ISO/TS 23031:2020

TECHNICAL ISO/TS
SPECIFICATION 23031
First edition
2020-08
Graphic technology — Assessment
and validation of the performance
of spectrocolorimeters and
spectrodensitometers
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 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
Foreword
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electrotechnical standardization.
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This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
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iv © ISO 2020 – All rights reserved

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.
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
...