ISO/TR 20791-1:2020
(Main)Photography — Photographic reflection prints — Part 1: Evaluation methods of image quality
Photography — Photographic reflection prints — Part 1: Evaluation methods of image quality
This document provides information and examples of measurement methods for evaluating the image quality of digital photographic reflection prints, including prints produced by ink jet, thermal dye transfer, electrophotography and silver halide (chromogenic) technologies. These measurement methods are intended especially to be applicable to small prints with a size of available picture area ranging from 35 mm × 45 mm to 360 mm × 450 mm, a popular and basic application of photographic print technology. NOTE The definition of "digital print" here is print made directly from digital data when there is no intermediate image carrier, or when the image carrier is refreshed for each impression, and thus each impression can be different in content if desired.
Photographie — Tirages photographiques par réflexion — Partie 1: Méthodes d'évaluation de la qualité de l'image
General Information
Buy Standard
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 20791-1
First edition
2020-08
Photography — Photographic
reflection prints —
Part 1:
Evaluation methods of image quality
Photographie — Tirages photographiques par réflexion —
Partie 1: Méthodes d'évaluation de la qualité de l'image
Reference number
ISO/TR 20791-1:2020(E)
©
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 20791-1:2020(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/TR 20791-1:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Terms . 1
3.2 Abbreviations . 3
4 Overview of the photographic print image quality . 3
4.1 General . 3
4.2 Density and colour . 4
4.3 Uniformity . 4
4.4 Tone . 4
4.5 Detail reproduction . 4
4.6 Glossiness . 4
4.7 Others . 4
5 Density . 5
6 Colour . 5
7 Uniformity . 7
7.1 General . 7
7.2 Micro uniformity . 8
7.2.1 Granularity . 8
7.2.2 Graininess . 8
7.3 Semi-micro uniformity .10
7.4 Macro uniformity .10
8 Tone .12
8.1 Tone reproduction .12
8.2 Capacity for tone reproduction .12
9 Detail reproduction .13
9.1 General .13
9.2 Resolution measured with patterns of fine lines.13
9.3 Resolution measured with contrast-resolution test target .15
10 Glossiness .16
10.1 General .16
10.2 Specular gloss .16
10.3 Sharpness of reflected light (Image clarity and distinctness of image) .17
Bibliography .20
© ISO 2020 – All rights reserved iii
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ISO/TR 20791-1:2020(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 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.
iv © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 20791-1:2020(E)
Introduction
Image quality of photographic prints is not based on any single feature but instead involves several
factors. There are so-called “five major properties” for representing photographic image quality, and
each one is essential: (1) density; (2) colour; (3) uniformity; (4) tone and (5) detail reproduction.
Glossiness represents another important factor. It affects the observer’s perception since it relates to
the light specularly reflected from prints. Though the specularly reflected light contains no information
from the printed image, it affects the observer’s perception. Furthermore, sharpness of the shape of
reflected light sources also affects the perception.
Several standards for image quality measurement have been documented by ISO/TC 42. Standards
intended for analogue prints fail to properly address the effect of half tone dots, which required to
produce inkjet and electrophotographic prints.
In recent years, the field of imaging science and technology has witnessed the investigation of
measurement methods for digital prints. In addition, standards have been developed by ISO/IEC JTC 1/
SC 28 and ISO/TC 130. Some of these standards cannot be applied broadly to photographic prints
because they focus on a specific technical or industrial field. However, the rest of these standards
handle common aspects and can be applied to photographic prints with or without modifying the
standard. This document provides information about the measurement methods for image quality
that are applicable to photographic prints, including digital prints produced using a range of printing
technologies.
ISO/TC 42 plans to develop test targets for image quality measurements on small size photographic
prints. This document provides methods which can be applied for the measurements.
© ISO 2020 – All rights reserved v
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TECHNICAL REPORT ISO/TR 20791-1:2020(E)
Photography — Photographic reflection prints —
Part 1:
Evaluation methods of image quality
1 Scope
This document provides information and examples of measurement methods for evaluating the
image quality of digital photographic reflection prints, including prints produced by ink jet, thermal
dye transfer, electrophotography and silver halide (chromogenic) technologies. These measurement
methods are intended especially to be applicable to small prints with a size of available picture area
ranging from 35 mm × 45 mm to 360 mm × 450 mm, a popular and basic application of photographic
print technology.
