SIST-TS ISO/TS 23031:2021

TECHNICAL ISO/TS
SPECIFICATION 23031
First edition
2020-08
Graphic technology — Assessment
and validation of the performance
of spectrocolorimeters and
spectrodensitometers
Reference number
ISO/TS 23031:2020(E)
©
ISO 2020

---------------------- Page: 1 ----------------------
ISO/TS 23031:2020(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/TS 23031:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Known practices for instrument characterization. 6
4.1 Repeatability . 6
4.1.1 General. 6
4.1.2 Procedures . 6
4.2 Reproducibility . 7
4.2.1 General. 7
4.2.2 Determination of temporal reproducibility . 8
4.2.3 Determination of instrument reproducibility .10
4.2.4 Data collection and analysis .11
4.3 Accuracy .12
4.4 Quality of the influx spectrum .12
5 Reference materials for assessment of performance .13
5.1 Reference materials for comparison to the manufacturer’s specifications .13
5.2 Reference materials for comparison between identical models .15
5.2.1 General.15
5.3 Reference materials for comparison between different models .15
5.3.1 General.15
5.3.2 Measurements .16
5.3.3 Determination of instrument differences .16
6 Reported performance results.16
6.1 Conformance to factory specifications .16
6.2 Inter-instrument agreement .17
6.3 Inter-model agreement .17
6.4 Repeatability .17
6.5 Reproducibility .17
6.6 Assessment of accuracy .17
Bibliography .18
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ISO/TS 23031:2020(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 ISO/TC 130, Graphic technology.
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

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ISO/TS 23031:2020(E)

Introduction
Instruments for the measurement of colour and colour difference have been in use since the middle of the
20th century. In the days before digital computers, converting spectral data into CIE tristimulus values
was a difficult, manual operation. Additionally, the optics and electronic components were large and
difficult to maintain. As a result, every instrument was supplied with a number of reference materials
that could be used to assess the performance of the instrument or to adjust the operating parameters.
These reference materials included coloured glass filters, rare earth glass filters, neutral density filters
and porcelain on steel plaques. Concepts such as accuracy, precision, bias and reproducibility had
special and unique applications to these instruments and reference materials.
As the optical and electronic technologies improved, the instruments became smaller, more precise
and more affordable. At the same time, the science of metrology matured to the point that the colour-
measuring instrument’s performance out-paced the ability of the national testing laboratories to
produce and certify standard materials suitable for testing. Modern optoelectronic instruments are
more precise and more stable than the materials used to assess their performance. Thus, it has become
problematic to determine if an instrument is within its factory specification or if two instruments
produce results that are in agreement with each other.
Several industries that produce coloured products have chosen to address this situation by adopting
and specifying a single brand and design of instrument. The paper and pulp industry have gone so far as
to capture one particular design from the 1960s and enshrine it in an International Standard. ISO 2469
describes the optics, the geometry and the operation of an instrument that is ideally suited and specially
designed for the measurement of the reflectance and colour of paper and pulp. Additionally, ISO/TC 6,
has established a series of authorized laboratories which issue certified reference materials (CRM) for
testing and calibrating the performance of an ISO 2469 compliant instrument. This was possible, in part,
as the instrument captured in ISO 2469 was widely available on the market and it had no competitive
designs and the authorized laboratories market sets of standards which are produced using materials
with similar physical and optical properties as production papers or pulps. The authorized laboratories
maintain a very close relationship to a single national standards laboratory and to each other. WG3
periodically audits these laboratories to verify that they have calibrated their instruments properly
against the scale of radiance factor developed by the national standards laboratory.
In contrast, modern graphic reproduction has moved from the era of artistic interpretation into a time
in which the image reproduction is driven by objective numerical assessments. With the availability
of modern electro-optics, the number of companies providing instruments and the range of models of
different size and capabilities has increased dramatically. Geometries utilized are nominally 45°:0°
but may be uniplanar, biplanar, circumferential or annular. While referred to as bidirectional, they are
always biconical and the sizes of the influx and efflux cones vary as much as the directionality.
Unfortunately, the national metrology laboratories have not been successful in defining a universally
accepted scale of diffuse reflectance factor or diffuse radiance factor for these biconical instruments,
especially when the sampling aperture is small. Without artefact standards that closely align with
the properties to be measured in the printing industry, the result can easily be a match between two
instruments on the reference material that does not correlate to a match on real world materials.
As a result, colour-measuring instruments from different manufacturers or with different design
intents do not provide adequate agreement on the determination of the colour values or methods for
the assessment of the performance of an instrument system relative to its manufacturer declared
performance specifications. Further, to make the instruments as simple as possible to operate, the end-
user is given little to no access the underlying operation of the instrument. The operator can select an
influx spectral quality (M0, M1, M2, M3) but has no way to determine or adjust the spectral quality of the
influx. The realization of the scale of 45°:0° reflectance factor or radiance factor is different than that
of hemispherical diffuse reflectance factor, even for nearly ideal materials. The operator only has the
ability to request that instrument adjust the scale of the instrument using a single reference standard
supplied with the instrument. The instrument scale is thus traceable only at the one point. Most do
not even offer the ability to set or verify the mid-scale value or the optical null value. Today, optical
metrologists refer to this process as standardization, since the instrument is forced to reproduce the
values of the one standard.
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ISO/TS 23031:2020(E)

