Graphic technology -- Image quality evaluation methods for printed matter

This document specifies the Contrast?Resolution test chart, the requirements on the printing process needed to reproduce this test chart, the required characteristics of a high resolution scanner needed to digitize the information reproduced on printed test charts, and the requirements on the interpretation of this digitized data. It also specifies the resolution-score method for evaluating the perceptual resolution of printed material using the Contrast?Resolution test chart. The procedure specified in this document is intended for a characterization of the perceived resolution of a graphic arts production printing system using the Contrast?Resolution test chart.

Technologie graphique -- Méthodes d’évaluation de la qualité d’image pour les imprimés

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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/TS 18621-31:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TS 18621-31:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TS 18621-31:2020(E)
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

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TS 18621-31:2020(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

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

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).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

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.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TS 18621-31:2020(E)
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.

© ISO 2020 – All rights reserved v
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ISO/TS 18621-31:2020(E)

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
6/6 visual limit
cy/mm at 300 mm
cy/mm at 400 mm
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
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ISO/TS 18621-31:2020(E)

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 perceived well at high contrast, but not as

well at low contrast.

For given viewing conditions (illumination, viewing distance, magnification), measurements at extreme

spatial frequencies are irrelevant to the characterization of the perceived resolution of a printing

system as their effects cannot be seen (e.g. the far right side of Figure 2 or Figure 3).

The illustration shown in Figure 3 also illustrates the peak in visual sensitivity in the mid spatial

frequency range and is a major motivation for the test chart design utilized in this method for evaluating

the perceived resolution of a printing system. A test chart that explores modulation or contrast along

one axis and spatial frequency along an orthogonal axis covers a large fraction of the major contributors

[4]

to the perceived resolution of a printing system. Figure 4 shows the Contrast–Resolution test chart .

© ISO 2020 – All rights reserved vii
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ISO/TS 18621-31:2020(E)
Key
Y resolution, line pairs per millimetre, log steps
X1 contrast
X2 reference tone value = 50%
NOTE Reproduced with permission from Sicofilm A.G.
Figure 4 — Elements of the Contrast–Resolution test chart

In Figure 3, contrast and spatial frequency vary continuously. In Figure 4, each circularly symmetric

element explores a particular sampled contrast and spatial frequency – the individual patches in

the target. The spatial frequency of separation of these circularly symmetric marks and spaces in

each patch is varied logarithmically along the vertical axis of the target and the contrast, or depth

of modulation, is varied logarithmically along the horizontal axis. This logarithmic spacing mimics

the largely logarithmic response characteristics of the human visual system. This representation of

contrast vs. spatial frequency resembles the Campbell and Robson illustration flipped on its side. The

circularly symmetric shape, and the range of values explored in the Contrast–Resolution test chart are

well suited to the characterization of digital printing workflows.

In a conventional printing system, there are processes at four spatial frequencies that interact with each

other to form an image on the substrate. The first frequency is the spatial frequency of detail in an imaged

scene (this is represented by the vertical axis of the Contrast–Resolution test chart). The second spatial

frequency is the sampling frequency of the pixel grid in the digital image to be reproduced. The Contrast–

Resolution test chart shown in Figure 4 is vector based, not a bitmap, therefore there are no image

pixels. The third spatial frequency is the addressability grid of the printing device. The printing system

raster image processor (RIP) maps the image pattern to the addressability grid and then decides, for

each individual addressability location, how to image that spot. For a binary printing device (e.g. offset or

flexo printing), the spot is either turned on or off. For a non-binary output device (e.g. some electrostatic

or inkjet systems), where the output spots can be imaged at more than one gray level, the RIP also

determines at which gray level the output spot needs to be imaged. These individual spots are utilized

viii © ISO 2020 – All rights reserved
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ISO/TS 18621-31:2020(E)

by the RIP to build the screening pattern that carries the tone scale of the image. The spatial repetition

frequency of this screen is the fourth frequency in this printing process. All of these frequencies have the

potential to interfere with one another, and hence have the potential to introduce moiré.

