ISO 14524:2009

INTERNATIONAL ISO
STANDARD 14524
Second edition
2009-02-15

Photography — Electronic still-picture
cameras — Methods for measuring opto-
electronic conversion functions (OECFs)
Photographie — Appareils de prises de vue électroniques — Méthodes
de mesure des fonctions de conversion opto-électroniques



Reference number
ISO 14524:2009(E)
©
ISO 2009

---------------------- Page: 1 ----------------------
ISO 14524:2009(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2009
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2009 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 14524:2009(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Test methods. 3
4.1 General. 3
4.2 Camera OECF measurement . 4
4.3 Focal plane OECF measurement (method A) . 4
4.4 Alternative focal plane OECF measurement (method B). 4
5 Illumination. 5
5.1 Focal plane OECF measurement (method A) . 5
5.2 Alternative focal plane (method B) and camera OECF measurement. 5
6 Test conditions . 5
6.1 Temperature and relative humidity. 5
6.2 White balance (only applicable to colour cameras) . 5
6.3 Infrared-blocking (IR-blocking) filter. 6
6.4 Focus. 6
7 Input . 7
7.1 Focal plane log exposures. 7
7.2 Camera OECF chart log luminances. 7
8 Data analysis . 8
9 Presentation of results. 8
9.1 General. 8
9.2 Tabular presentation . 9
9.3 Graphical presentation. 11
Annex A (normative) ISO 14524 camera OECF test chart. 13
Annex B (informative) Relevance of the ISO 7589 spectral distribution index. 20
Annex C (informative) Reporting of the camera OECF in relative reflectance units. 22
Bibliography . 23

© ISO 2009 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 14524:2009(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 14524 was prepared by Technical Committee ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 14524:1999), which has been technically
revised.
iv © ISO 2009 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 14524:2009(E)
Introduction
This International Standard establishes standard methods for measuring the functional relationship between
the focal plane log exposures or scene luminances, and the digital output levels of a digital camera. This
information is required for the development and testing of digital cameras, is used in other electronic
still-picture camera measurement standards and may be helpful in the processing of digital image data.
An opto-electronic conversion function (OECF) measurement standard is required for several reasons, as
outlined below.
a) Well-established measurement methods have been used to determine the characteristic curves for
television cameras, where the characteristic curve is known as the “gamma correction” curve, and for
silver halide photography, where the characteristic curve is known as the “H&D” or “DlogH” curve.
However, these methods cannot be easily or unambiguously applied to the characterization of electronic
still-picture cameras.
b) The sampling and quantization processes found in digital systems present fundamental issues that need
to be addressed in a standardized manner.
c) The flexibility of digital systems complicates the determination and presentation of the functional
relationship between the camera's optical input and digital output levels. This International Standard
attempts to account for all the variables and ensure that results are presented in a consistent fashion.
The OECF of a digital camera might appear to be the analogue of the characteristic curve used in
photography and television, but this observation is only partly true. Characteristic curves show the relationship
between a physical input, such as log exposure or reflectance, and a physical output, such as density or volts.
The OECF, on the other hand, shows the relation between a similar physical input and a digital code value
assigned to the physical response produced by that input. Since this assignation can be arbitrary, digital
values themselves do not have physical meaning or units. For example, a change of a factor of two in digital
values could correspond to a doubling of the physical response to the input, to an order of magnitude change,
or to something else, depending on how the code values are assigned.
In digital photography applications, it is generally not necessary to know the physical response produced in a
digital camera. It is sufficient to know what digital values will be produced by a variety of inputs. Consequently,
this International Standard does not specify how to measure the true characteristic curve of a digital camera.
Rather, it specifies how to measure the relationship between the input to a digital camera and the digital code
values produced. These values are only absolutely meaningful in that they represent information. The
graphical reporting formats specified in this International Standard support this viewpoint by allowing OECFs
to be reported with either digital code values or bits on the vertical axis. This is the convention in information
theory. Users of this International Standard are advised that the actual physical response of a digital camera,
or of a complete digital photography system, can be linear, logarithmic, or something else, regardless of the
form of the OECF plot and whether digital code values or bits are reported on the vertical axis.
NOTE In accordance with the rules given in the ISO/IEC Directives, Part 2, commas are used rather than full-stops
as the decimal radix in this International Standard.

