ISO/TR 17321-5:2021

TECHNICAL ISO/TR
REPORT 17321-5
First edition
2021-01
Graphic technology and
photography — Colour
characterization of digital still
cameras (DSCs) —
Part 5:
Colour targets including saturated
colours for colour characteristic
evaluation test for colorimetric
image capture
Technologie graphique et photographie — Caractérisation de la
couleur des appareils photonumériques —
Partie 5: Cibles de couleurs incluant des couleurs saturées pour l’essai
d’évaluation des caractéristiques chromatiques pour la capture
d’images en mode colorimétrique
Reference number
©
ISO 2021
© ISO 2021
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ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Highly-saturated colour targets . 2
4.1 General . 2
4.2 Extension of real existing spectra using eigenvector method . 2
4.2.1 General. 2
4.2.2 Selection of spectra database . 2
4.2.3 Spectral reconstruction from the eigenvectors . 3
4.3 Artificial (LED-based) spectra whose wavelength peak is on colour-difference-
sensitive wavelength (CDSW) . 4
4.3.1 General. 4
4.3.2 The method to define the colour-difference-sensitive wavelength (CDSW) . 4
4.3.3 Selection of LED for CDSW targets . 6
5 FOM metric for evaluation of overall sensor spectral sensitivities, used in the digital
cameras . 8
5.1 General . 8
5.2 Evaluation metrics for OSSS . 8
5.3 Advantages and disadvantages of ΔE (deltaE) evaluation . 9
5.4 How 17321-5 datasets can be used for FOMs.9
5.5 Worked examples .11
Annex A (informative) Selection and eigenvectors of spectral distribution set .13
Annex B (informative) Colour gamut of boundary colour .15
Annex C (informative) Worked example for spectral distribution generation of Pointer’s
surface colours .16
Annex D (informative) Background information for defining CDSW .26
Annex E (informative) Additional 410nm to colour-difference-sensitive wavelengths (CDSW) .29
Annex F (informative) Colour differences of patches of CDSW target.30
Annex G (informative) Spectral distribution of CDSW target for ITU-R BT.2020 .31
Annex H (informative) Spectral distribution dataset for users to download .34
Bibliography .35
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
A list of all parts in the ISO 17321 series can be found on the ISO website.
iv © ISO 2021 – All rights reserved

Introduction
There are many application areas such as medical imaging, cosmetics, e-commerce, sales catalogue,
fine art reproduction, art archive etc. where colorimetric image capture and colorimetric image
reproduction are desired. When precise colorimetric reproduction is required for the subjects that
include highly-saturated colours, it is desirable that overall sensor spectral sensitivities are close to
linear combinations of CIE 1931 colour matching functions.
On the other hand, real DSCs have overall sensor spectral sensitivities that deviates from linear
combination of CIE 1931 colour matching functions, and yet reproduces reasonable colours for general
low-saturated colour objects. This is because most of spectral distribution of real-existing objects are
well self-correlated in the wavelength direction. This is also true for the frequently-used colour target
such as X-rite colour checker classic.
Therefore, when the precise colour reproduction is required for highly-saturated colour objects, it is
important to use spectral distribution that are less self-correlated in the wavelength direction, for the
evaluation of overall sensor spectral sensitivities.
For this purpose, Clause 3 proposes two methods for generating highly-saturated colour targets. The
first method is statistical extension of existing objects spectra, and the second one is selection from
artificial (LED-based) spectra.
Clause 4 then describes how these highly-saturated colour targets can be used for goodness evaluation
of overall sensor spectral sensitivities. Applicability of several existing evaluation metrics (such as
Voraʼs μ-factor and Sharmaʼs FOM) are compared, using highly-saturated targets generated by the
methods proposed in Clause 4.
Annex B gives details on colour gamut of boundary colour and Annex F gives more information on
colour differences of patches of CDSW target.
TECHNICAL REPORT ISO/TR 17321-5:2021(E)
Graphic technology and photography — Colour
characterization of digital still cameras (DSCs) —
Part 5:
Colour targets including saturated colours for colour
characteristic evaluation test for colorimetric image capture
1 Scope
This document describes sample methods to generate spectra for colour targets comprised of highly
saturated colours for colour characteristic evaluation of colorimetric image capture capability of digital
still cameras (DSCs).
2 Normative references
There are no normative references in this document.
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
colour-difference-sensitive wavelength
CDSW
wavelength sensitive to colour difference
3.2
colour matching functions
tristimulus values of monochromatic stimuli of equal radiant power
[SOURCE: CIE Publication 17.4, 845-03-23]
3.3
digital still camera
DSC
device which incorporates an image sensor and produces a digital signal representing a still picture
[SOURCE: ISO 12232:2012, 3.40, modified — Notes 1 and 2 to entry have been deleted.]
3.4
light-emitting diode
LED
semiconductor diode that emits non coherent optical radiation through stimulated emission resulting
from the recombination of electrons and photons, when excited by an electric current
[SOURCE: IEC 60050-521, 521-04-39]
3.5
overall sensor spectral sensitivities
OSSS
spectral sensitivities of overall sensor components, which could be derived as spectral sensitivities’
product of optical elements (including IR/UV-cut filter), colour filter sets, and image sensor
3.6
tristimulus values
amounts of the three reference colour stimuli, in a given trichromatic system, required to match the
colour of the stimulus considered. (see colour matching functions)
[SOURCE: CIE Publication 17.4, 845-03-22]
4 Highly-saturated colour targets
4.1 General
This document proposes two different methods for generating highly-saturated colour targets.
First one is “Extension of real existing spectra using an eigenvector method”. Naturally existing
saturated colour spectra are usually very difficult to obtain by measurement. Therefore, computed
spectra are extended from the eigenvectors generated from spectral databases.
Second one is “Artificial (LED-based) spectra whose wavelength peak is on colour-difference-sensitive
wavelength (CDSW)”. Mathematical analysis was performed to select the wavelengths which were
colour-difference-sensitive. Artificial (LED-based) spectra were then generated whose peak matches
colour-difference-sensitive wavelength (CDSW).
4.2 Extension of real existing spectra using eigenvector method
4.2.1 General
The eigenvector-based procedure for generating highly-saturated colour targets is as follows:
— selection of spectral database (see 4.2.2);
— spectral reconstruction from the eigenvectors (see 4.2.3);
— calculation of boundary colours (see 4.2.3.2);
— calculation of saturated-colours using reference spectral distribution (see 4.2.3.3).
4.2.2 Selection of spectra database
[7]
The wavelength range and wavelength increment are user-definable. ISO 17321-1 described that
the wavelength range is 380 nm to 730 nm with a sampling interval of 10 nm or less. The spectral
distribution set selec
...