer in the influx
and efflux portions of the optical path with their principal axes of polarization in the orthogonal or
“crossed” orientation. For specimens in which the fluorescence is primarily that of a UV activated blue
emission, it is possible to use the method of a virtual fluorescent standard reported by Imura of Konica
[24][25]
Minolta to determine the total radiance factors for M0, M1 and M2 conditions. In this revision,
Annex A has been revised providing a slightly narrower and more realistic set of spectral tolerances on
the white backing materials. The properties of the white backing material are critical to reproducibility
of readings of packaging printing on clear or translucent films.
Finally, as the CIE has been recommending the use of 5 nm intervals for practical tristimulus
integration since the second revision of CIE Publication 15 and as graphic images can be composed
of colour stimulus functions with very narrow transitions from the low values to the high values, this
revision recommends that tristimulus values be based on spectral data collected with a 5 nm interval
and a 5 nm bandpass. Since many of the instruments now in use in the field are equipped with 10 nm
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ISO 13655:2017(E)

intervals and 10 nm bandpass spectrometers, such readings are allowed with the recommendation
that tristimulus calculations be preceded by applying bandpass correction to the spectral data as
specified in ASTM E2729. The use of instruments with wider sampling intervals and bandpass has
been deprecated with the exception of the use of such non-standard instruments to monitor the state of
previously characterized materials or objects.
The requirements of this document are focused on colorimetric measurement equipment intended for
use in the graphic arts environment. Helpful information on issues such as substrate backing materials,
reporting, standardization, standard and improved colour difference metrics, fluorescence and ways
to improve the inter-instrument agreement are included. These will be useful to technical advisors
of graphic arts associations, specialized graphic arts research institutes, and practitioners with an
interest in the basics of measurement and process control.
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INTERNATIONAL STANDARD ISO 13655:2017(E)
Graphic technology — Spectral measurement and
colorimetric computation for graphic arts images
1 Scope
This document specifies procedures for the measurements and colorimetric computations appropriate
to objects that reflect, transmit and emit light, such as flat-panel displays. It also specifies procedures
for computation of colorimetric parameters for graphic arts images. Graphic arts include, but are not
limited to, the preparation of material for, and volume production by, production printing processes
that include offset lithography, letterpress, flexography, gravure, screen and digital printing.
This document does not address spectral measurements appropriate to other specific application needs,
such as those used during the production of materials, for example, printing paper and proofing media.
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 5-2, Photography and graphic technology — Density measurements — Part 2: Geometric conditions for
transmittance density
ISO 5-4:2009, Photography and graphic technology — Density measurements — Part 4: Geometric
conditions for reflection density
ISO 3664, Graphic technology and photography — Viewing conditions
ISO 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 11664-3, Colorimetry — Part 3: CIE tristimulus values
ISO 11664-4, Colorimetry — Part 4: CIE 1976 L*a*b* Colour space
ISO 28178, Graphic technology — Exchange format for colour and process control data using XML or
ASCII text
CIE Publication 15:2004, Colorimetry, 3rd ed.
CIE Publication 167:2005, Recommended practice for tabulating spectral data for use in colour
computations
CIE Publication 176:2006, Geometric Tolerances for Colour Measurements
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:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
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ISO 13655:2017(E)

3.1
adopted white
spectral radiance distribution as seen by an image capture or measurement device and converted
to colour signals that are considered to be perfectly achromatic and to have an observer adaptive
luminance factor of unity, i.e. colour signals that are considered to correspond to a perfect white diffuser
[SOURCE: ISO 22028-1]
3.2
bandwidth
width of the spectral response function of the instrument, measured between the half-power points
often termed full width at half maximum (FWHM)
3.3
calibration
set of operations that establish, under specified conditions, the relationship between values of
quantities indicated by a measuring instrument or measuring system, or values represented by a
material measure or a reference material, and the corresponding values realized by standards
Note 1 to entry: Contrary to a common usage, calibration is not the process of adjusting a measurement system
such that it produces values that are believed to be correct. Calibration permits either the assignment of values of
measurands to the indications (creating a reference table) or the decision to reset or adjust the device. Following
the resetting or adjusting of the device, a calibration needs to be verified to ensure that the new device setting(s)
provide indications within the accepted values.
[SOURCE: ISO/IEC Guide 99 (VIM)]
3.4
CIE illuminant
illuminant (3.6) defined by the International Commission on Illumination (CIE) in terms of relative
spectral power distribution
[SOURCE: IEC 60050-845-03-12]
EXAMPLE CIE illuminants A, C, and various D illuminants.
3.5
CIELAB chromaticness difference
ΔC
h
difference between two colours of approximately the same lightness projected onto a constant lightness
plane in the CIELAB colour space
2 2
** **
Note 1 to entry: This is calculated as DCb=−CIECααIE +−CIECIEb
() ()
h 12 12
3.6
illuminant
numeric tabulation of the relative spectral distribution of the radiant (light) flux incident on the
specimen surface
Note 1 to entry: The CIE defines an illuminant as “radiation with a relative spectral power distribution defined
over the wavelength range that influences object colour perception”. In everyday English, the term is more widely
used to mean any kind of light falling on a body or scene. See IEC 60050-845 for further information.
[SOURCE: IEC 60050-845-03-10]
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ISO 13655:2017(E)