NOTE The definition of “digital print” here is print made directly from digital data when there is no
intermediate image carrier, or when the image carrier is refreshed for each impression, and thus each impression
can be different in content if desired.
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 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 Terms
3.1.1
reflectance factor
ratio of the reflected flux as measured to the reflected flux under the same geometrical and spectral
conditions for an ideal 100 % diffuse reflecting surface
[SOURCE: ISO/IEC 24790:2017, 3.30]
3.1.2
spots per inch
spi
spots per 25,4 mm
[SOURCE: ISO/IEC 24790:2017, 3.34]
3.1.3
visual transfer function
VTF
function used to represent human visual characteristics when dividing the system into parts and
evaluating it as a superposition of transfer functions in the spatial frequency domain
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ISO/TR 20791-1:2020(E)
3.1.4
mura
non-uniformity in the image that should be of uniform density or uniform colour
Note 1 to entry: Mura defects are larger than graininess or mottle, and it generally has ambiguous boundaries.
3.1.5
addressability
number of uniquely identifiable printable spot positions per unit distance
[SOURCE: ISO/IEC 29112:2018, 3.1.1]
3.1.6
area modulation
method of expressing gradation by changing the size or the frequency of dots
3.1.7
density modulation
method of expressing gradation by changing the density of pixels
3.1.8
contrast transfer function
CTF
ratio of the image contrast to the object contrast of a square-wave pattern as a function of spatial
frequency
[SOURCE: ISO/TR 19319:2013, 2.2, modified — Notes 1 and 2 to entry were deleted.]
3.1.9
modulation transfer function
MTF
spatial frequency response
SFR
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
[SOURCE: ISO/IEC 29112:2018, 3.1.16]
3.1.10
line spread function
LSF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely thin line
[SOURCE: ISO 12231:2012, 3.94, modified — Note 1 entry was deleted.]
3.1.11
specular gloss
gloss observed or measured at the specular angle
[SOURCE: ISO 8254-1:2009, 3.8]
3.1.12
image clarity
degree of sharpness of an image reflected by a specimen or transmitted through a specimen
[SOURCE: ISO 17221:2014, 3.1, modified — Note 1 to entry was deleted.]
2 © ISO 2020 – All rights reserved
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ISO/TR 20791-1:2020(E)
3.2 Abbreviations
R red
G green
B blue
V visual
CIE International Commission on Illumination
CIELAB CIE 1976 (L*a*b*) colour space
dpi dots per inch
spi spots per inch
cy/mm cycle per millimetre
DOI distinctness of image
4 Overview of the photographic print image quality
4.1 General
It is commonly understood that there are five major properties for representing the image quality of
photographic prints, and each one is essential in representing the overall image quality. Each property
can be further classified into several sub-properties. Table 1 presents these categorised properties and
some related examples of measurements.
Table 1 — Overview of properties and measurement methods
Property Sub-property Related measurement methods
— Optical density R, G, B and V values for white, grey,
(1) Density a) Optical density
black and each colour step
a) Chromaticity — Colorimetric value in colour space
(2) Colour
b) Colour gamut — Coverage in colour space (area, volume)
— Granularity and graininess (tens to hundreds of
a) Micro uniformity
micrometres)
b) Semi-micro uniformity — Mottle (hundreds to thousands of micrometres)
(3) Uniformity
— Non-uniformity in print area, such as mura,
c) Macro uniformity banding and shading (thousands of micrometres
to the whole printing area)
— A characteristic of a printing system how colour
a) Tone reproduction or density is modulated by the change in the
intensity of input image signal to the system.
(4) Tone
b) Capacity for tone reproduction — Number of tonal steps
(5) Detail — Contrast transfer function (CTF) property and
a) Detail reproduction
reproduction several “spatial resolution” values
The following aspects should be included when evaluating and measuring digital photographic print
image quality, in comparison to that of traditional analogue photographic prints.
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ISO/TR 20791-1:2020(E)
4.2 Density and colour
For density and colour measurement, additional colorants should be considered. In addition to cyan,
magenta and yellow colorants, black colorant is typically used in many digital printing technologies.
In some cases, more colorants, for example, orange or violet colorant, are used to improve density and
colour reproduction. Of course, these additional colorants affect density and colorimetric value itself.