This document has been prepared to provide the users of portable spectrocolorimeters and
spectrodensitometers with guidance on the methods for validation of the performance of those
instruments. Since calibration is not possible, the use of a series of certified reference materials (CRM)
or a series of stable, idealized reference materials is required. ISO 15790 provides guidance on the
development of CRM standards for the scale of optical density. But optical density is a more forgiving
measurement than tristimulus colorimetry. Measurement of colour is inherently more complicated than
the measurement of optical density, since the logarithmic function compresses the measurement scale
and the associated errors. Computing colorimetric tristimulus values from spectral data requires the
use of the entire range of reflectance factor values while ISO status density is based on the response of
the spectral product. Bright colours, useful for producing a large gamut of colour in image reproduction,
possess large differences between the spectral regions of absorption and non-absorption of light but
density is only assessing the spectral regions of maximum absorbance. While the human visual system
has broad spectral responses, in terms of the cone fundamentals, the post receptor processing allows
an observer to perceive hue differences as small as 1 nm. So, the instrumentation for colour assessment
needs to have an accuracy several times small than the human visual system.
There is a need to use a set of 10 to 20 physical standards to sample the visible spectrum with materials
possessing both high and low reflectance levels and that transition between the two extremes over a
very small range of wavelengths. Those materials are stable and nearly opaque to avoid the problems of
lateral diffusion observed when the sampling aperture are small. The procedures described here have
been shown to provide end-users with methods to quantify the performance of spectrocolorimeters
on the day it arrives from the manufacturer or distributor until the day it is retired from service. The
methods may also be used to validate the instrument system against manufacturer’s specifications and
against the requirements for product quality.
National measurement laboratories (NML) continue to develop new scales and new methods of assessing
artefacts with the goal of providing certified standard materials for establishing the level of traceability
and reproducibility of commercial instruments. Unfortunately, these standards have historically been
too expensive for routine use. Only recently have the NMLs began developing automated methods for
characterizing reference colours or even user supplied materials. Currently, only large corporations or
instrument makers can afford to own such materials. Practical users rely on secondary laboratories
and reference standards designed specifically for the end use case. In the graphic arts, that should be
some form of printed material with a relatively short duty lifetime.
Finally, even after the CRM has been obtained, the methods for assessing the measurement data are not
well described. A spectral reflectance factor curve should include 31, 36, 40 or more measurements.
Trying to assign values, tolerances and uncertainties to the individual wavelengths is a challenge.
For example, it is possible that measurements of an artefact are consistent for 28 wavelengths and
inconsistent at 3 others. Should these instruments be considered as acceptable or failures? Converting
the measured data to colorimetric values (XYZ or L*a*b*) improves the situation slightly, but the
dilemma of comparing 3 individual readings from one lab or instrument to 3 individual values from
another lab, remains a problem not conveniently described in the standards literature. It is the intent of
this document to document and describe objective ways of assessing and comparing the performance of
a colour-measuring instrument with the ultimate goal of identifying an optimum method for application
in the graphic reproduction workflow.
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TECHNICAL SPECIFICATION ISO/TS 23031:2020(E)
Graphic technology — Assessment and validation
of the performance of spectrocolorimeters and
spectrodensitometers
1 Scope
This document describes procedures for the assessment and validation of the performance of an optical
spectrometer intended for use in capturing the spectral reflectance factor or the spectral radiance
factor of printed areas comprised of non-fluorescent or fluorescent materials, respectively. While it
does not describe the application to transmitting materials directly, many of the procedures can be
applied to transmitting systems by backing them with a reflective white backing material.
This document does not address spectral measurements appropriate to other specific application
needs, such as those used during the production of materials (e.g. printing paper and proofing media),
which are well described by ISO standards under the jurisdiction of ISO/TC 6. It does not describe the
special requirements for testing instruments that make in-process or online colour measurements.
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 13655:2017, Graphic technology — Spectral measurement and colorimetric computation for graphic
arts images
ISO 15790:2004, Graphic technology and photography — Certified reference materials for reflection and
transmission metrology — Documentation and procedures for use, including determination of combined
standard uncertainty
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 http:// www .electropedia .org/
3.1
accuracy
closeness of agreement between a test result and an accepted reference value
Note 1 to entry: The qualitative term accuracy, when applied to a set of observed values, is a combination of a
random precision component and a systematic error or bias component. Since, in routine use, random components
and bias components cannot be completely separated, the reported “accuracy” is interpreted as a combination of
these two elements.
[SOURCE: ASTM E 284]
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ISO/TS 23031:2020(E)