The Contrast–Resolution test chart was designed for visual evaluation. Evaluation starts at the top

of column A (lowest spatial frequency and highest contrast) and moves down the target towards

higher spatial frequencies – note how a moiré pattern gradually develops between the circular lines

and addressability grid of the printer. The observer is tasked to find, for each column of the target,

the patch at the highest spatial frequency at which the circular lines in the patch are still recognizably

reproduced – where no lines or spaces are missing or overlap and where the level of moiré interference

does not obscure the circles. For each column in the target, an index value that is the row number

(each row is a single spatial frequency) of the last recognizable patch is recorded. This operation maps

the threshold curve along columns in the Contrast–Resolution target where circular elements are no

longer recognizable. The area enclosed by this threshold curve can be used as a capability score for the

printing process. In observation, the circular nature of the lines in each pattern tends to average out

any angular dependencies in system resolution.
Figure 5 — Enlarged portion of a Contrast–Resolution target print

Figure 5 shows an enlarged portion of a print made with a 1 200 spot per inch addressability, utilizing

a 133 line per inch dot screen. The circular patterns of the 2,91 cy/mm Row in Columns A through E

are clear. The circular patterns of the 3,76 cy/mm Row in Columns A and B are clear, but are not legible

in Columns C, D or E. The circular pattern of the 4,85 cy/mm Row in Column A is present with some

aliasing. The circular pattern of the 6,25 cy/mm Row in Column A is barely legible with significant

aliasing. The resolution capability of this printer configuration degrades significantly as the contrast is

lowered – none of the other patches in Figure 5 shows a recognizably circular pattern. An illustration

of an index value threshold curve (white line) and its enclosed area (above the white line) is shown in

Figure 6.

The procedure specified in this document provides an automated, objective measurement surrogate of

the detailed visual examination process previously used in the evaluation of the Contrast–Resolution

[5]

test chart. The initial form of this procedure, developed by Liensberger , provided a single valued score

(L-score) that correlated well with subjective impression, based upon the area of a threshold curve

© ISO 2020 – All rights reserved ix
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ISO/TS 18621-31:2020(E)

derived from normalized cross-correlation coefficients. A refinement of this automated procedure

[6]

proposed by Uno and Sasahara and called resolution-score forms the basis for this document. An

international verification test was conducted, involving both objective measurements, using this

improved procedure, and subjective evaluation of Contrast–Resolution test charts printed with a variety

of printing systems. These experiments showed very good correlation of objective measurements with

subjective evaluations using the improved resolution-score procedure.
Figure 6 — Enclosed area above an index value threshold curve

Both objective measurement and subjective evaluation of Contrast–Resolution test charts printed

with process colorants are minimally affected by the low levels of colorant mis-registration present in

modern, well maintained printing systems. The level of colorant mis-registration in printed test charts

should be verified to be low when utilizing the procedure specified in this document with process color

printing.

Clause 4 specifies the requirements of the workflow settings needed to effectively print the Contrast–

Resolution test chart, the setup requirements of the printer utilized to reproduce these test charts, the

requirements of the scanner characteristics needed to effectively digitize the information reproduced

on the printed test charts, and the requirements of the scanner data processing path needed to properly

represent this information for automated evaluation.
Clause 5 specifies the resolution-score measurement procedure.

Clause 6 specifies the reporting of results obtained with the process specified in Clause 5.

x © ISO 2020 – All rights reserved
---------------------- Page: 10 ----------------------
TECHNICAL SPECIFICATION ISO/TS 18621-31:2020(E)
Graphic technology — Image quality evaluation methods
for printed matter —
Part 31:
Evaluation of the perceived resolution of printing systems
with the Contrast–Resolution chart
1 Scope

This document specifies the Contrast–Resolution test chart, the requirements on the printing process

needed to reproduce this test chart, the required characteristics of a high resolution scanner needed to

digitize the information reproduced on printed test charts, and the requirements on the interpretation

of this digitized data. It also specifies the resolution-score method for evaluating the perceptual

resolution of printed material using the Contrast–Resolution test chart.