Moreover, additional colorants can expand colour space because they enable vivid colour reproduction
with less turbidity.
4.3 Uniformity
Uniformity measurement includes micro, semi-micro and macro uniformity. Because some digital
photographic prints contain a structure of microdots, the dots themselves can be perceived as a sort
of micro structure. Displacement of dots or colour registration failure can cause misalignment of
microstructure. Misalignment negatively impacts the micro or sub-micro uniformity. Such attributes
should be a concern, especially for digital photographic prints.
NOTE Dot size (or dot area), dot gain, dot placement accuracy, and colour bleed are important parameters
for analyzing the image quality described in this document. Rosettes, moiré and reticulation streaking are also
important parameters to consider when assessing the image quality of digital prints.
4.4 Tone
For tone measurement, the capacity for tone reproduction should be addressed in addition to tone
curve. Depending on the original digital image file format, the image’s capacity to differentiate pixel
tone is limited; for example, 256 levels for 8-bit format, 1 024 levels for 10-bit format. It is important to
measure the distinguishability of each of the available levels in the printed image.
4.5 Detail reproduction
Regarding detail reproduction, micro image structures directly affects how finely an image can be
represented. However, there are many kinds of micro image structure now present in digital imaging
across the variety of print technologies. For digital prints especially, one should be concerned with how
the image is perceived by human eye. This is crucial when comparing the properties of prints produced
with different printing technologies.
4.6 Glossiness
In addition to the five major properties above, reflective properties such as glossiness need to be
addressed for all photographic prints but especially for digital photographic reflection prints. In
traditional silver halide prints, colour images are generated inside the surface gelatin layer. However, in
many types of digital print technologies, colorants are set on substrates, so that colorants themselves
form some parts of the print surface. Thus, the intensity and uniformity of the light reflected from such
prints will change depending on the illumination under which a print is observed.
4.7 Others
The above measurements address the quality of a given print, but the variation or repeatability of image
quality among multiple prints is another important aspect. Colour variation affects not only identity
in print duplication but also overall quality of a print series; for example, a photo book that includes
various scenes in a specified sequence.
In this document, measurement methods for five major properties are described in the following
clauses, from Clause 5 to Clause 9. In addition to the five major properties, glossiness measurement
methods are described in Clause 10. Colour variation in printing will be described in another document
of the ISO 20791 series (future ISO/TS 20791-2).
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ISO/TR 20791-1:2020(E)
5 Density
[7] [8]
Density can be measured according to ISO 5-3 and ISO 5-4 . With these methods, the red (R), green
(G), blue (B) and Visual (V) density values can be assessed in a single measurement. A test chart for
density measurements consists of several patches of sufficient size to be measured with an applied
densitometer. A typical test chart is shown in Figure 1. It has grey, cyan, magenta, yellow, red, green and
blue tone patches, with the numerical digital value at equal intervals. The maximum density of each
colour and the density of D patch in the test chart are especially important. The results of density
min
measurements can also be used to evaluate the gamma attribute, as well as tone curve.
[9]
Figure 1 — An example of a test chart for density and colour measurements (ISO 18944 )
6 Colour
[10] [11] [12]
Chromaticity can be measured according to ISO 11664-1 , ISO 11664-4 and ISO 13655 . With
these methods, colorimetric values L*, a*, and b* in CIELAB 1976 colour space can be assessed in a single
measurement and subsequent calculations. Conforming to ISO 13655, CIELAB values are computed
using CIE illuminant D50 and the CIE 1931 standard colourimetric observer (often referred to as the
2° standard observer). Other illuminants can be used depending on circumstances and purpose of the
measurement. A test chart for chromaticity measurements consists of several patches of sufficient size
to be measured with an applied colorimeter. An example test chart is shown in Figure 1. Grey, cyan,
magenta, yellow, red, green, blue and white patches are fundamental components of the test chart.
Special colours can be added, for example, skin tones, leaf green and sky blue.
In addition to colorimetric value itself, coverage in colour space is an important concern for digital
photographic prints. Since a*-b* chromaticity diagrams represent considerable information while
remaining visually simple, they have been used widely in photography analysis. Examples of an a*-b*
chromaticity diagram are shown in Figure 2.
