ISO 17957:2015

INTERNATIONAL ISO
STANDARD 17957
First edition
2015-04-01
Photography — Digital cameras —
Shading measurements
Photographie — Caméras numériques — Mesurages d’ombrage
Reference number
ISO 17957:2015(E)
©
ISO 2015

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ISO 17957:2015(E)

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© ISO 2015
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Published in Switzerland
ii © ISO 2015 – All rights reserved

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ISO 17957:2015(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test conditions and methods . 1
4.1 General . 1
4.2 Apparatus and hardware . 2
4.3 Lighting . 2
4.4 Test chart. 2
4.5 Image/camera settings . 2
5 Analytical approach. 3
5.1 General . 3
5.2 Lightness non-uniformity . 4
5.3 Luminance non-uniformity . 4
5.4 Chrominance non-uniformity . 5
5.5 Total colour non-uniformity . 5
6 Presentations of results . 5
Annex A (normative) Recommended measurement condition . 7
Annex B (informative) Example of shading metric calculation . 8
Bibliography .13
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ISO 17957:2015(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 meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT), see the following URL: Foreword — Supplementary information.
The committee responsible for this document is ISO/TC 42, Photography.
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ISO 17957:2015(E)

Introduction
One common type of image phenomenon seen in digital cameras is a systematic intensity variation
across an image known as luminance shading. There are several causes of luminance shading in a digital
camera: pixels having non-uniform sensitivity across a sensor array, illumination non-uniformity,
lens shading, pixel geometry, and electronic non-uniformity. Characteristics of lens shading include
symmetry about the optical sensor, uniformity across each colour plane, and variation as a function of
n
cos Θ. Luminance shading only affects intensity falloff.
Colour variations within the image can be induced by several factors, all of which will cause varying
degrees of non-neutrality within an ideal uniform-gray exposure. Some of the primary factors include,
(a) a mismatch between the CMOS micro-lens optical acceptance angle and the lens’ chief ray angle over
the field of view, (b) spatially varying spectral transmittance differences from the infrared rejection
filter (usually most significant at the cutoff wavelength), and (c) inherent spectral sensitivity differences
across the array. Consistent, systematic variations can be reduced through correction via image
processing, but there might remain residual colour variations.
The information that follows defines the recommended approach for creating a test scene to use for
the evaluation of luminance shading and colour variations, as well as specifying the conditions for
illumination and exposure. The currently proposed analysis approach to quantify the magnitude of
colour variations is described. It is important to understand that this specification is related to the degree
of colour variations that are apparent in the image, not the specific colour accuracy. Thus, an image that
appeared pink but was uniformly pink over the entire image would be equivalent to a perfectly uniform
gray field relative to this colour non-uniformity specification.
Some part of this International Standard is based on the work done by the Camera Phone Image Quality
group (CPIQ) within the International Imaging Industry Association (I3A), now part of the Institute of
Electrical and Electronics Engineers (IEEE), whose contribution is greatly acknowledged.
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INTERNATIONAL STANDARD ISO 17957:2015(E)
Photography — Digital cameras — Shading measurements
1 Scope
This International Standard defines a method for measuring shading for digital cameras (including camera
phones). The method includes procedures for measuring colour and luminance signal components separately.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 7589, Photography — Illuminants for sensitometry — Specifications for daylight, incandescent
tungsten and printer
ISO 11664-2/CIE, S 014-2/E: Joint ISO/CIE Standard: Colourimetry — Part 2: CIE Standard Illuminants
for Colourimetry
ISO 11664-4/CIE, S 014-4/E: Joint ISO/CIE Standard: Colourimetry — Part 4: CIE 1976 L*a*b* Colour Space
ISO 12231, Photography — Electronic still picture imaging — Vocabulary
IEC 61966-2-1:1999+A1:2003: Multimedia systems and equipment — Colour measurement and management
— Part 2-1: Colour management – Default RGB colour space — sRGB
CIE 15:2004, Colourimetry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
luminance shading
luminance non-uniformity
gradual variation of the luminance signal components within the image field
3.2
colour shading
colour non-uniformity
gradual variation of the chrominance signal components within the image field
4 Test conditions and methods
4.1 General
The following measurement conditions should be used as nominal conditions when measuring the
shading of a digital camera. If it is not possible or appropriate to achieve these nominal operating
conditions, the actual operating conditions shall be listed along with the reported results.
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ISO 17957:2015(E)

