ISO/TS 18621-31:2020

TECHNICAL ISO/TS
SPECIFICATION 18621-31
First edition
2020-12
Graphic technology — Image quality
evaluation methods for printed
matter —
Part 31:
Evaluation of the perceived resolution
of printing systems with the Contrast–
Resolution chart
Technologie graphique — Méthodes d’évaluation de la qualité
d’image pour les imprimés —
Partie 31: Évaluation de la résolution perçue des systèmes
d’impression avec un graphique de contraste–résolution
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 Requirements . 3
4.1 General . 3
4.2 Apparatus requirements . 3
4.2.1 Printing system requirements . 3
4.2.2 Scanning system requirements . 3
4.3 Procedure . 3
4.3.1 Test chart . 3
4.3.2 Evaluation intent . 4
4.3.3 Printing and scanning . . 5
4.3.4 Evaluation process . 6
5 Resolution-Score processing . 6
5.1 General . 6
5.2 Element identification . 6
5.3 Scanning signal interpretation . 6
5.4 Spatial filtering . 6
5.5 Normalized 2-D cross-correlation . 7
5.6 Resolution-score computation . 8
6 Reporting .12
Annex A (normative) Test chart and reference files — Availability .14
Annex B (normative) Printing process and data path requirements .17
Annex C (normative) Linearization .21
Annex D (normative) Scanner conformance requirements .23
Annex E (normative) Evaluation process conformance.29
Annex F (normative) Spatial filter .31
Bibliography .35
Foreword
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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
Perceived resolution, the capability to perceive fine detail, is a measure of full system capability and
depends upon characteristics of the printing system (substantially more than just its addressability),
characteristics of the substrate, of the viewing conditions, and of the observer. Perceived resolution
depends critically upon tonal differences between elements of an image – there is no perceived
detail, hence no measure of resolution, when there is no tonal difference in an image. The three
major contributors to the perceived resolution of a printing system are the capability of a printing
system to maintain a desired spatial separation between nearby elements printed on a substrate
(the addressability of a printing system indicates what the minimum spatial separation can be), the
capability of the printing system to carry tonal differences (contrast) between these nearby printed
elements, and the capability of the human visual system to perceive the printed detail. The design of
a test chart and an evaluation process for measuring a printing system’s capability to carry fine detail
must reflect these major contributors.
Fourier analysis has proven very useful in analysing the reproduction capability of image forming
[1]
systems . In this formalism, spatial separation is measured in terms of spatial frequency (e.g. cycles
per millimetre) and contrast is measured in terms of modulation (the dimensionless ratio of a change
in perceived luminance to its average luminance) at a particular spatial frequency. The ratio of the
reproduced modulation to the original (desired) modulation can be used to describe the capability of a
printing system to reproduce a sinusoidal input at a particular spatial frequency. This ratio, taken over
a range of spatial frequencies is called the modulation transfer function (MTF).
Key
X spatial fequency
1 modulation of original (constant amplitude)
2 modulation of reproduction (with limited resolution)
3 modulation transfer function (decreases due to limited resolution)
Figure 1 — Modulation transfer function of a printing system
The MTF characteristic shows the ratio of the reproduced modulation to the original (input) modulation
as a function of spatial frequency and provides a very useful description of printing system capability.
The decrease at high frequencies of the modulation transfer function shown in Figure 1 characterizes
the common degradation in printing system image detail capability at high spatial frequencies.
In characterizing perceived resolution, a single component of the imaging chain cannot be isolated
since we look at the results of the complete system. The printing system imaging chain starts with the
process of placing marks on a substrate. In many printing systems, the individual marks can provide
only a limited number of tonal levels and the full tonal range is provided by subsequent area modulation
(screening) of the marks. This screening process can strongly affect the image detail capability of a
printing system. The characteristics of the substrate can affect both the effectiveness of placing these
marks (e.g. surface roughness) and affect the interplay between the placed marks and the illumination
required for viewing the printed image (e.g. light scattering in the substrate). Finally, perceived
resolution depends upon the viewing conditions (illumination, viewing distance, and magnification) as
well as the capability of the human observer to perceive detail. The capability of normal human vision
to perceive spatial detail can be characterized by a modulation transfer function (see Reference [2]).
This is shown in Figure 2.
Key
Y relative contrast sensitivity
X spatial frequency
a
6/6 visual limit
b
cy/mm at 300 mm
c
cy/mm at 400 mm
d
cy/degree
Figure 2 — Contrast sensitivity function of a human observer
The natural units for the perceptual contrast sensitivity function are cycles per degree, which are
independent of viewing distance. Shown as a dotted line on the right of Figure 2 is the ophthalmological
limit of visual acuity known as 6/6 vision in metric units which means a person being examined can see
the same level of detail at 6 m as a person with "normal" visual acuity would see at 6 m distance. This
visual limit corresponds to a spatial frequency of about 6 cy/mm at 300 mm viewing distance or about
vi © ISO 2020 – All rights reserved

4,5 cy/mm at a viewing distance of 400 mm. At a viewing distance of 400 mm the human visual system
response to spatial detail peaks at about 0,4 cy/mm (0,5 cy/mm at 300 mm), decreasing in sensitivity at
both higher and lower spatial frequencies.
Key
Y contrast
X spatial frequency
Figure 3 — Illustrative contrast sensitivity function (Reference [3])
A visual illustration of the dependence of perceptual detail reproduction capability on both spatial
frequency (horizontal axis) and contrast (vertical axis) is shown in Figure 3 (see Reference [3]). The
perception of fine detail is frequency dependent and can be perceiv
...