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ISO/TR 20791-1:2020(E)
Figure 2 — Examples of an a*-b* chromaticity diagram
[13]
Coverage in three-dimensional colour space is measured according to ISO/TS 18621-11 . Colour space
coverage can thus be calculated as volume in CIELAB 1976 colour space. A sufficient number of patches
is needed for the calculation of volume. An example test chart including 756 patches, as stipulated in
ISO/TS 18621-11, is shown in Figure 3. If the print size is not large enough to contain the full test chart
with suitable patch sizes, the test chart should be divided and printed across multiple prints
Figure 3 — An example of a test chart for measurement of coverage in colour space
For measuring volume in CIELAB 1976 colour space, ISO/TS 18621-11 provides a way to obtain
measurement values; for example, the volume of a single gamut, or the volume of the interaction of
two gamuts. A schematic image of coverage in colour space is shown in Figure 4. For evaluation of a
photographic image, reporting the value of colour gamut area or colour gamut volume is not sufficient,
because:
a) the value does not indicate the dependence of luminance range, and
b) the value does not indicate the hue direction which is sometimes more important than the value of
the colour gamut.
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ISO/TR 20791-1:2020(E)
Figure 4 — Schematic 3D image of coverage in colour space
In those cases, a set of graphs of a*-b* plane at a range of L* values is effective, providing a clear visual
depiction of features of colour reproduction from shadow to highlight. Examples of such graph sets are
shown in Figure 5. This type of graph is more preferable for quantitative and scientific representation
of photographic image than a schematic 3D image such as Figure 4.
a) L*=20 b) L*=40 c) L*=60 d) L*=80
Key
system A
system B
Figure 5 — Example graphs of area coverage in the a*-b* plane at specific L* values
7 Uniformity
7.1 General
For uniformity measurement, spatial dimensions are important. Uniformity is often sub-categorised
at three levels: micro, semi-micro and macro uniformity. Variation of density, reflectance factor or
colorimetric value is measured in each sub-category and calculated according to its specific size range.
— Micro uniformity: spatial fluctuation from tens to hundreds of micrometres.
— Semi-micro uniformity: spatial fluctuation from hundreds to thousands of micrometres.
— Macro uniformity: spatial fluctuation from thousands of micrometres to the whole printing area.
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ISO/TR 20791-1:2020(E)
Both micro uniformity and semi-micro uniformity are sometimes called “noise” if the fluctuations
are aperiodic. Macro uniformity is sometimes named according to its obvious visual feature such as
“banding,” “streak,” “shading” and so on.
7.2 Micro uniformity
7.2.1 Granularity
[14]
Micro uniformity is sometimes measured with “granularity.” ISO 10505 provides a photographic
film measurement method called root mean square granularity. Density fluctuation is measured with
a microdensitometer, and the root mean square value is calculated. A similar method can be applied to
reflection analogue prints. However, this method is not appropriate for modern digital prints that have
two-dimensional micro structures.
A simple way to measure two-dimensional granularity is the “graininess metric”. Instead of
a microdensitometer, this method uses a scanner with a minimum of 600 spi to obtain a two-
[15]
dimensional density map as stipulated in ISO/IEC 13660 , which has been withdrawn and replaced
[1]
by ISO/IEC 24790 mentioned below. The two-dimensional density map is divided into small tiles, as
shown in Figure 6. Granularity is then calculated as the root mean square deviation from the mean of
density among tiles.
NOTE Within each tile of Figure 6, there are 900 evenly-spaced scanned pixels and the standard deviation of
900 density data is calculated. The size of each scanned pixel is 42 μm. It is a little bit larger than other referred
methods but within the range of micro uniformity in this document (several tens to hundreds of micrometres).
Figure 6 — Schema for measuring “graininess metric”
7.2.2 Graininess
In imaging science and technology fields, several methods have been proposed that take human visual
[16][17][18]
perception into account . Results of these methods are called “graininess” because they are
a type of psychophysical property. Using these methods, the print image is scanned with a flat bed
scanner. Two-dimensional digitised data from the scanned image are processed, and a graininess value
is calculated. Power spectrum analysis and filtering using a visual transfer function are the essential
components of the calculation. A schematic of this calculation process is shown in Figure 7. This
process was originally proposed by R. P. Dooley and R. Shaw for perceptual noise measurements in
[16]
electrophotography .