The procedure specified in this document is intended for a characterization of the perceived resolution

of a graphic arts production printing system using the Contrast–Resolution test chart.

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 5 (all parts), Photography and graphic technology — Density measurements

ISO 14524, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic

conversion functions (OECFs)

ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic

arts images

ISO 16067-1, Photography — Spatial resolution measurements of electronic scanners for photographic

images — Part 1: Scanners for reflective media
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
aliasing

output image artifacts that occur in a sampled imaging system for input images having significant

energy at frequencies higher than one half the spatial sampling frequency of the system

© ISO 2020 – All rights reserved 1
---------------------- Page: 11 ----------------------
ISO/TS 18621-31:2020(E)
3.2
luminance factor
CIE Y

dimensionless ratio of the luminance of the surface element, in the given direction, to that of the perfect

reflecting diffuser identically illuminated and viewed
Note 1 to entry: It is defined by the CIE and denoted as CIE Y.

Note 2 to entry: The luminance factor of the perfect reflecting diffuser identically illuminated is 100.

3.3
CIE L*
metric lightness

function of luminance factor, defined by the CIE which approximates the human visual system response

to achromatic stimuli.
1/3

Note 1 to entry: For luminance factors greater than 0.008856, L* = 116(Y/Y ) – 16. For luminance factors less

than or equal to 0,008 856, L* =903,3 (Y/Y ).

Note 2 to entry: Y is the luminance factor of a white achromatic reference, typically the perfect reflecting

diffuser.
3.4
ICC profile

International Color Consortium's file format used to store transforms from one colour encoding to another,

e.g. from device colour coordinates to profile connection space, as part of a colour management system

[7]
Note 1 to entry: The colour management system is standardized as ISO 15076-1 .
3.5
modulation

difference between the minimum and maximum signal levels divided by the sum of these levels

3.6
modulation transfer function
MTF

ratio, as a function of spatial frequency, of the measured modulation response in a print produced by a

printing system, to the stimulus modulation presented to that printing system
3.7
opto-electronic conversion function
OECF

relationship between the input levels and the corresponding digital output levels for an opto-electronic

digital image capture system
3.8
perceived resolution

subjective impression of the capability of an imaging system to depict fine detail

3.9
reflectance factor

dimensionless ratio of the radiant or luminous flux reflected in the directions delimited by the given

cone to that reflected in the same directions by a perfect reflecting diffuser identically irradiated or

illuminated
3.10
resolution
measure of the ability of a digital imaging system to depict fine detail
2 © ISO 2020 – All rights reserved
---------------------- Page: 12 ----------------------
ISO/TS 18621-31:2020(E)
3.11
scanner dynamic range

difference of the maximum density where the incremental gain is higher than 0,5 and the minimum

density that appears unclipped
Note 1 to entry: The dynamic range is determined according to ISO 21550.
4 Requirements
4.1 General

The method specified in this document provides an evaluation of the fine detail carrying capability of

a printing system comprising both workflow and printing that correlates with perceived resolution.

Sample prints of the Contrast–Resolution test chart, ISO_ConRes20 , produced by a printing system are

scanned by a high resolution scanner. Evaluation of the fine detail carrying capability of the printing

system is done by comparing these high resolution scans of the Contrast–Resolution test chart prints

with a defined reference. Effective practice of this method places requirements on both the printing

system and the scanning system. This method can be applied to most printing technologies and

substrates.
4.2 Apparatus requirements
4.2.1 Printing system requirements

Effective interpretation of the Contrast–Resolution test chart depends upon printing a set of sample

prints of Contrast_Resolution test chart with minimal mis-registration between the different colorants

that may be utilized in the printing process. This requirement is conventionally met by either printing

the single colorant version of the Contrast–Resolution test chart using a single separation colorant, or

by using the well controlled registration of a modern, well maintained, colour managed process colour

printing system to print one of the process colour versions of the Contrast-Resolution test chart. Refer

to Annex B and Annex C for details.
4.2.2 Scanning system requirements

In the evaluation method specified in this document, a high resolution scanning system is utilized

as a surrogate for a human observer. This imposes strict requirements on the setup, calibration and

performance of the scanning system used in the evaluation process. In simplest form, the measurement

device shall be calibrated and able to accurately capture luminance factor (CIE Y) or lightness

information (CIE L*) at a high optical resolution. A scanning system with a useful optical resolution of

1 200 ppi in both the horizontal and vertical direction is required. If the Contrast–Resolution test chart

has bee
...

