ISO/TS 18621-22:2024

Technical
Specification
ISO/TS 18621-22
First edition
Graphic technology — Image quality
2024-05
evaluation methods for printed
matter —
Part 22:
Evaluation of colour graininess
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 2
4.1 Principles .2
4.2 Apparatus .2
4.3 Procedure .2
4.3.1 Testform .2
4.3.2 Printing and measuring .2
4.4 Evaluation .3
4.5 Reporting .4
Annex A (informative) Interpreting graininess, S . 6
CG
Annex B (informative) Spatial filter . 7
Bibliography . 9

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
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iv
Introduction
The subject of image quality is broad and complex, due to its multidimensionality. A large number of
[1]
measurement methods have been developed to describe attributes of printed image quality . Many
different methods may be available to provide a measure of a particular image quality attribute, usually on
completely different numerical scales and, with few exceptions, providing no well-defined correlation with
visual perception to establish the visual significance of a measured difference. A fraction of these methods
has been developed in a manner that is independent of marking technology, permitting general, technology-
independent measurement of an image quality attribute.
The evaluation of perceived image quality in prints is an active field of research. Definitions of measurements
of print quality attributes that correlate with visual perception by technology-independent means, even
across many printing technologies, is under current scrutiny. Nevertheless, these evaluations are complex
due to subjectivity and dimensionality. It is influenced by a number of different quality attributes. It is
often difficult and complicated to evaluate the influence of all attributes on overall image quality, and their
influence on other attributes.
Graininess measurements provide an indication of the apparent high frequency image noise in a digital
printing system and typically refers to aperiodic fluctuations of density at a spatial frequency greater than
0,4 cycles per millimetre in all directions for standard viewing distance of 400 mm. Many methods have
[2]
been developed over the years, for instance, the method defined in ISO/IEC 24790 , which is restricted to
luminance based variations. In this document this approach has been extended for colour variations, while
default viewing distance is 40 cm (reading distance).

v
Technical Specification ISO/TS 18621-22:2024(en)
Graphic technology — Image quality evaluation methods for
printed matter —
Part 22:
Evaluation of colour graininess
1 Scope
This document defines a process independent measurement method for the evaluation of graininess of
printed products. It provides requirements for the test form and measurement devices to be used as well as
the formulas to compute the colour graininess score (S ).
CG
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 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
CIEDE2000 colour difference
method of/formula for calculating colour difference, ΔE /CIEDE
00 00
[3]
Note 1 to entry: See ISO/CIE 11664-6 for details.
Note 2 to entry: The default weights for lightness, chroma and hue are (1:1:1).
Note 3 to entry: The unit is 1.
3.2
CIELAB colour space
CIE 1976 L* a* b* colour space
three-dimensional, approximately uniform colour space produced by plotting L*, a*, b* in Cartesian co-
ordinates
[4]
[SOURCE: adapted from 17-23-077 of CIE S 012 ]
3.3
graininess
appearance of unintended microscopic but visible aperiodic fluctuations of colour (microscopic means
variations with spatial frequencies greater than about 0,4 cy/mm)

4 Requirements
4.1 Principles
The graininess score is evaluated using a calibrated scanner. After a Gaussian filter that roughly
...