8 © ISO 2020 – All rights reserved
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ISO/TR 20791-1:2020(E)
Figure 7 — Schematic calculation process for granularity value
[1]
The measurement of a modified “graininess metric” was proposed in ISO/IEC 24790 . In ISO/IEC 24790,
graininess is defined as appearance of unintended microscopic, but visible aperiodic fluctuations of
lightness. It is noted that microscopic means variations with spatial frequencies greater than about
0,4 cy/mm. In this modified method, the print image is scanned with a scanner with a minimum
1 200 spi. Power spectrum an
...
TECHNICAL ISO/TR
REPORT 20791-1
First edition
Photography — Photographic
reflection prints —
Part 1:
Evaluation methods of image quality
Photographie — Tirages photographiques par réflexion —
Partie 1: Méthodes d'évaluation de la qualité de l'image
PROOF/ÉPREUVE
Reference number
ISO/TR 20791-1:2020(E)
©
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 20791-1:2020(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/TR 20791-1:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Terms . 1
3.2 Abbreviations . 3
4 Overview of the photographic print image quality . 3
4.1 General . 3
4.2 Density and colour . 4
4.3 Uniformity . 4
4.4 Tone . 4
4.5 Detail reproduction . 4
4.6 Glossiness . 4
4.7 Others . 4
5 Density . 5
6 Colour . 5
7 Uniformity . 7
7.1 General . 7
7.2 Micro uniformity . 8
7.2.1 Granularity . 8
7.2.2 Graininess . 8
7.3 Semi-micro uniformity .10
7.4 Macro uniformity .10
8 Tone .12
8.1 Tone reproduction .12
8.2 Capacity for tone reproduction .12
9 Detail reproduction .13
9.1 General .13
9.2 Resolution measured with patterns of fine lines.13
9.3 Resolution measured with contrast-resolution test target .15
10 Glossiness .16
10.1 General .16
10.2 Specular gloss .16
10.3 Sharpness of reflected light (Image clarity and distinctness of image) .17
Bibliography .20
© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
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ISO/TR 20791-1:2020(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 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.
iv PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 20791-1:2020(E)
Introduction
Image quality of photographic prints is not based on any single feature but instead involves several
factors. There are so-called “five major properties” for representing photographic image quality, and
each one is essential: (1) density; (2) colour; (3) uniformity; (4) tone and (5) detail reproduction.
Glossiness represents another important factor. It affects the observer’s perception since it relates to
the light specularly reflected from prints. Though the specularly reflected light contains no information
from the printed image, it affects the observer’s perception. Furthermore, sharpness of the shape of
reflected light sources also affects the perception.
Several standards for image quality measurement have been documented by ISO/TC 42. Standards
intended for analogue prints fail to properly address the effect of half tone dots, which required to
produce inkjet and electrophotographic prints.
In recent years, the field of imaging science and technology has witnessed the investigation of
measurement methods for digital prints. In addition, standards have been developed by ISO/IEC JTC 1/
SC 28 and ISO/TC 130. Some of these standards cannot be applied broadly to photographic prints
because they focus on a specific technical or industrial field. However, the rest of these standards
handle common aspects and can be applied to photographic prints with or without modifying the
standard. This document provides information about the measurement methods for image quality
that are applicable to photographic prints, including digital prints produced using a range of printing
technologies.
ISO/TC 42 plans to develop test targets for image quality measurements on small size photographic
prints. This document provides methods which can be applied for the measurements.
© ISO 2020 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 20791-1:2020(E)
Photography — Photographic reflection prints —
Part 1:
Evaluation methods of image quality
1 Scope
This document provides information and examples of measurement methods for evaluating the
image quality of digital photographic reflection prints, including prints produced by ink jet, thermal
dye transfer, electrophotography and silver halide (chromogenic) technologies. These measurement
methods are intended especially to be applicable to small prints with a size of available picture area
ranging from 35 mm × 45 mm to 360 mm × 450 mm, a popular and basic application of photographic
print technology.
NOTE The definition of “digital print” here is print made directly from digital data when there is no
intermediate image carrier, or when the image carrier is refreshed for each impression, and thus each impression
[1]
can be different in content if desired. (See ISO 18913 ).