TECHNICAL ISO/TS
SPECIFICATION 18621-31
First edition
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
Member bodies are requested to consult relevant national interests in ISO/IEC

JTC 1/SC 28,ISO/TC 42 before casting their ballot to the e-Balloting application.

PROOF/ÉPREUVE
Reference number
ISO/TS 18621-31:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TS 18621-31:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TS 18621-31:2020(E)
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 ....................................................................................................................................................................22

Annex D (normative) Scanner conformance requirements ......................................................................................................24

Annex E (normative) Evaluation process conformance.................................................................................................................31

Annex F (normative) Spatial filter .......................................................................................................................................................................33

Bibliography .............................................................................................................................................................................................................................37

© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/TS 18621-31:2020(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

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

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).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

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.
iv PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TS 18621-31:2020(E)
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

[ ]

systems 1 . 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
2 reproduction with limited resolution
3 original (constant amplitude)
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.

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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
6/6 visual limit
cy/mm at 300 mm
cy/mm at 400 mm
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

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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 — Contrast sensitivity function of a human observer

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 perceived well at high contrast, but not as

well at low contrast.

For given viewing conditions (illumination, viewing distance, magnification), measurements at extreme

spatial frequencies are irrelevant to the characterization of the perceived resolution of a printing

system as their effects cannot be seen (e.g. the far right side of Figure 2 or Figure 3).

The illustration shown in Figure 3 also illustrates the peak in visual sensitivity in the mid spatial

frequency range and is a major motivation for the test chart design utilized in this method for evaluating

the perceived resolution of a printing system. A test chart that explores modulation or contrast along

one axis and spatial frequency along an orthogonal axis covers a large fraction of the major contributors

[4]

to the perceived resolution of a printing system. Figure 4 shows the Contrast–Resolution test chart .

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Key
Y resolution, line pairs per millimetre, log steps
X1 contrast
X2 reference tone value = 50%
NOTE Reproduced with permission from the Rochester Institute of Technology.
Figure 4 — Elements of the Contrast–Resolution test chart

In Figure 3, contrast and spatial frequency vary continuously. In Figure 4, each circularly symmetric

element explores a particular sampled contrast and spatial frequency – the individual patches in

the target. The spatial frequency of separation of these circularly symmetric marks and spaces in

each patch is varied logarithmically along the vertical axis of the target and the contrast, or depth

of modulation, is varied logarithmically along the horizontal axis. This logarithmic spacing mimics

the largely logarithmic response characteristics of the human visual system. This representation of

contrast vs. spatial frequency resembles the Campbell and Robson illustration flipped on its side. The

circularly symmetric shape, and the range of values explored in the Contrast–Resolution test chart are

well suited to the characterization of digital printing workflows.

In a conventional printing system, there are processes at four spatial frequencies that interact with each

other to form an image on the substrate. The first frequency is the spatial frequency of detail in an imaged

scene (this is represented by the vertical axis of the Contrast–Resolution test chart). The second spatial

frequency is the sampling frequency of the pixel grid in the digital image to be reproduced. The Contrast–

Resolution test chart shown in Figure 4 is vector based, not a bitmap, therefore there are no image

pixels. The third spatial frequency is the addressability grid of the printing device. The printing system

raster image processor (RIP) maps the image pattern to the addressability grid and then decides, for

each individual addressability location, how to image that spot. For a binary printing device (e.g. offset or

flexo printing), the spot is either turned on or off. For a non-binary output device (e.g. some electrostatic

or inkjet systems), where the output spots can be imaged at more than one gray level, the RIP also

determines at which gray level the output spot needs to be imaged. These individual spots are utilized

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by the RIP to build the screening pattern that carries the tone scale of the image. The spatial repetition

frequency of this screen is the fourth frequency in this printing process. All of these frequencies have the

potential to interfere with one another, and hence have the potential to introduce moiré.