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.
Note 2 to entry: 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. Verification of a measuring device
requires determination of the uncertainty of the calibration.
[SOURCE: ISO/IEC Guide 99:2007:2.39, modified — The definition has been editorially revised and the
original Notes to entry have been replaced.]
3.4
certified reference material
CRM
reference material, accompanied by a certificate, one or more of whose property values are certified by
a procedure which establishes traceability to an accurate realization of the unit in which the property
values are expressed, and for which each certified value is accompanied by an uncertainty at a stated
level of confidence
[SOURCE: ISO 15790:2004, 3.1.2]
3.5
combined standard uncertainty
u
c
standard uncertainty of the result of a measurement when that result is obtained from the values of
a number of other quantities, equal to the positive square root of a sum of terms, the terms being the
variance or covariance of these other quantities weighted according to how the measurement result
varies with changes in these quantities
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.4]
3.6
coverage factor
k
numerical factor used as a multiplier of the combined standard uncertainty (3.5) in order to obtain an
expanded uncertainty
Note 1 to entry: The coverage factor is chosen based on the level of confidence desired. This coverage factor, k, is
typically in the range of 2 to 3. A coverage factor (k) of 2 generally results in a level of confidence of approximately
95 %, and a coverage factor of 3 generally results in a level of confidence of approximately 99 %. This association
of confidence level and coverage factor is based on assumptions regarding the probability distribution of
measurement results. For a more thorough explanation, see the Guide to the Expression of Uncertainty in
[13]
Measurement .
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.6, modified — Note 1 to entry has been elaborated.]
3.7
CRM reference value
value of the certified property of a Certified Reference Material (CRM), reported in the documentation
supplied with it
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ISO/TS 23031:2020(E)