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 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 Terms
3.1.1
reflectance factor
ratio of the reflected flux as measured to the reflected flux under the same geometrical and spectral
conditions for an ideal 100 % diffuse reflecting surface
[SOURCE: ISO/IEC 24790:2017, 3.30]
3.1.2
spots per inch
spi
spots per 25,4 mm
[SOURCE: ISO/IEC 24790:2017, 3.34]
3.1.3
visual transfer function
VTF
function used to represent human visual characteristics when dividing the system into parts and
evaluating it as a superposition of transfer functions in the spatial frequency domain
© ISO 2020 – All rights reserved PROOF/ÉPREUVE 1
---------------------- Page: 6 ----------------------
ISO/TR 20791-1:2020(E)
3.1.4
mura
non-uniformity in the image that should be of uniform density or uniform colour
Note 1 to entry: Mura defects are larger than graininess or mottle, and it generally has ambiguous boundaries.
3.1.5
addressability
number of uniquely identifiable printable spot positions per unit distance
[SOURCE: ISO/IEC 29112:2018, 3.1.1]
3.1.6
area modulation
method of expressing gradation by changing the size or the frequency of dots
3.1.7
density modulation
method of expressing gradation by changing the density of pixels
3.1.8
contrast transfer function
CTF
ratio of the image contrast to the object contrast of a square-wave pattern as a function of spatial
frequency
[SOURCE: ISO/TR 19319:2013, 2.2, modified — Notes 1 and 2 to entry were deleted.]
3.1.9
modulation transfer function
MTF
spatial frequency response
SFR
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
[SOURCE: ISO/IEC 29112:2018, 3.1.16]
3.1.10
line spread function
LSF
normalized spatial signal distribution in the linearized output of an imaging system resulting from
imaging a theoretical infinitely thin line
[SOURCE: ISO 12231:2012, 3.94, modified — Note 1 entry was deleted.]
3.1.11
specular gloss
gloss observed or measured at the specular angle
[SOURCE: ISO 8254-1:2009, 3.8]
3.1.12
image clarity
degree of sharpness of an image reflected by a specimen or transmitted through a specimen
[SOURCE: ISO 17221:2014, 3.1, modified — Note 1 to entry was deleted.]
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3.2 Abbreviations
R red
G green
B blue
V visual
CIE International Commission on Illumination
CIELAB CIE 1976 (L*a*b*) colour space
dpi dots per inch
spi spots per inch
cy/mm cycle per millimetre
DOI distinctness of image
4 Overview of the photographic print image quality
4.1 General
It is commonly understood that there are five major properties for representing the image quality of
photographic prints, and each one is essential in representing the overall image quality. Each property
can be further classified into several sub-properties. Table 1 presents these categorised properties and
some related examples of measurements.
Table 1 — Overview of properties and measurement methods
Property Sub-property Related measurement methods
— Optical density R, G, B and V values for white, grey,
(1) Density a) Optical density
black and each colour step
a) Chromaticity — Colorimetric value in colour space
(2) Colour
b) Colour gamut — Coverage in colour space (area, volume)
— Granularity and graininess (tens to hundreds of
a) Micro uniformity
micrometers)
b) Semi-micro uniformity — Mottle (hundreds to thousands micrometers)
(3) Uniformity
— Non-uniformity in print area, such as mura,
c) Macro uniformity banding and shading (thousands of micrometers
to the whole printing area)
— A characteristic of a printing system how colour
a) Tone reproduction or density is modulated by the change in the
intensity of input image signal to the system.
(4) Tone
b) Capacity for tone reproduction — Number of tonal steps
(5) Detail — Constrat transfer function (CTF) property and
a) Detail reproduction
reproduction several “spatial resolution” values
The following aspects should be included when evaluating and measuring digital photographic print
image quality, in comparison to that of traditional analogue photographic prints.
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4.2 Density and colour
For density and colour measurement, additional colorants should be considered. In addition to cyan,
magenta and yellow colorants, black colorant is typically used in many digital printing technologies.
In some cases, more colorants, for example, orange or violet colorant, are used to improve density and
colour reproduction. Of course, these additional colorants affect density and colorimetric value itself.
Moreover, additional colorants can expand colour space because they enable vivid colour reproduction
with less turbidity.
