ISO 24585-1:2023

INTERNATIONAL ISO
STANDARD 24585-1
First edition
2023-07
Graphic technology — Multispectral
imaging measurement and
colorimetric computation for graphic
arts and industrial application —
Part 1:
Parameters and measurement
methods
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Multispectral measurement requirements . 4
4.1 Instrument calibration and adjustment . 4
4.2 Illumination requirements . 4
4.2.1 Wavelength range . . 4
4.2.2 Measurement conditions . 4
4.3 Specimen preparation and backing requirements . 5
4.4 Light capturing requirements . 5
4.4.1 Spatial resolution . 5
4.4.2 Wavelength range, wavelength interval and bandwidth . 6
4.5 Multispectral image storage . 6
4.6 Tristimulus image computation requirements. 6
4.7 Requirements for comparing multispectral images . 6
4.8 Requirements for comparing tristimulus images derived from multispectral
images. 6
5 Data reporting requirements.6
6 Conformance . 7
Bibliography . 8
iii
Foreword
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iv
Introduction
Multispectral imaging is an image capture technology with more than three channels or wavelengths.
This document focuses on multispectral image capture of surfaces with spatially varying colour. To
achieve this, a contiguous band of wavelengths are sampled including the visible range and, in some
cases, including ultraviolet and infrared.
NOTE The term hyperspectral imaging is typically used for systems capturing multiple wavelength bands
[1]
such as infrared, visible, ultraviolet and other electromagnetic ranges . Usually, these bands are non-contiguous
or are overlapping. The boundary between multispectral and hyperspectral in practice is sometimes not
precise and the usage of each term is often inaccurate, often for marketing reasons. To avoid any confusion, this
document uses the term multispectral imaging to mean capture of image data across a single continuous band
of wavelengths sampled at regular intervals across the visible range and which can extend into the infrared or
ultraviolet.
In the past, assessment of object surfaces with spatially varying colour was carried out by visual
inspection by experienced personnel. In some cases, area-averaging (pointwise) measurements were
made to obtain the average value of the spatially varying colour of the object surfaces. The object
surface was sampled at regular spatial intervals, however low spatial resolution and imprecise re-
positioning between repeated measurements limited the success of such approaches. With the advent
of modern lighting and 2D-sensor technologies, multispectral imaging devices are increasingly being
used for this purpose.
Many laminations, decorations and materials containing patterns of various colours are produced by
printing processes that include offset lithography, letterpress, flexography, gravure, screen and digital
printing. To characterize the appearance of these objects effectively, the spectral reflectance factor
must be read at many points across and down the object. Multispectral imaging devices can do this
very effectively and provide an unprecedented advantage allowing pixel-wise evaluation of colour
image difference to be used for quality assessment. Improved digital colour transforms based on
this assessment can be used to reduce spatial colour differences leading to better reproductions. One
benefit over RGB-based imaging systems is the improved colour accuracy when characterizing printing
systems that use process colorants beyond CMYK, for example including orange, green and violet inks.
There are only a few choices allowed by the CIE when making spectral measurements and performing
colorimetric computations. The specific choice will result in numerical values for the specific property
for a specific specimen that are in agreement with the results from national standardizing laboratories.
Similarly, two instruments from different manufacturers, that are both different from the CIE
recommendations can be different from each other.
ISO 13655 specifies a limited number of such choices for the measurement and computation of the
colorimetric characteristics of single printed patches providing a solid foundation for consistent colour
communication within the graphic arts community. There is a need to have multispectral measurement
system attributes documented and standardized in order to achieve a similar solid foundation so that
multispectral images can reliably be compared.
Spectral imaging systems aim for spectrally resolved pixel data of an image of a scene. The objectives
of these systems include object or material identification, and monitoring of manufacturing processes.
Current multispectral imaging technologies restrict the number of spectral image channels to a set that
is smaller than the minimum required by ISO 13655 for spectral measurements (400 nm to 700 nm at
20 nm intervals, requiring 16 channels). While such systems might be become more widely available in
future, this document provides requirements for systems with fewer channels.
There are many commercially available multispectral imaging systems which provide spectral diffuse
reflectance factor measurements at many pixels, from which colorimetric values can be calculated. The
majority of these instruments do not conform to ISO 13655 guidelines for spectral measurements. In
practice, it might not be possible to achieve all requirements of speed, spatial resolution and ISO 13655
spectral sampling in a cost-effective multispectral imaging system. One trade-off is to decrease the
number of spectral channels. This can result in different numerical values for the colorimetry of a
specimen when compared to measurements conforming to ISO 13655. Unless the data being compared
v
are all based on the same set of measurement and computational choices it is not be possible to make
reliable comparisons.
This document is aligned with some aspects of ISO 13655, making use of many definitions such as light
sources and tables for colorimetric computation. When using a non-standard number of spectral bands,
[2]
ASTM E2022-16 provides a method to be used to calculate tables of weighting factors for tristimulus
integration using custom spectral power distributions and spectral sampling intervals and ranges.
This document is part of a series of standards that allows for different choices for measurement and
computation for different application areas. Subsequent parts set out the specification needed for each
application area following the structure defined in this document.
In light of the variation in the number of spectral channels, the inter-instrument agreement between
a multispectral imaging system and ISO 13655 compliant spectrophotometers can be significantly
poorer than the agreement between ISO 13655 compliant instruments. The user of multispectral
imaging devices is advised to weigh the benefits of an imaging device with colour fidelity better than an
RGB camera with the drawback of measurements that do not agree with spectrophotometers.
vi
INTERNATIONAL STANDARD ISO 24585-1:2023(E)
Graphic technology — Multispectral imaging measurement
and colorimetric computation for graphic arts and
industrial application —
Part 1:
Parameters and measurement methods
1 Scope
This document establishes procedures for the spatially resolved spectral measurement of reflecting
flat objects, including inline colour measurements. It also establishes procedures for computation of
colorimetric parameters for graphic arts multispectral imaging devices.
Graphic arts includes, but is not limited to, the preparation of material for and volume production by
production printing processes that include offset lithography, letterpress, flexography, gravure, screen
and all kind of digital printing.
This document does not address spatially resolved spectral measurements of transmitting or self-
illuminating objects including flat-panel displays. This document is not applicable to step and repeat
spot reading scanning instruments. It does not address printing on a metallic or interference foil.
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, Graphic technology — Spectral measurement and colorimetric computation for graphic arts
images
ISO 23603, Standard method of assessing the spectral quality of daylight simulators for visual appraisal
and measurement of colour
CE Publication No. 15, Colorimetry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology 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
multispectral imaging device
digital imaging system, capturing spectral data with more than three channels or bands of wavelengths
Note 1 to entry: Although the spectral range is typically found to be from 400 nm to 700 nm, energy slightly
below and above that range contributes to the visual signal. Therefore, multispectral imaging systems can be
considered that cover a spectral range from 380 nm to 730 nm.
Note 2 to entry: The channels are usually of limited spectral bandwidth, also known as narrowband, from
which the spectral distribution of a captured image can be estimated for colorimetric evaluation. Typically, the
differences between estimated and original spectral or colorimetric data o
...