The Contrast–Resolution test chart was designed for visual evaluation. Evaluation starts at the top

of column A (lowest spatial frequency and highest contrast) and moves down the target towards

higher spatial frequencies – note how a moiré pattern gradually develops between the circular lines

and addressability grid of the printer. The observer is tasked to find, for each column of the target,

the patch at the highest spatial frequency at which the circular lines in the patch are still recognizably

reproduced – where no lines or spaces are missing or overlap and where the level of moiré interference

does not obscure the circles. For each column in the target, an index value that is the row number

(each row is a single spatial frequency) of the last recognizable patch is recorded. This operation maps

the threshold curve along columns in the Contrast–Resolution target where circular elements are no

longer recognizable. The area enclosed by this threshold curve can be used as a capability score for the

printing process. In observation, the circular nature of the lines in each pattern tends to average out

any angular dependencies in system resolution.
Figure 5 — Enlarged portion of a Contrast–Resolution target print

Figure 5 shows an enlarged portion of a print made with a 1 200 spot per inch addressability, utilizing

a 133 line per inch dot screen. The circular patterns of the 2,91 cy/mm Row in Columns A through E

are clear. The circular patterns of the 3,76 cy/mm Row in Columns A and B are clear, but are not legible

in Columns C, D or E. The circular pattern of the 4,85 cy/mm Row in Column A is present with some

aliasing. The circular pattern of the 6,25 cy/mm Row in Column A is barely legible with significant

aliasing. The resolution capability of this printer configuration degrades significantly as the contrast is

lowered – none of the other patches in Figure 5 shows a recognizably circular pattern. An illustration

of an index value threshold curve (white line) and its enclosed area (above the white line) is shown in

Figure 6.

The procedure specified in this document provides an automated, objective measurement surrogate of

the detailed visual examination process previously used in the evaluation of the Contrast–Resolution

[5]

test chart. The initial form of this procedure, developed by Liensberger , provided a single valued score

(L-score) that correlated well with subjective impression, based upon the area of a threshold curve

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derived from normalized cross-correlation coefficients. A refinement of this automated procedure

[6]

proposed by Uno and Sasahara and called resolution-score forms the basis for this document. An

international verification test was conducted, involving both objective measurements, using this

improved procedure, and subjective evaluation of Contrast–Resolution test charts printed with a variety

of printing systems. These experiments showed very good correlation of objective measurements with

subjective evaluations using the improved resolution-score procedure.
Figure 6 — Enclosed area of an index value threshold curve

Both objective measurement and subjective evaluation of Contrast–Resolution test charts printed

with process colorants are minimally affected by the low levels of colorant mis-registration present in

modern, well maintained printing systems. The level of colorant mis-registration in printed test charts

should be verified to be low when utilizing the procedure specified in this document with process color

printing.

Clause 4 specifies the requirements of the workflow settings needed to effectively print the Contrast–

Resolution test chart, the setup requirements of the printer utilized to reproduce these test charts, the

requirements of the scanner characteristics needed to effectively digitize the information reproduced

on the printed test charts, and the requirements of the scanner data processing path needed to properly

represent this information for automated evaluation.
Clause 5 specifies the resolution-score measurement procedure.

Clause 6 specifies the reporting of results obtained with the process specified in Clause 5.

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TECHNICAL SPECIFICATION ISO/TS 18621-31:2020(E)
Graphic technology — Image quality evaluation methods
for printed matter —
Part 31:
Evaluation of the perceived resolution of printing systems
with the Contrast–Resolution chart
1 Scope

This document specifies the Contrast–Resolution test chart, the requirements on the printing process

needed to reproduce this test chart, the required characteristics of a high resolution scanner needed to

digitize the information reproduced on printed test charts, and the requirements on the interpretation

of this digitized data. It also specifies the resolution-score method for evaluating the perceptual

resolution of printed material using the Contrast–Resolution test chart.