3.8
expanded uncertainty
U
quantity defining an interval about the result of a measurement that may be expected to encompass a
large fraction of values that could reasonably be attributed to the measurand
Note 1 to entry: Expanded uncertainty is the product of the combined standard uncertainty (u ) and the chosen
c
coverage factor (k).
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5, modified — Notes to entry 1 and 2 have been omitted.]
3.9
inter-instrument agreement
expected level of reproducibility between two or more instruments of exactly the same design and
manufacturer
3.10
inter-model agreement
expected level of reproducibility between two or more instruments of different designs, models or
manufacturer
3.11
manufacturer's calibration reference material
physical device or material, certified or non-certified, supplied by the instrument manufacturer, which
is used to standardize a specific instrument to the manufacturer’s scale calibrated to a reference
material
3.12
mean colour difference from the mean
MCDM
measure of the dispersion of the results of a series of colour measurements
Note 1 to entry: The MCDM quantifies the average colour difference between each reading and the mean of the
group of readings
Note 2 to entry: MCDM is a better single number indicator of the dispersion of a set of colour readings than is
the standard deviation of the colour difference (ΔE). This is because the distribution of colour difference is not
Normally distributed.
3.13
measurand
particular quantity subject to measurement
Note 1 to entry: Examples are: density, lightness, transmittance, reflectance factor.
[SOURCE: ISO/IEC Guide 99:2007, 2.3, modified — The notes to entry have been deleted.]
3.14
measurement uncertainty
parameter, associated with the result of a measurement, that characterizes the dispersion of the values
that could reasonably be attributed to the measurand (3.13)
Note 1 to entry: Each component of the uncertainty is assumed to have a normal distribution. For cases where
this assumption may not be valid, users follow the concepts and rules shown in the Guide to the Expression of
[13]
Uncertainty in Measurement .
Note 2 to entry: The result of a measurement is only an approximation or estimate of the value of the measurand
and thus is complete only when accompanied by a statement of the uncertainty of that estimate (see 3.3 and
ISO 15790:2004, 7.1.6).
Note 3 to entry: Colour values are three dimensional variables. The uncertainties of a colour are derived from the
propagation of the uncertainty from the spectral readings. The method for this process has been documented in
[1]
the literature .
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ISO/TS 23031:2020(E)

[SOURCE: ISO/IEC Guide 99:2007, 2.26, modified — The definition has been slightly modified and the
Notes to entry have been replaced.]
3.15
precision
closeness of agreement between test results obtained under prescribed conditions
[SOURCE: ASTM E 284]
3.16
radiance factor
ratio of the radiance from a point on a specimen, in a given direction, to that from the perfect reflecting
or transmitting diffuser, similarly irradiated and viewed
Note 1 to entry: For fluorescent media, the radiance factor is the sum of two quantities, to that from the perfect
reflecting or transmitting diffuser, similarly irradiated and viewed.
[SOURCE: ASTM E 284]
3.17
reference material
material or substance one or more of whose property values are sufficiently homogeneous and well
established to be used for the calibration (3.3) of an apparatus, the assessment of a measurement
method, or for assigning values to other materials
[SOURCE: ISO Guide 30:2015, 2.1.1, modified — Notes to entry have been omitted.]
3.18
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, but incorrectly, 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 CIE Publication 176.
[SOURCE: IEC 60050-845-04-64]
3.19
repeatability
closeness of the agreement between the results of successive
measurements on that single specimen using a single instrument by the same operator, in the same
location and in a short period of time
3.20
reproducibility
closeness of the agreement between the results of measurements of the
same measurand carried out under changed conditions of measurement
Note 1 to entry: Reproducibility is distinct from repeatability. Conditions of measurement may include operator,
specimen — including repositioning the same material standard, longer spans of time between readings —
including hours, days, weeks, etc.
[SOURCE: ISO International Vocabulary of Basic and General Terms in Metrology]
4 © ISO 2020 – All rights reserved

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ISO/TS 23031:2020(E)

3.21
spectrocolorimeter
spectrometer, one component of which is
...