© ISO 2009 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 14524:2009(E)

Photography — Electronic still-picture cameras — Methods for
measuring opto-electronic conversion functions (OECFs)
1 Scope
This International Standard specifies methods for the measurement of opto-electronic conversion functions
(OECFs) of electronic still-picture cameras whose output is encoded as a digital image file. The OECF is
defined as the relationship between the focal plane log exposures or scene log luminances, and the digital
output levels of an opto-electronic digital image capture system.
This International Standard applies to both monochrome and colour electronic still-picture cameras.
2 Normative references
The following referenced documents are indispensable for the application 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 5-1, Photography — Density measurements — Part 1: Terms, symbols, and notations
ISO 5-2, Photography — Density measurements — Part 2: Geometric conditions for transmission density
ISO 5-3, Photography — Density measurements — Part 3: Spectral conditions
ISO 5-4, Photography — Density measurements — Part 4: Geometric conditions for reflection density
ISO 516, Photography — Camera shutters — Timing
ISO 554, Standard atmospheres for conditioning and/or testing — Specifications
ISO 7589:2002, Photography — Illuminants for sensitometry — Specifications for daylight, incandescent
tungsten and printer
3 Terms and definitions
1)
For the purposes of this document, the following terms and definitions apply .
3.1
camera opto-electronic conversion function
camera OECF
relationship between the input scene log luminances and the digital output levels for an opto-electronic digital
image capture system
NOTE The units of measurement for this function are log candelas per square metre.
10