4.2 Apparatus and hardware
The following hardware is necessary to control and report the test conditions:
— a uniform field target;
— light source(s);
— luminance meter.
The total uniformity required for the target and for the light source(s) is specified below.
4.3 Lighting
For lightness non-uniformity and luminance non-uniformity measurement, average outdoor daylight (type
D) illuminant shall be used. For chrominance non-uniformity measurement and total colour non-uniformity
measurement, three standard illuminants shall be used to represent the characteristics of (a) average
outdoor daylight (type D), (b) indoor incandescent lighting (type A), and (c) indoor fluorescent lighting
(type F). Other illuminants may be used optionally. The type of illuminant shall be reported in any case.
The recommended daylight illuminant is CIE/D55 (5 503K) with a tolerance of 5 000K to 6 504K. The
recommended indoor incandescent illuminant is CIE/A (2 856K) with a tolerance of 2 700K to 3 200K.
The recommended fluorescent illuminant is type CIE/F2 (4 230K) with a tolerance of 3 900K to 4 600K.
The light source(s) shall be positioned to provide uniform illumination and produce no glare or reflections
from the target.
4.4 Test chart
The target shall provide a spectrally neutral, uniform luminance distribution over the entire imaged field
of view. Spectral neutrality is achieved if the spectral transmittance or reflectance of the target does
not vary by more than 5 % over the range between 420 nm and 750 nm. The construction of the target
can be either transmissive or reflective. Total luminance uniformity of the lighting and the test chart
is very important for this measurement. The luminance uniformity should be within 2,5 %; (i.e. [|Max-
Ave|/Ave < 0,025 and |Min-Ave|/Ave < 0,025]), and shall be within 5 %; (i.e. [|Max-Ave|/Ave < 0,05 and
2
|Min-Ave|/Ave < 0,05]) across the entire imaged area, with an average luminance between 50 cd/m and
2
120 cd/m . The light flux from the target shall be diffuse and shall not include any specular component.
A target containing patterns may also be used, provided the same results as the uniform chart are
obtained by performing a suitable procedure to remove the patterns. When a target containing patterns
is used, exposure shall be adjusted to give an average output value of the central measurement block,
between 110 and 130 in sRGB encoding values.
4.5 Image/camera settings
Exposure shall be adjusted to give an average output value of the central measurement block, between
110 and 130 in sRGB encoding values, when exposure adjustment is available. When average output
values of the peripheral measurement blocks are very low (e.g. in the case of very wide-angle lens), the
exposure level may be adjusted higher. The average output value of the central measurement block shall
be reported in any case.
White balance should be adjusted to render the centre of the image as neutral as possible. Recommended
focus distance is equal to or larger than “(35 mm-equivalent) focal length x 10”. When the focus may
manually be adjusted, focus does not have to be on the test chart. F-number, focal length, focus distance,
ISO sensitivity, and exposure time shall be reported. Images should be saved at maximum pixel number
using the highest quality (i.e. with least compression) setting.
For the recommended measurement conditions, see Annex A.
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ISO 17957:2015(E)

Test target
~ 45° ~ 45°
Illumination
Illumination
Figure 1 — Diagram of recommended measurement condition for a reflective target
5 Analytical approach
5.1 General
For non-uniformity calculation, the image is divided into (2N+1) x (2N+1) non-overlapping rectangular
blocks (N ≥ 5), where N should be determined to include sufficient number of pixels for averaging noise
and local disturbance for each measurement block. The sRGB values of pixels within one block are
averaged. When precipitous variations in the image are visible, it is desirable to increase the “N” value
so that the precipitous variations will not be averaged out. Mean values of the data should be preserved
as floating point or rational numbers as rounding numbers to the nearest code value could result in
significant quantization errors.
CIELAB values are calculated from sRGB values as follows:
a) First, the sRGB values of the pixels: RG,,B in the ith measurement block will be averaged to
jj j
Ri ,,Gi Bi .
() () ()
AveAve Ave
n n n
ii ii
11 1
Ri =⋅ RG i =⋅ GB ()i =⋅ B (1)
() ()
Ave ∑∑j Ave j Ave ∑ j
n n n
i i i
j==11j j=1
where
jth pixel value in measurement block ( j=1 to n );
i
RG,,B
jj j
n is the number of pixels in ith measurement block.
i
b) Convert averaged sRGB values: Ri ,,Gi Bi to linear RGB values in the range of 0 to 1:
() () ()
AveAve Ave
Ri ,,Gi Bi
() () ()
LinLin Lin
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ISO 17957:2015(E)

Ri'( )/=Ri 255 Gi'( )/=Gi 255 Bi'( )/=Bi 255
...