4.3 Uniformity
Uniformity measurement includes micro, semi-micro and macro uniformity. Because some digital
photographic prints contain a structure of microdots, the dots themselves can be perceived as a sort
of micro structure. Displacement of dots or colour registration failure can cause misalignment of
microstructure. Misalignment negatively impacts the micro or sub-micro uniformity. Such attributes
should be a concern, especially for digital photographic prints.
NOTE Dot size (or dot area), dot gain, dot placement accuracy, and colour bleed are important parameters
for analyzing the image quality described in this document. Rosettes, moiré and reticulation streaking are also
important parameters to consider when assessing the image quality of digital prints.
4.4 Tone
For tone measurement, the capacity for tone reproduction should be addressed in addition to tone
curve. Depending on the original digital image file format, the image’s capacity to differentiate pixel
tone is limited; for example, 256 levels for 8-bit format, 1 024 levels for 10-bit format. It is important to
measure the distinguishability of each of the available levels in the printed image.
4.5 Detail reproduction
Regarding detail reproduction, micro image structures directly affects how finely an image can be
represented. However, there are many kinds of micro image structure now present in digital imaging
across the variety of print technologies. For digital prints especially, one should be concerned with how
the image is perceived by human eye. This is crucial when comparing the properties of prints produced
with different printing technologies.
4.6 Glossiness
In addition to the five major properties above, reflective properties such as glossiness need to be
addressed for all photographic prints but especially for digital photographic reflection prints. In
traditional silver halide prints, colour images are generated inside the surface gelatin layer. However, in
many types of digital print technologies, colorants are set on substrates, so that colorants themselves
form some parts of the print surface. Thus, the intensity and uniformity of the light reflected from such
prints will change depending on the illumination under which a print is observed.
4.7 Others
The above measurements address the quality of a given print, but the variation or repeatability of image
quality among multiple prints is another important aspect. Colour variation affects not only identity
in print duplication but also overall quality of a print series; for example, a photo book that includes
various scenes in a specified sequence.
In this document, measurement methods for five major properties are described in the following
clauses, from Clause 5 to Clause 9. In addition to the five major properties, glossiness measurement
methods are described in Clause 10. Colour variation in printing will be described in another document
of the ISO 20791 series (future ISO/TS 20791-2).
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5 Density
[8] [9]
Density can be measured according to ISO 5-3 and ISO 5-4 . With these methods, the red (R), green
(G), blue (B) and Visual (V) density values can be assessed in a single measurement. A test chart for
density measurements consists of several patches of sufficient size to be measured with an applied
densitometer. A typical test chart is shown in Figure 1. It has grey, cyan, magenta, yellow, red, green and
blue tone patches, with the numerical digital value at equal intervals. The maximum density of each
colour and the density of D patch in the test chart are especially important. The results of density
min
measurements can also be used to evaluate the gamma attribute, as well as tone curve.
[10]
Figure 1 — An example of a test chart for density and colour measurements (ISO 18944 )
6 Colour
[11] [12] [13]
Chromaticity can be measured according to ISO 11664-1 , ISO 11664-4 and ISO 13655 . With
these methods, colorimetric values L*, a*, and b* in CIELAB 1976 colour space can be assessed in a single
measurement and subsequent calculations. Conforming to ISO 13655, CIELAB values are computed
using CIE illuminant D50 and the CIE 1931 standard colourimetric observer (often referred to as the
2° standard observer). Other illuminants can be used depending on circumstances and purpose of the
measurement. A test chart for chromaticity measurements consists of several patches of sufficient size
to be measured with an applied colorimeter. An example test chart is shown in Figure 1. Grey, cyan,
magenta, yellow, red, green, blue and white patches are fundamental components of the test chart.
Special colours can be added, for example, skin tones, leaf green and sky blue.
In addition to colorimetric value itself, coverage in colour space is an important concern for digital
photographic prints. Since a*-b* chromaticity diagrams represent considerable information while
remaining visually simple, they have been used widely in photography analysis. Examples of an a*-b*
chromaticity diagram are shown in Figure 2.