TECHNICAL ISO/TS
SPECIFICATION 23031
First edition
Graphic technology — Assessment
and validation of the performance
of spectrocolorimeters and
spectrodensitometers
PROOF/ÉPREUVE
Reference number
ISO/TS 23031:2020(E)
©
ISO 2020

---------------------- Page: 1 ----------------------
ISO/TS 23031:2020(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 PROOF/ÉPREUVE © ISO 2020 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/TS 23031:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Known practices for instrument characterization. 6
4.1 Repeatability . 6
4.1.1 General. 6
4.1.2 Procedures . 6
4.2 Reproducibility . 7
4.2.1 General. 7
4.2.2 Determination of temporal reproducibility . 8
4.2.3 Determination of instrument reproducibility .10
4.2.4 Data collection and analysis .11
4.3 Accuracy .12
4.4 Quality of the influx spectrum .12
5 Reference materials for assessment of performance .13
5.1 Reference materials for comparison to the manufacturer’s specifications .13
5.2 Reference materials for comparison between identical models .15
5.2.1 General.15
5.3 Reference materials for comparison between different models .15
5.3.1 General.15
5.3.2 Measurements .16
5.3.3 Determination of instrument differences .16
6 Reported performance results.16
6.1 Conformance to factory specifications .16
6.2 Inter-instrument agreement .17
6.3 Inter-model agreement .17
6.4 Repeatability .17
6.5 Reproducibility .17
6.6 Assessment of accuracy .17
Bibliography .18
© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii

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ISO/TS 23031:2020(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 ISO/TC 130, Graphic technology.
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 PROOF/ÉPREUVE © ISO 2020 – All rights reserved

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ISO/TS 23031:2020(E)

Introduction
Instruments for the measurement of colour and colour difference have been in use since the middle of the
20th century. In the days before digital computers, converting spectral data into CIE tristimulus values
was a difficult, manual operation. Additionally, the optics and electronic components were large and
difficult to maintain. As a result, every instrument was supplied with a number of reference materials
that could be used to assess the performance of the instrument or to adjust the operating parameters.
These reference materials included coloured glass filters, rare earth glass filters, neutral density filters
and porcelain on steel plaques. Concepts such as accuracy, precision, bias and reproducibility had
special and unique applications to these instruments and reference materials.
As the optical and electronic technologies improved, the instruments became smaller, more precise
and more affordable. At the same time, the science of metrology matured to the point that the colour-
measuring instrument’s performance out-paced the ability of the national testing laboratories to
produce and certify standard materials suitable for testing. Modern optoelectronic instruments are
more precise and more stable than the materials used to assess their performance. Thus, it has become
problematic to determine if an instrument is within its factory specification or if two instruments
produce results that are in agreement with each other.
Several industries that produce coloured products have chosen to address this situation by adopting
and specifying a single brand and design of instrument. The paper and pulp industry have gone so far as
to capture one particular design from the 1960s and enshrine it in an International Standard. ISO 2469
describes the optics, the geometry and the operation of an instrument that is ideally suited and specially
designed for the measurement of the reflectance and colour of paper and pulp. Additionally, ISO/TC 6,
has established a series of authorized laboratories which issue certified reference materials (CRM) for
testing and calibrating the performance of an ISO 2469 compliant instrument. This was possible, in part,
as the instrument captured in ISO 2469 was widely available on the market and it had no competitive
designs and the authorized laboratories market sets of standards which are produced using materials
with similar physical and optical properties as production papers or pulps. The authorized laboratories
maintain a very close relationship to a single national standards laboratory and to each other. WG3
periodically audits these laboratories to verify that they have calibrated their instruments properly
against the scale of radiance factor developed by the national standards laboratory.
In contrast, modern graphic reproduction has moved from the era of artistic interpretation into a time
in which the image reproduction is driven by objective numerical assessments. With the availability
of modern electro-optics, the number of companies providing instruments and the range of models of
different size and capabilities has increased dramatically. Geometries utilized are nominally 45°:0°
but may be uniplanar, biplanar, circumferential or annular. While referred to as bidirectional, they are
always biconical and the sizes of the influx and efflux cones vary as much as the directionality.
Unfortunately, the national metrology laboratories have not been successful in defining a universally
accepted scale of diffuse reflectance factor or diffuse radiance factor for these biconical instruments,
especially when the sampling aperture is small. Without artefact standards that closely align with
the properties to be measured in the printing industry, the result can easily be a match between two
instruments on the reference material that does not correlate to a match on real world materials.
As a result, colour-measuring instruments from different manufacturers or with different design
intents do not provide adequate agreement on the determination of the colour values or methods for
the assessment of the performance of an instrument system relative to its manufacturer declared
performance specifications. Further, to make the instruments as simple as possible to operate, the end-
user is given little to no access the underlying operation of the instrument. The operator can select an
influx spectral quality (M0, M1, M2, M3) but has no way to determine or adjust the spectral quality of the
influx. The realization of the scale of 45°:0° reflectance factor or radiance factor is different than that
of hemispherical diffuse reflectance factor, even for nearly ideal materials. The operator only has the
ability to request that instrument adjust the scale of the instrument using a single reference standard
supplied with the instrument. The instrument scale is thus traceable only at the one point. Most do
not even offer the ability to set or verify the mid-scale value or the optical null value. Today, optical
metrologists refer to this process as standardization, since the instrument is forced to reproduce the
values of the one standard.
© ISO 2020 – All rights reserved PROOF/ÉPREUVE v

