ISO 17321-1:2006

INTERNATIONAL ISO
STANDARD 17321-1
First edition
2006-12-01

Graphic technology and photography —
Colour characterisation of digital still
cameras (DSCs) —
Part 1:
Stimuli, metrology and test procedures
Technologie graphique et photographie — Caractérisation de la couleur
des appareils photonumériques —
Partie 1: Stimuli, métrologie et modes opératoires d'essai




Reference number
ISO 17321-1:2006(E)
©
ISO 2006

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ISO 17321-1:2006(E)
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ISO 17321-1:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 DSC colour characterization methods. 2
4.1 General. 2
4.2 Spectral sensitivity-based characterization — Method A. 3
4.3 Target-based characterization — Method B. 4
Annex A (informative) Recommended laboratory set-up for photographing a reflection colour test
target . 8
Annex B (informative) Digital still camera / sensitivity metamerism index (DSC/SMI) . 10
Annex C (informative) Characterization target considerations . 15
Annex D (informative) Calculating natural scene element responses from spectral
characterization data. 22
Bibliography . 25

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ISO 17321-1:2006(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 17321-1 was prepared by Technical Committee ISO/TC 42, Photography in collaboration with
ISO/TC 130, Graphic technology.
ISO 17321 consists of the following parts, under the general title Graphic technology and photography —
Colour characterization of digital still cameras (DSCs):
⎯ Part 1: Stimuli, metrology and test procedures

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ISO 17321-1:2006(E)
Introduction
The spectral responses of the colour analysis channels of digital still cameras (DSCs) do not, in general,
match those of a typical human observer, such as defined by the CIE standard colorimetric observer. Nor do
the responses of different DSCs ordinarily match each other. In characterizing DSCs, it is therefore necessary
to take account of the DSC spectral sensitivities, illumination, and encoding colour space. This part of
ISO 17321 will begin to address these considerations. This part of ISO 17321 defines stimuli (spectral
illumination or a colour target), metrology and photographic test procedures for acquiring DSC
characterization data. It specifies test procedures for “scenes”, the most general picture taking conditions
where metameric colours and a range of illumination sources are encountered. It also specifies test
procedures for hardcopy “originals”, a more narrowly defined picture-taking condition in which the illumination
source and the colorants being imaged are pre-defined.
The ISO 17321 series will distinguish among several possible image representations in different colour
encodings as depicted in Figure 1 which shows the diagram of a generic image workflow for digital
photography.

Figure 1 — Generic image workflow for digital photography
The DSC characterizations obtained using this part of the ISO 17321 will be applicable to raw (sensor-
referred) DSC data. Two alternative methods are described for obtaining these characterization data.
Method A, the spectral method, uses spectral lights as stimuli for measuring the colour performance of a DSC.
Method B, the target method, involves the use of a physical colour test target under specific lighting conditions
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ISO 17321-1:2006(E)
to measure DSC colour performance. Annexes A to C recommend a laboratory set-up for photographing
reflection targets, provide target patch selection criteria, and provide a digital still camera metamerism index.
Some operations (colour pixel reconstruction, flare removal, white balancing) can be performed without
disqualifying the DSC data as being raw. However, operations that render the image data so that they become
output-referred (ready to display or to print) generally do disqualify the data. With such cameras, this standard
can only be applied if the capability exists to extract or to regenerate raw data, e.g. by applying the inverse of
the rendering transform or by tapping the appropriate signals internal to the camera.
The technical experts who have developed this part of ISO 17321 recognize that a standard that could be
applied generally to any (not just raw) DSC output would be desirable. Such a standard is problematic for
DSCs that employ colour-rendering algorithms in order to produce output-referred image data. For such DSCs,
it would frequently be impossible to determine if colour analysis errors relative to the scene or original
captured were due to sensor image encoding errors or to proprietary colour rendering algorithms. The only
way to make this distinction is if the colour rendering used is well documented and available, and the rendered
data can be converted to un-rendered data by inverting the colour rendering. This situation is unlikely to occur
because one of the major differentiators in DSC performance is the colour rendering. Sophisticated colour-
rendering algorithms can be image dependent, and locally varying within an image. This makes it extremely
difficult to reliably determine the exact colour rendering used by analysing captured test scenes.
The purpose of this part of ISO 17321 is both to assist in the characterization of DSCs for colour management
purposes and to assist camera manufacturers in the determination of the colour analysis capabilities of DSCs
that they are developing. This standard is applicable to any DSC intended for photographic or graphic
technology applications. However, for many users it is not practical to apply this part of ISO 17321 to
individual DSCs. Some of the measurements described in this part of ISO 17321 require complex, expensive
measurement equipment. In the case of test targets that are commercially produced, spectral as well as
colorimetric measurement data would ideally accompany the target.
Those unfamiliar with this part of ISO 17321 are encouraged to read through the entire standard (in particular
the informative annexes) before proceeding with DSC characterization, in order to verify appropriateness for
their particular application. In some cases, the procedures described in the multimedia standard,
[5]
IEC 61966-9 might be more applicable.
It is proposed that other parts of ISO 17321 will be developed in the future to deal with other aspects of the
colour characterization of digital still cameras.