1) Additional definitions of interest can be found in ISO 12232.
© ISO 2009 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 14524:2009(E)
3.2
digital output level
digital code value
numerical value assigned to a particular output level
3.3
electromechanical shutter
mechanical shutter which is electronically controlled
3.4
electronic still-picture camera
camera incorporating an image sensor that outputs an analogue or digital signal representing a still picture
and/or records an analogue or digital signal representing a still picture on a removable medium, such as a
memory card or magnetic disc
3.5
focal plane opto-electronic conversion function
focal plane OECF
relationship between the input focal plane log exposures and the digital output levels for an opto-electronic
digital image capture system
NOTE The units of measurement for this function are log lux seconds.
10
3.6
illuminance scale exposure series
series of exposures produced using a constant exposure time and a varying focal plane illuminance
3.7
incremental gain function
change in the output level (digital code value) divided by the change in the input level (luminance or exposure)
as a function of input level
NOTE 1 For the determination of incremental gain values, log input values are not used.
NOTE 2 If the input exposure points are very finely spaced and the output noise is small compared to the quantization
interval, the incremental gain function can have a jagged shape. Such behaviour is an artefact of the quantization process
and it is advisable to remove this by using an appropriate smoothing algorithm or by fitting a smooth curve to the data. In
some cases, it might be desirable to fit a curve to the input-output data and then determine the incremental gain function
by taking the first derivative of the function used for the curve fit.
3.8
incremental output signal
input level (luminance or exposure; not logged) multiplied by system incremental gain at that level
3.9
maximum exposure limit
smallest exposure which produces the digital output level corresponding to the maximum detectable exposure
NOTE The maximum detectable exposure is also known as the saturation or quantization ceiling.
3.10
minimum exposure limit
largest exposure below saturation which produces an incremental output signal equal in magnitude to the
output noise
3.11
opto-electronic conversion function
OECF
relationship between the log of the input levels and the corresponding digital output levels for an opto-
electronic digital image capture system
2 © ISO 2009 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 14524:2009(E)
NOTE If the input log exposure points are very finely spaced and the output noise is small compared to the
quantization interval, the OECF can have a step-like character. Such behaviour is an artefact of the quantization process
and it is advisable to remove this by using an appropriate smoothing algorithm or by fitting a smooth curve to the data.
3.12
opto-electronic digital image capture system
system which converts either a light exposure at the focal plane, or a spatial arrangement of luminances (a
scene) to digital information
3.13
output noise
root-mean-square fluctuation about the mean in the digital output level for a constant input level
3.14
scene luminance ratio
ratio of the highest (highlight) luminance value to the lowest (shadow) luminance value in a scene
3.15
time scale exposure series
series of exposures produced using a constant focal plane illuminance and a varying exposure time
3.16
white balance
adjustment of electronic still-picture colour channel gains or image processing so that radiation with relative
spectral power distribution equal to that of the scene illumination source is rendered as a visual neutral
4 Test methods
4.1 General
This International Standard describes test methods for measuring both camera OECFs and focal plane
OECFs. Camera OECFs include the effects of the camera lens and associated flare, while focal plane OECFs
do not. These image-formation effects vary with the overall scene luminance ratio, the amounts of each of the
different luminances present in the scene and the spatial arrangement of these luminances. This variability
can be quite large and, consequently, it is possible to determine a repeatable camera OECF only for a specific
scene, such as a test chart. The camera OECF measurement method described in this International Standard
allows for the determination of different camera OECFs based on test charts with different luminance ratios,
but does not allow for the effects of different amounts or spatial arrangements of scene luminances. The
camera OECF test charts are designed to simulate the image formation effects produced by a scene with a
specific luminance ratio and average distribution of luminances; however many scenes are significantly
different from average. When determining camera OECFs, it is important to keep in mind that the OECF
characteristics measured may be quite different from those exhibited by the camera in capturing specific
scenes. The reasons for inclusion of a camera OECF measurement method are as follows:
a) the mandatory automatic exposure control found in some cameras precludes the determination of focal
plane OECFs;
b) the camera OECF measurement method allows for one-step determination of the camera system
characteristics for the scene simulated by the test chart used;
c) focal plane OECF values can be estimated from camera OECF values for the midtone and highlight
regions of most images, provided the range of interest is covered by the test chart used.
The focal plane OECF is a characteristic of the camera only and is not dependent on the scene.
NOTE Some cameras and/or supporting software can contain scene-dependent rendering algorithms. These
algorithms are generally bypassed when performing focal plane OECF measurements because of the approximately
uniform illumination incident on the focal plane. In situations where it is impossible or undesirable to bypass the rendering
algorithms, it is more appropriate to perform camera OECF measurements.
© ISO 2009 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 14524:2009(E)
Two methods are described for focal plane OECF measurement, although both methods should give the
same result. The preferred method (method A) allows for a higher degree of accuracy than the alternative
method (method B). Method B should be used only with cameras that have fixed lenses. The advantages of
focal plane OECFs are as described below.
⎯ Separation of the optical image formation stage from the focal plane image to output stage allows each
stage of the image capture to be dealt with independently. These two stages behave quite differently. The
image formation stage is strongly scene dependent, while the focal plane image to output stage depends
only on the sensor and camera electronics’ characteristics. On the other hand, the response of pictorial
cameras tends to be highly non-linear, complicating the subsequent analysis of optical image formation
effects if the focal plane OECF is not known. The analysis of camera systems is much easier if the two
stages are dealt with independently.
⎯ Traditionally, only the density versus log exposure relation, or characteristic curve, is measured for film.
This curve is analogous to the focal plane OECF.
⎯ The predominant factor affecting camera OECF values in the darker areas of a scene is the camera flare.
These values are, therefore, primarily scene dependent and do not provide much information about the
general camera characteristics.
⎯ Focal plane OECFs cover the entire usable range of the camera and are not limited by the test chart
luminance ratio.
The methods for measurement of the OECFs described above are given in 4.2 to 4.4.
4.2 Camera OECF measurement
The OECF may be determined for the entire camera opto-electronic digital image capture system using a