---------------------- Page: 5 ----------------------
ISO/TS 23031:2020(E)

This document has been prepared to provide the users of portable spectrocolorimeters and
spectrodensitometers with guidance on the methods for validation of the performance of those
instruments. Since calibration is not possible, the use of a series of certified reference materials (CRM)
or a series of stable, idealized reference materials is required. ISO 15790 provides guidance on the
development of CRM standards for the scale of optical density. But optical density is a more forgiving
measurement than tristimulus colorimetry. Measurement of colour is inherently more complicated than
the measurement of optical density, since the logarithmic function compresses the measurement scale
and the associated errors. Computing colorimetric tristimulus values from spectral data requires the
use of the entire range of reflectance factor values while ISO status density is based on the response of
the spectral product. Bright colours, useful for producing a large gamut of colour in image reproduction,
possess large differences between the spectral regions of absorption and non-absorption of light but
density is only assessing the spectral regions of maximum absorbance. While the human visual system
has broad spectral responses, in terms of the cone fundamentals, the post receptor processing allows
an observer to perceive hue differences as small as 1 nm. So, the instrumentation for colour assessment
needs to have an accuracy several times small than the human visual system.
There is a need to use a set of 10 to 20 physical standards to sample the visible spectrum with materials
possessing both high and low reflectance levels and that transition between the two extremes over a
very small range of wavelengths. Those materials are stable and nearly opaque to avoid the problems of
lateral diffusion observed when the sampling aperture are small. The procedures described here have
been shown to provide end-users with methods to quantify the performance of spectrocolorimeters
on the day it arrives from the manufacturer or distributor until the day it is retired from service. The
methods may also be used to validate the instrument system against manufacturer’s specifications and
against the requirements for product quality.
National measurement laboratories (NML) continue to develop new scales and new methods of assessing
artefacts with the goal of providing certified standard materials for establishing the level of traceability
and reproducibility of commercial instruments. Unfortunately, these standards have historically been
too expensive for routine use. Only recently have the NMLs began developing automated methods for
characterizing reference colours or even user supplied materials. Currently, only large corporations or
instrument makers can afford to own such materials. Practical users rely on secondary laboratories
and reference standards designed specifically for the end use case. In the graphic arts, that should be
some form of printed material with a relatively short duty lifetime.
Finally, even after the CRM has be obtained, the methods for assessing the measurement data are not
well described. A spectral reflectance factor curve should include 31, 36, 40 or more measurements.
Trying to assign values, tolerances and uncertainties to the individual wavelengths is a challenge.
For example, it is possible that measurements of an artefact are consistent for 28 wavelengths and
inconsistent at 3 others. Should these instruments be considered as acceptable or failures? Converting
the measured data to colorimetric values (XYZ or L*a*b*) improves the situation slightly, but the
dilemma of comparing 3 individual readings from one lab or instrument to 3 individual values from
another lab, remains a problem not conveniently described in the standards literature. It is the intent of
this document to document and describe objective ways of assessing and comparing the performance of
a colour-measuring instrument with the ultimate goal of identifying an optimum method for application
in the graphic reproduction workflow.
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TECHNICAL SPECIFICATION ISO/TS 23031:2020(E)
Graphic technology — Assessment and validation
of the performance of spectrocolorimeters and
spectrodensitometers
1 Scope
This document describes procedures for the assessment and validation of the performance of an optical
spectrometer intended for use in capturing the spectral reflectance factor or the spectral radiance
factor of printed areas comprised of non-fluorescent or fluorescent materials, respectively. While it
does not describe the application to transmitting materials directly, many of the procedures can be
applied to transmitting systems by backing them with a reflective white backing material.
This document does not address spectral measurements appropriate to other specific application
needs, such as those used during the production of materials (e.g. printing paper and proofing media),
which are well described by ISO standards under the jurisdiction of ISO/TC 6. It does not describe the
special requirements for testing instruments that make in-process or online colour measurements.
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 13655:2017, Graphic technology — Spectral measurement and colorimetric computation for graphic
arts images
ISO 15790:2004, Graphic technology and photography — Certified reference materials for reflection and
transmission metrology — Documentation and procedures for use, including determination of combined
standard uncertainty
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 http:// www .electropedia .org/
3.1
accuracy
closeness of agreement between a test result and an accepted reference value
Note 1 to entry: The qualitative term accuracy, when applied to a set of observed values, is a combination of a
random precision component and a systematic error or bias component. Since, in routine use, random components
and bias components cannot be completely separated, the reported “accuracy” is interpreted as a combination of
these two elements.
[SOURCE: ASTM E 284]
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ISO/TS 23031:2020(E)