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Figure 2 — Examples of an a*-b* chromaticity diagram
[14]
Coverage in three-dimensional colour space is measured according to ISO/TS 18621-11 . Colour space
coverage can thus be calculated as volume in CIELAB 1976 colour space. A sufficient number of patches
is needed for the calculation of volume. An example test chart including 756 patches, as stipulated in
ISO/TS 18621-11, is shown in Figure 3. If the print size is not large enough to contain the full test chart
with suitable patch sizes, the test chart should be divided and printed across multiple prints
Figure 3 — An example of a test chart for measurement of coverage in colour space
For measuring volume in CIELAB 1976 colour space, ISO/TS 18621-11 provides a way to obtain
measurement values; for example, the volume of a single gamut, or the volume of the interaction of
two gamuts. A schematic image of coverage in colour space is shown in Figure 4. For evaluation of a
photographic image, reporting the value of colour gamut area or colour gamut volume is not sufficient,
because:
a) the value does not indicate the dependence of luminance range, and
b) the value does not indicate the hue direction which is sometimes more important than the value of
the colour gamut.
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Figure 4 — Schematic 3D image of coverage in colour space
In those cases, a set of graphs of a*-b* plane at a range of L* values is effective, providing a clear visual
depiction of features of colour reproduction from shadow to highlight. Examples of such graph sets are
shown in Figure 5. This type of graph is more preferable for quantitative and scientific representation
of photographic image than a schematic 3D image such as Figure 4.
a) L*=20 b) L*=40 c) L*=60 d) L*=80
Key
system A
system B
Figure 5 — Example graphs of area coverage in the a*-b* plane at specific L* values
7 Uniformity
7.1 General
For uniformity measurement, spatial dimensions are important. Uniformity is often sub-categorised
at three levels: micro, semi-micro and macro uniformity. Variation of density, reflectance factor or
colorimetric value is measured in each sub-category and calculated according to its specific size range.
— Micro uniformity: spatial fluctuation from 10 μm to 100 μm.
— Semi-micro uniformity: spatial fluctuation from 100 μm to 1 000 μm.
— Macro uniformity: spatial fluctuation from 1 000 μm to the whole printing area.
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Both micro uniformity and semi-micro uniformity are sometimes called “noise” if the fluctuations
are aperiodic. Macro uniformity is sometimes named according to its obvious visual feature such as
“banding,” “streak,” “shading” and so on.
7.2 Micro uniformity
7.2.1 Granularity
[15]
Micro uniformity is sometimes measured with “granularity.” ISO 10505 provides a photographic
film measurement method called root mean square granularity. Density fluctuation is measured with
a microdensitometer, and the root mean square value is calculated. A similar method can be applied to
reflection analogue prints. However, this method is not appropriate for modern digital prints that have
two-dimensional micro structures.
A simple way to measure two-dimensional granularity is the “graininess metric”. Instead of
a microdensitometer, this method uses a scanner with a minimum of 600 spi to obtain a two-
[16]
dimensional density map as stipulated in ISO/IEC 13660 , which has been withdrawn and replaced
[2]
by ISO/IEC 24790 mentioned below. The two-dimensional density map is divided into small tiles, as
shown in Figure 6. Granularity is then calculated as the root mean square deviation from the mean of
density among tiles.
NOTE Within each tile of Figure 6, there are 900 evenly-spaced scanned pixels and the standard deviation of
900 density data is calculated. The size of each scanned pixel is 42 μm. It is a little bit larger than other referred
methods but within the range of micro uniformity in this document (several 10 μm to 100 μm).
Figure 6 — Schema for measuring “graininess metric” (ISO/IEC 24790)
7.2.2 Graininess
In imaging science and technology fields, several methods have been proposed that take human visual
[17][18][19]
perception into account . Results of these methods are called “graininess” because they are
a type of psychophysical property. Using these methods, the print image is scanned with a flat bed
scanner. Two-dimensional digitised data from the scanned image are processed, and a graininess value
is calculated. Power spectrum analysis and filtering using a visual transfer function are the essential
components of the calculation. A schematic of this calculation process is shown in Figure 7. This
process was originally proposed by R. P. Dooley and R. Shaw for perceptual noise measurements in
[17]
electrophotography .
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Figure 7 — Schematic calculation process for granularity value
[2]
The measurement of a modified “graininess metric” was proposed in ISO/IEC 24790 . In ISO/IEC 24790,
graininess is defined as appearance of unintended microscopic, but visible aperiodic fluctuations of
lightness. It is noted that microscopic means variations with spatial frequencies greater th
...
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