camera OECF test chart as defined in this International Standard. This determination is accomplished by
using the camera system to capture an image of the chart under controlled conditions. It should be noted that
the independent variable for the camera OECF is scene log luminance, not focal plane log exposure as with
the focal plane (method A) and alternative focal plane (method B) measurement methods.
4.3 Focal plane OECF measurement (method A)
This method involves the exposure of the electronic still-picture camera sensor directly to specific quantities of
uniform illumination with the camera lens removed. The illumination shall have the spectral characteristics
specified in 5.1 and shall be produced by a small source at a distance, such that the largest dimensions of the
source and the sensor are no greater than 1/20 the distance from the source to the sensor. In addition,
reflective surfaces shall not be placed where they could cause additional illumination to be incident on the
sensor.
4.4 Alternative focal plane OECF measurement (method B)
If a particular electronic still-picture camera does not allow the lens to be removed, method B may be
employed. This method involves the use of a uniformly emissive, approximately Lambertian target (reflective
surface or illuminator), which is then imaged by the camera lens on the sensor. If method B is used, the
illuminance falling on the sensor, E , expressed in lux, shall be assumed to be as calculated from Equation (1)
s
(see Reference [9]):
0,65 L
t
E = (1)
s
2
f
e
where
L is the arithmetic mean luminance of the target in candela per square metre;
t
f is the effective f-number of the lens.
e
If method B is used, the target shall be measured to verify that it is approximately Lambertian and uniform in
luminance. Luminance readings of the target shall be within 2 % of the arithmetic mean value for readings
4 © ISO 2009 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 14524:2009(E)
taken normal to the target at all four corners and at the centre of the field of view of the camera, and also for
readings taken at an angle of 30° to normal of the centre of the target. The surface of the target shall be
normal to the optical axis of the camera (± 5°) when the test image is captured, and shall extend out at least
15° beyond the edge of the camera field of view. The spectral radiance characteristics of the target shall be as
described in 5.2.
OECFs obtained using method B of this International Standard shall be designated as such.
5 Illumination
5.1 Focal plane OECF measurement (method A)
OECF measurements shall indicate whether the daylight or tungsten illuminant was used. ISO 7589 describes
the procedures for determining whether the illumination used for OECF measurements is an acceptable match
to the daylight and tungsten sensitometric illuminants.
5.2 Alternative focal plane (method B) and camera OECF measurement
Since these test methods involve measurements with the camera lens in place, the spectral radiance
characteristics of the target, for the alternative focal plane OECF, or the chart illumination source, for the
camera OECF, should be equivalent to either the daylight or tungsten source specified in ISO 7589. The
relative spectral power distributions for these sources are provided in the second column of Tables 1 and 2 in
ISO 7589:2002. In order to apply the ISO 7589 spectral distribution index criterion to these sources, the
spectral radiance of the source or target shall be measured and then multiplied by the relative spectral
transmittance of the ISO 7589 standard lens (also described in ISO 7589), prior to multiplying by the weighted
spectral sensitivities. See Annex B.
With these test methods, the target or chart, and camera lens, shall be shielded from external illumination
sources and reflective surfaces, including the walls, ceiling and floor of the test room, using black shielding
materials. The wall behind the target or chart shall be black and the only illumination sources present shall be
those used to illuminate the chart. For reflective targets or charts, the illumination sources shall be positioned
so that the angular distribution of influx radiance is at its maximum at 45° to the target or chart normal, and is
negligible at angles less than 40° or more than 50° to the normal at any point on the target or chart.
ISO 12233 may be consulted for recommendations for reflection chart illumination geometries.
6 Test conditions
6.1 Temperature and relative humidity
The ambient temperature during the acquisition of the test data shall be (23 ± 2) °C, as specified in ISO 554,
and the relative humidity shall be (50 ± 20) %.
6.2 White balance (only applicable to colour cameras)
6.2.1 Single fixed white balance setting
If a camera has only one fixed white balance setting, either in the camera circuitry or supporting software
supplied with the camera, this setting shall be used for all OECF determinations and the white balance for
these OECFs shall be designated as “fixed”.
6.2.2 Daylight and/or tungsten fixed white balance settings
If a camera has fixed white balance settings designated as “daylight” and/or “tungsten”, either in the camera
circuitry or supporting software, the white balance adjustment may be set at either of these fixed settings. If
© ISO 2009 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO 14524:2009(E)
this white balance option is chosen, the white balance for the OECF(s) determined shall be designated as
“daylight” and/or “tungsten”, depending on the setting chosen.
6.2.3 Variable white balance
6.2.3.1 General
If the camera white balance can be adjusted using a variable white balance adjustment, either in the camera
circuitry or supporting software, the variable white balance option may be chosen. In this case, the white
balance for the OECF(s) determined shall be designated as “variable” and shall be set to provide respectively
neutral digital output levels for the colour channels. Neutral digital output levels means equal Red-Green-Blue
(RGB) levels, or luminance-chrominance levels indicating no chrominance.
6.2.3.2 Focal plane OECFs
The white balance shall be set to provide neutral digital output levels for a focal plane exposure which is
greater than one-half of the maximum exposure limit.
6.2.3.3 Camera OECFs
The white balance shall be set to provide neutral digital output levels for the camera OECF test-chart
background.
6.2.4 Automatic white balance
If a camera automatically adjusts its white balance with every exposure, the white balance for the OECFs
determined shall be designated as “automatic”. It should be noted that in this case the camera OECF may
provide colour information not found in the focal plane OECFs.
6.3 Infrared-blocking (IR-blocking) filter
If required, one or more IR-blocking filters shall be placed in front of the camera lens. These filters are
required if the output signal level of the camera with a visible light blocking, IR-transmitting filter is greater than
5 % of the maximum digital output level, at the exposure or luminance which produces the maximum camera
digital output level without the IR-transmitting filter. The IR-transmitting filter used to test to see whether an IR-
blocking filter is required shall have a maximum edge (50 % transmittance) wavelength of 780 nm and a
transmittance of less than 1 % at wavelengths between 350 nm and 740 nm. The IR-blocking filters should be
chosen to provide the required reduction in IR sensitivity without changing the photometric sensitivity any
more than absolutely necessary. If IR-blocking filters are required, the filter types used shall be indicated. The
purpose of the IR-blocking filters is to prevent the OECF measurement from being significantly affected by
infrared radiation.
6.4 Focus
6.4.1 Alternative focal plane OECF measurement (method B)
If the camera lens focus is adjustable, it shall be set on infi
...