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.
Note 2 to entry: 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. Verification of a measuring device
requires determination of the uncertainty of the calibration.
[SOURCE: ISO/IEC Guide 99:2007:2.39, modified — The definition has been editorially revised and the
original Notes to entry have been replaced.]
3.4
certified reference material
CRM
reference material, accompanied by a certificate, one or more of whose property values are certified by
a procedure which establishes traceability to an accurate realization of the unit in which the property
values are expressed, and for which each certified value is accompanied by an uncertainty at a stated
level of confidence
[SOURCE: ISO 15790:2004, 3.1.2]
3.5
combined standard uncertainty
u
c
standard uncertainty of the result of a measurement when that result is obtained from the values of
a number of other quantities, equal to the positive square root of a sum of terms, the terms being the
variance or covariance of these other quantities weighted according to how the measurement result
varies with changes in these quantities
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.4]
3.6
coverage factor
k
numerical factor used as a multiplier of the combined standard uncertainty (3.5) in order to obtain an
expanded uncertainty
Note 1 to entry: The coverage factor is chosen based on the level of confidence desired. This coverage factor, k, is
typically in the range of 2 to 3. A coverage factor (k) of 2 generally results in a level of confidence of approximately
95 %, and a coverage factor of 3 generally results in a level of confidence of approximately 99 %. This association
of confidence level and coverage factor is based on assumptions regarding the probability distribution of
measurement results. For a more thorough explanation, see the Guide to the Expression of Uncertainty in
[13]
Measurement .
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.6, modified — Note 1 to entry has been elaborated.]
3.7
CRM reference value
value of the certified property of a Certified Reference Material (CRM), reported in the documentation
supplied with it
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ISO/TS 23031:2020(E)

