Photography -- Photographic reflection prints

Photographie -- Tirages photographiques par réflexion

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18-Dec-2019
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ISO/PRF TS 20791-2 - Photography -- Photographic reflection prints
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TECHNICAL ISO/TS
SPECIFICATION 20791-2
First edition
Photography — Photographic
reflection prints —
Part 2:
Evaluation of colour variation in
printing
Photographie — Tirages photographiques par réflexion —
Partie 2: Évaluation de la variation de couleur dans l'impression
PROOF/ÉPREUVE
Reference number
ISO/TS 20791-2:2021(E)
ISO 2021
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ISO/TS 20791-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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.
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Email: copyright@iso.org
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Published in Switzerland
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ISO/TS 20791-2:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

3.1 Terms ............................................................................................................................................................................................................... 2

3.2 Abbreviations ........................................................................................................................................................................................... 2

4 Test target .................................................................................................................................................................................................................... 2

5 Printing .......................................................................................................................................................................................................................... 4

5.1 Environmental conditions ............................................................................................................................................................. 4

5.2 Procedures .................................................................................................................................................................................................. 5

6 Sampling ........................................................................................................................................................................................................................ 5

6.1 Reproducibility within a job ........................................................................................................................................................ 5

6.2 Colour variation between jobs .................................................................................................................................................. 5

6.3 Colour variation between printed images located in different areas on a large sheet ............. 5

7 Measurement ........................................................................................................................................................................................................... 6

7.1 Measurement device .......................................................................................................................................................................... 6

7.2 Timing of measurement .................................................................................................................................................................. 6

7.3 Measurement conditions ......... ....................................................................................................................................................... 6

8 Calculation .................................................................................................................................................................................................................. 6

8.1 General ........................................................................................................................................................................................................... 6

8.2 Calculation of colour difference ............................................................................................................................................... 7

8.3 Colour variation between jobs .................................................................................................................................................. 9

9 Test report ................................................................................................................................................................................................................10

9.1 In laboratory notebook .................................................................................................................................................................10

9.2 For business communications ................................................................................................................................................11

Bibliography .............................................................................................................................................................................................................................12

© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii
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ISO/TS 20791-2:2021(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 on 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 the following

URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
A list of all parts in the ISO 20791 series can be found on the ISO website.
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ISO/TS 20791-2:2021(E)
Introduction

In photographic reflection colour prints, density and colour are essential quality attributes of an image.

However, the density and colour of prints can fluctuate during repeated continuous printing or vary

between two printings done at different times or days, even if the same input data and printer settings

are used. This is a critical issue, particularly in the following cases:

a) When the same photographic image is reprinted to make an extra copy and there is obvious

difference in density and colour between the original and the reprint when they are compared side

by side, the customer might be dissatisfied with this difference.

b) When a photo book is produced by binding up photographic prints into different pages in a book,

and the difference in the density and colour between adjacent pages is easily noticeable, this

difference might be considered unsatisfactory by the customer.

c) When a customer and a print shop agree to the image quality of prints based on a sample or a proof

print, and the difference between the proof sample and the actual prints is discernible, this might

lead to customer dissatisfaction.

Moreover, colour management for high quality printing is attainable only with a stable printing system.

If the colour and density of the actual print differ from that of the designated proof print or sample used

as the basis for the colour management, the quality of the actual print may be different from the desired

image quality.

To improve the reproducibility of printing, or to select the appropriate printing system, the evaluation

of colour variation in printing is crucial. Although the evaluation of colour variation in graphic printing

is described in ISO/TS 15311-1, standardized evaluation methods and procedures for quantifying

colour variation specific to photographic prints are also necessary.
— test target design for the print ranging from a small size to large size;
— sampling procedures (within a job and between jobs);
— measurements and evaluation of colour variation and
— reporting (evaluation results, parameter and mode of printing).

The objective of this document is to provide standard procedures to evaluate colour variation in

photographic printing.
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TECHNICAL SPECIFICATION ISO/TS 20791-2:2021(E)
Photography — Photographic reflection prints —
Part 2:
Evaluation of colour variation in printing
1 Scope

This document describes the procedures for evaluation of colour variation in the printing of

photographic reflection colour prints. The following procedures are described to evaluate colour

variation in printing within a consecutive print job, between several print jobs, or between multiple

photographic images printed in different areas of a large sheet:
a) test targets for small and large photographic prints;
b) printing procedures and conditions;
c) sampling in correspondence to the production scale;
d) measurement of colour;
e) calculation and analysis of colour variation and
f) reporting.