The procedure specified in this document is intended for a characterization of the perceived resolution

of a graphic arts production printing system using the Contrast–Resolution test chart.

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 5 (all parts), Photography and graphic technology — Density measurements

ISO 14524, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic

conversion functions (OECFs)

ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic

arts images

ISO 16067-1, Photography — Spatial resolution measurements of electronic scanners for photographic

images — Part 1: Scanners for reflective media
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
aliasing

output image artifacts that occur in a sampled imaging system for input images having significant

energy at frequencies higher than one half the spatial sampling frequency of the system

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3.3
luminance factor
CIE Y

dimensionless ratio of the luminance of the surface element, in the given direction, to that of the perfect

reflecting diffuser identically illuminated and viewed
Note 1 to entry: It is defined by the CIE and denoted as CIE Y.

Note 2 to entry: The luminance factor of the perfect reflecting diffuser identically illuminated is 100.

3.4
CIE L*
metric lightness

function of luminance factor, defined by the CIE which approximates the human visual system response

to achromatic stimuli.
1/3

Note 1 to entry: For luminance factors greater than 0.008856, L* = 116(Y/Y ) – 16. For luminance factors less

than or equal to 0,008 856, L* =903,3 (Y/Y ).

Note 2 to entry: Y is the luminance factor of a white achromatic reference, typically the perfect reflecting

diffuser.
3.2
ICC profile

International Color Consortium's file format used to store transforms from one colour encoding to another,

e.g. from device colour coordinates to profile connection space, as part of a colour management system

[7]
Note 1 to entry: The colour management system is standardized as ISO 15076-1 .
3.5
modulation

difference between the minimum and maximum signal levels divided by the sum of these levels

3.6
modulation transfer function
MTF

ratio, as a function of spatial frequency, of the measured modulation response in a print produced by a

printing system, to the stimulus modulation presented to that printing system
3.7
opto-electronic conversion function
OECF

relationship between the input levels and the corresponding digital output levels for an opto-electronic

digital image capture system
3.8
perceived resolution

subjective impression of the capability of an imaging system to depict fine detail

3.9
reflectance factor

dimensionless ratio of the radiant or luminous flux reflected in the directions delimited by the given

cone to that reflected in the same directions by a perfect reflecting diffuser identically irradiated or

illuminated
3.10
resolution
measure of the ability of a digital imaging system to depict fine detail
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3.11
scanner dynamic range

difference of the maximum density where the incremental gain is higher than 0,5 and the minimum

density that appears unclipped
Note 1 to entry: The dynamic range is determined according to ISO 21550.
4 Requirements
4.1 General

The method specified in this document provides an evaluation of the fine detail carrying capability of a

printing system comprising both workflow and printing that correlates well with perceived resolution.

Sample prints of the Contrast–Resolution test chart, ISO_ConRes20 (available at https:// standards

.iso .org/ iso/ ts/ 18621/ -31/ ed -1/ en/ ), produced by a printing system are scanned by a high resolution

scanner. Evaluation of the fine detail carrying capability of the printing system is done by comparing

these high resolution scans of the Contrast–Resolution test chart prints with a defined reference.

Effective practice of this method places requirements on both the printing system and the scanning

system. This method can be applied to most printing technologies and substrates.
4.2 Apparatus requirements
4.2.1 Printing system requirements

Effective interpretation of the Contrast–Resolution test chart depends upon printing a set of sample

prints of this chart with minimal mis-registration between the different colorants that may be utilized

in the printing process. This requirement is conventionally met by either printing the single colorant

version of the Contrast–Resolution test chart using a single separation colorant, or by using the well

controlled registration of a modern, well maintained, colour managed process colour printing system

to print the sRGB version of the Contrast-Resolution test chart. Refer to Annex B and Annex C for details.

4.2.2 Scanning system requirements

In the evaluation method specified in this document, a high-resolution scanning system is utilized

as a surrogate for a human observer. This imposes strict requirements on the setup, calibration and

performance of the scanning system used in the evaluation process. In simplest form, the measurement

device shall be calibrated and able to a
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