ISO/TS 18621-21:2020

TECHNICAL ISO/TS
SPECIFICATION 18621-21
First edition
Graphic technology — Image quality
evaluation methods for printed
matter —
Part 21:
Measurement of 1D distortions of
macroscopic uniformity utilizing
scanning spectrophotometers
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JTC 1/SC 28,ISO/TC 42 before casting their ballot to the e-Balloting application.
PROOF/ÉPREUVE
Reference number
ISO/TS 18621-21:2020(E)
©
ISO 2020

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ISO/TS 18621-21:2020(E)

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ISO/TS 18621-21:2020(E)

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
4.2.1 General. 2
4.2.2 Measurement condition . . 3
4.3 Procedure . 3
4.3.1 Test pattern . 3
4.3.2 Printing and measuring . 3
4.4 Evaluation . 4
4.5 Reporting . 5
Annex A (informative) Recommended tone value combinations . 6
Bibliography . 7
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ISO/TS 18621-21:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
A list of all parts in the ISO 18621 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO/TS 18621-21:2020(E)

Introduction
The subject of image quality is broad and complex, due to its multidimensionality and the apparent
characteristics of human vision. Many different methods can be available to provide a measure of a
particular visual attribute in some particular viewing context and aimed at a particular printing
technology. It is a challenge to have image quality evaluation methods that are independent of the
marking technology, i.e. correlate with human perception to provide visual significance to measured
differences across many printing technologies. The evaluation of perceived image quality is an active
field of research.
The uniformity that can be achieved, or in fact the lack thereof, is an important factor in the evaluation of
the overall print quality. Uniformity refers to the subjective impression of the homogeneity of the colour
in extended areas up to the document size. Colour uniformity refers to all types of unintended but visible
variations in colour, that may go in any direction in colour space and may have any spatial pattern. Spatial
patterns include 1D, 2D, periodic, aperiodic, localized, large-scale and small-scale variations. They can be
identified as streaks or streakiness, banding, gradients, mottle, moiré and others.
When evaluating perceived uniformity, the intended viewing distance should be taken into account. For
practical application it is also common practice to distinguish 2 categories of uniformity that depend on
the spatial frequency (or actually the angular frequency as seen by the eye):
— Microscopic uniformity such as graininess that is related to the imaging process and generally
consists of a 2D random noise pattern. It is visually relevant for image objects as small as few square
mm in size in case of normal reading distance (40 cm).
— Macroscopic uniformity involves distortions in the homogeneity that extend beyond few mm in one
or both geometric dimensions. It is generally visible across the document page size with examples
called banding, cording stripes or streaks.
This document focuses on the macroscopic uniformity that exhibits 1D type patterns that extend more
or less across the printed area of a page. It uses well established colour measurement instruments as
the basic measurement device, especially systems that can be combined with automated XY-tables for
performing well defined measurements in a complete 2D grid of measurement locations in an easy
way. These systems generally have a minimum pitch in both dimensions of 6 mm. This method takes
a much more rigorous approach than the 9-point sample method that is defined in ISO 12647-7 that is
applicable to proofing systems.
The measurement method derives a single valued Macro-Uniformity-Score on a scale that ranges
from 100 (“perfect uniformity”) to 0 (“extremely poor uniformity”). It is based on the evaluation of
the average colour differences that occur in horizontal and vertical rows separately and adds them up
to arrive at a single value. Then a formula is applied to compute the Macro-Uniformity-Score that is
shown to correlate well with the perception of representative streaks or stripes of toner-based printing
systems as well as inkjet based systems.
This document describes a methodology in such a way that other documents can apply it for specific
use cases. Such documents will typically need to apply additional constraints on test pages and process
control in order to ensure that the resulting Macro-Uniformity-Score can be compared between
different printing devices, substrates and ink sets.
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TECHNICAL SPECIFICATION ISO/TS 18621-21:2020(E)
Graphic technology — Image quality evaluation methods
for printed matter —
Part 21:
Measurement of 1D distortions of macroscopic uniformity
utilizing scanning spectrophotometers
1 Scope
This document defines a measurement method for the evaluation of distortions in the macroscopic
uniformity of printed areas that are oriented in the horizontal and vertical direction, such as streaks
and bands.
It provides requirements for the layout of the test form, the use of a colour measurement device taking
measurements in a 2D sampling grid, and the formula to compute the Macro-Uniformity-Score.
This document does not cover any non-adjacent or non-horizontal nor ver
...