3.8
expanded uncertainty
U
quantity defining an interval about the result of a measurement that may be expected to encompass a
large fraction of values that could reasonably be attributed to the measurand
Note 1 to entry: Expanded uncertainty is the product of the combined standard uncertainty (u ) and the chosen
c
coverage factor (k).
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5, modified — Notes to entry 1 and 2 have been omitted.]
3.9
inter-instrument agreement
expected level of reproducibility between two or more instruments of exactly the same design and
manufacturer
3.10
inter-model agreement
expected level of reproducibility between two or more instruments of different designs, models or
manufacturer
3.11
manufacturer's calibration reference material
physical device or material, certified or non-certified, supplied by the instrument manufacturer, which
is used to standardize a specific instrument to the manufacturer’s scale calibrated to a reference
material
3.12
mean colour difference from the mean
MCDM
measure of the dispersion of the results of a series of colour measurements
Note 1 to entry: The MCDM quantifies the average colour difference between each reading and the mean of the
group of readings
Note 2 to entry: MCDM is a better single number indicator of the dispersion of a set of colour readings than is
the standard deviation of the colour difference (ΔE). This is because the distribution of colour difference is not
Normally distributed.
3.13
measurand
particular quantity subject to measurement
Note 1 to entry: Examples are: density, lightness, transmittance, reflectance factor.
[SOURCE: ISO/IEC Guide 99:2007, 2.3, modified — The notes to entry have been deleted.]
3.14
measurement uncertainty
parameter, associated with the result of a measurement, that characterizes the dispersion of the values
that could reasonably be attributed to the measurand (3.13)
Note 1 to entry: Each component of the uncertainty is assumed to have a normal distribution. For cases where
this assumption may not be valid, users follow the concepts and rules shown in the Guide to the Expression of
[13]
Uncertainty in Measurement .
Note 2 to entry: The result of a measurement is only an approximation or estimate of the value of the measurand
and thus is complete only when accompanied by a statement of the uncertainty of that estimate (see 3.3 and
ISO 15790:2004, 7.1.6).
Note 3 to entry: Colour values are three dimensional variables. The uncertainties of a colour are derived from the
propagation of the uncertainty from the spectral readings. The method for this process has been documented in
[1]
the literature .
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ISO/TS 23031:2020(E)

[SOURCE: ISO/IEC Guide 99:2007, 2.26, modified — The definition has been slightly modified and the
Notes to entry have been replaced.]
3.15
precision
closeness of agreement between test results obtained under prescribed conditions
[SOURCE: ASTM E 284]
3.16
radiance factor
ratio of the radiance from a point on a specimen, in a given direction, to that from the perfect reflecting
or transmitting diffuser, similarly irradiated and viewed
Note 1 to entry: For fluorescent media, the radiance factor is the sum of two quantities, to that from the perfect
reflecting or transmitting diffuser, similarly irradiated and viewed.
[SOURCE: ASTM E 284]
3.17
reference material
material or substance one or more of whose property values are sufficiently homogeneous and well
established to be used for the calibration (3.3) of an apparatus, the assessment of a measurement
method, or for assigning values to other materials
[SOURCE: ISO Guide 30:2015, 2.1.1, modified — Notes to entry have been omitted.]
3.18
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, but incorrectly, 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 CIE Publication 176.
[SOURCE: IEC 60050-845-04-64]
3.19
repeatability
closeness of the agreement between the results of successive
measurements on that single specimen using a single instrument by the same operator, in the same
location and in a short period of time
3.20
reproducibility
closeness of the agreement between the results of measurements of the
same measurand carried out under changed conditions of measurement
Note 1 to entry: Reproducibility is distinct from repeatability. Conditions of measurement may include operator,
specimen — including repositioning the same material standard, longer spans of time between readings —
including hours, days, weeks, etc.
[SOURCE: ISO International Vocabulary of Basic and General Terms in Metrology]
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