3.7
opacity
measure of the property that describes the ability of a specimen to hide a surface
behind and in contact with it
Note 1 to entry: The numerical value of opacity as used in this document is 100 times the ratio of the luminous
reflectance factor of the substrate over black backing (as defined in A.2) to the luminous reflectance factor
over white backing (as defined in A.3). This is different from the measurement of opacity used by the paper
manufacturing industry and defined in ISO 2471.
3.8
opaque substrate
substrate whose opacity (3.7), measured according to A.3, is 0,99 or greater
3.9
transparent substrate
clear material having minimal absorption or scattering of transmitted visible light
Note 1 to entry: Clear packaging film is an example of this type of material.
3.10
reflectance factor
ratio of the radiant or luminous flux reflected in the directions delimited by the given cone to that
reflected in the same directions by a perfect reflecting diffuser identically irradiated or illuminated
Note 1 to entry: The industry commonly uses the term reflectance rather than reflectance factor.
Note 2 to entry: It is important to specify the geometry that establishes the given conditions of measurement. See
Annex B.
[SOURCE: IEC 60050-845-04-64]
3.11
radiance factor
of irradiation> ratio of the radiance of the surface element in the given direction to that of the perfect
reflecting or transmitting diffuser identically irradiated and viewed
Note 1 to entry: For photoluminescent (fluorescent) media, the radiance factor contains 2 components, the
reflected radiance factor, β , and the luminescent radiance factor, β . The sum of the reflected and luminescent
R L
radiance factors is the total radiance factor, β : β = β + β . The subscript R is used here for the reflected
T T R L
radiance factor because it is more intuitive than the traditional S and avoids confusion with the use of S to denote
a state of polarization.
[SOURCE: IEC 60050-845-04-68]
3.12
specimen backing
material placed behind and in contact with the specimen during measurement
Note 1 to entry: For this document, this can be either white or black.
3.13
spectrophotometer
instrument for measuring the relative spectral reflectance factor (3.10) or transmittance factor (3.18) of
a material across the visible spectrum in order to derive colorimetric quantities
3.14
spectroradiometer
instrument for measuring radiometric quantities in narrow wavelength intervals over a given
spectral region
[SOURCE: IEC 60050-845-05-07]
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ISO 13655:2017(E)

3.15
standardization
process of forcing or adjusting a measurement system to produce readings that correspond to a
previously established calibration (3.3) using one or more homogeneous specimens or certified
reference materials
Note 1 to entry: As defined here, standardization is normally carried out by an instrument user.
3.16
telespectroradiometer
spectroradiometer (3.14) that uses an optical relay component to allow measurements to be made at a
distance from the specimen
3.17
transmittance (for incident radiation of a given spectral composition, polarization, and
geometrical distribution)
ratio of the transmitted radiant or luminous flux to the incident flux in the given conditions
Note 1 to entry: It is important to specify the geometry that establishes the given conditions of measurement,
for example, rectilinear geometry produces regular transmittance and integrating sphere produces diffuse
transmittance. But an opal glass diffuser does not produce the same readings as an integrating sphere geometry.
See Annex B.
[SOURCE: IEC 60050-845-04-59]
3.18
transmittance factor
ratio of flux transmitted by a specimen in a given optical system to the flux transmitted when the
specimen is removed from the sampling aperture
Note 1 to entry: For example, this is the case when radiation penetrating a slide situated in a projector and
reaching a screen is compared to the radiation when the slide is removed from a projector and only an empty
slide mount is in the projector.
4 Spectral measurement requirements
4.1 Instrument standardization and adjustment
The measurement device or system shall be verified (standardized and possibly adjusted) in accordance
with its manufacturer’s instructions. See also Annexes C and D.
NOTE 1 ISO 15790 defines the use of a certified reference material (CRM) to check calibration of a
measurement system. It also provides additional information relating to the use of CRMs, the determination of
combined standard uncertainty and data reporting.
NOTE 2 Where multiple instruments are used for measurement, there can be differences in the resulting data
due to the individual characteristics of the instruments and variations in measurement conditions. Annexes C
and D provide information on the improvement of inter-instrument agreement and the use of certified reference
materials.
4.2 Reflectance factor measurement
4.2.1 Wavelength range, wavelength interval and bandwidth
The data should be measured from 380 nm to 780 nm and shall be measured from 400 nm to 700 nm,
inclusive. Data should be measured at 5 nm intervals with a spectral response function that is
triangular with a 5 nm bandwidth at the half-power point, as specified in ISO 11664-3. Where unknown
data are measured at other intervals and bandwidths, the sampling interval and bandwidth shall not
exceed 10 nm. For applications where the spectral properties of the specimens are well established,
conformance to process requirements may be assessed using 20 nm sampling. Measurement data taken
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ISO 13655:2017(E)

at greater
...