The procedures presented in this document are applicable for prints with a size of available picture

2 2

area ranging from 35 cm (e.g. 5 cm × 7 cm) to 5 400 cm (e.g. 60 cm × 90 cm). This document is

applicable to any of the photographic printing technologies, including inkjet, thermal dye transfer,

electrophotography and silver halide (chromogenic) technologies.
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 5-4, Photography and graphic technology — Density measurements — Part 4: Geometric conditions for

reflection density
ISO 11664-1, Colorimetry - Part 1: CIE standard colorimetric observers
ISO 11664-4, Colorimetry - Part 4: CIE 1976 L*a*b* Colour space

ISO 18944, Imaging materials — Reflection colour photographic prints — Test print construction and

measurement

ISO/TS 21139-1:2019, Permanence and durability of commercial prints — Part 1: Definition of use profiles

and guiding principles for specifications
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
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ISO/TS 20791-2:2021(E)
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Terms
3.1.1
job
one sequence of printing initiated by the order from the print operation system

Note 1 to entry: Even if a pause is automatically inserted between printings, one sequence ordered from the

operating system is considered as one job.
3.2 Abbreviations
CIE International Commission on Illumination
CIELAB CIE 1976 (L*a*b*) colour space
4 Test target

The test target shall be composed of the colour patches in Macbeth Color Checkers shown in Figure 1.

[4]
The colour codes in sRGB as defined in IEC 61966-2-1 are shown in Table 1.

The size of each square colour patch area shall be large enough to cover measured area plus positioning

error. The appropriate size depends on the equipment used. Aperture size requirement shall comply

with the geometric conditions in ISO 5-4.
Table 1 — Colour patch of the test target (RGB value in 8 bit)
R G B R G B
1 Dark skin 115 82 68 13 Blue 56 61 150
2 Light skin 194 150 130 14 Green 70 148 73
3 Blue sky 98 122 157 15 Red 175 54 60
4 Foliage 87 108 67 16 Yellow 231 199 31
5 Blue flower 133 128 177 17 Magenta 187 86 149
6 Bluish green 103 189 170 18 Cyan 8 133 161
7 Orange 214 126 44 19 White 243 243 242
8 Purple blue 80 91 166 20 Neutral 8 200 200 200
9 Moderate red 193 90 99 21 Neutral 6.5 160 160 160
10 Purple 94 60 108 22 Neutral 5 122 122 121
11 Yellow green 157 188 64 23 Neutral 3.5 85 85 85
12 Orange yel- 24
224 163 46 Black 52 52 52
low
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ISO/TS 20791-2:2021(E)

NOTE The numbers shown in the figure are only added here as reference and do not represent an integral

part of the test target when printed.
Figure 1 — An example of the test target design
2 2

When the print size is larger than 35 cm (e.g. 5 cm × 7 cm) and smaller than 2 600 cm (e.g.

43 cm × 60 cm), the 24 solid patches shall be positioned at the centre of the print, as shown in Figure 2.

Key
a, b, c, d
The distance between each edge of the print and the test target.

Figure 2 — An example of the test target arrangement for small (left) and medium (right) print

2 2

When the print size is equal to or larger than 2 600 cm (e.g. 43 cm × 60 cm), and smaller than 5 400 cm

(e.g. 60 cm × 90 cm), the 24 solid patches shall be positioned at the leading edge of the print, as shown in

Figure 3. Other layouts may be used, but the layout used shall be reported.
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ISO/TS 20791-2:2021(E)
Key
The distance between the edge and the patch, g = 25 mm.
The printing direction.
Figure 3 — An example of the test target arrangement for large size print

When multiple photographic images (pages) need to be printed on one larger sheet described in the

previous paragraph, the above test target shall be printed at the centre of each location corresponding

to where each image (page) will be printed on the large sheet. When the locations of the images to be

printed are not specified, the locations may be the centre and near each of the four corners of the large

sheet.
ISO 18944 shall be referred to for more details concerning dimensions.
5 Printing
5.1 Environmental conditions

The standard ambient environmental conditions shall be the temperature of 23 °C ± 3 °C, relative

humidity of 50 % RH ± 10 % RH and air pressure of 101 kPa ± 20 kPa.

The printing test shall be done under this standard ambient environmental condition. However, if it is

not possible to conduct the test under the standard condition, the test may be done under uncontrolled

environmental conditions. In this case, the temperature and relative humidity shall be measured

regularly every 30 min and shall be reported.

In addition to the defined standard ambient environment, it is recommended that the test be performed

under different temperature and relative humidity conditions, for example, at the higher end (e.g.,

35 °C, 90 % RH) and the lower end (e.g., 5 °C, 10 % RH) of the “operating environment” specified for the

printing system. The temperature and relative humidity conditions of the test shall be reported.

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ISO/TS 20791-2:2021(E)
5.2 Procedures
The test target described in Clause 4 shall be printed consecutive
...

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