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INTERNATIONAL STANDARD ISO 17321-1:2006(E)

Graphic technology and photography — Colour
characterisation of digital still cameras (DSCs) —
Part 1:
Stimuli, metrology and test procedures
1 Scope
This part of ISO 17321 specifies colour stimuli, metrology, and test procedures for the colour characterization
of a digital still camera (DSC) to be used for photography and graphic technology. Two methods are provided,
one using narrow spectral band illumination and the other using a spectrally and colorimetrically calibrated
target. Except for a specific set of permitted data operations, these DSC data are raw.
This part of ISO 17321 does not specify the methods for deriving transformations from raw DSC data in order
to estimate scene colorimetry.
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 7589:2002, Photography — Illuminants for sensitometry — Specifications for daylight, incandescent
tungsten and printer
ISO 13655:1996, Graphic technology — Spectral measurement and colorimetric computation for graphic arts
images
ISO 14524:1999, Photography — Electronic still-picture cameras — Methods for measuring opto-electronic
conversion functions (OECFs)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
adopted white
spectral radiance distribution as seen by an image capture or measurement device and converted to colour
signals that are considered to be perfectly achromatic and to have an observer adaptive luminance factor of
unity; i.e. colour signals that are considered to correspond to a perfect white diffuser
NOTE 1 The adopted white can vary within a scene.
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ISO 17321-1:2006(E)
NOTE 2 No assumptions can be made concerning the relation between the adapted or adopted white and
measurements of near perfectly reflecting diffusers in a scene, because measurements of such diffusers will depend on
the illumination and viewing geometry, and other elements in the scene that can affect perception.
[ISO 22028-1]
3.2
digital still camera
DSC
device that incorporates an image sensor and that produces a digital signal representing a still picture
NOTE A digital still camera is typically a portable, hand-held device. The digital signal is usually recorded on a
removable memory, such as a solid-state memory card or magnetic disk.
3.3
opto-electronic conversion function
OECF
relationship between log of input levels and corresponding digital output levels for an opto-electronic digital
image capture system
NOTE If the input log exposure points are very finely spaced and the output noise is small compared to the
quantization interval, the OECF possibly has a step-like character. Such behaviour is an artefact of the quantization
process and needs to be removed by using an appropriate smoothing algorithm or by fitting a smooth curve to the data.
3.4
raw DSC image data
image data produced by, or internal to, a DSC that has not been processed, except for A/D conversion and
the following optional steps:
⎯ linearization,
⎯ dark current/frame subtraction,
⎯ shading and sensitivity (flat field) correction,
⎯ flare removal,
⎯ white balancing (e.g. so the adopted white produces equal RGB values or no chrominance),
⎯ missing colour pixel reconstruction (without colour transformations).
3.5
spectrally non-selective
exhibiting reflective or transmissive characteristics that are constant over the wavelength range of interest
4 DSC colour characterization methods
4.1 General
Two methods are specified for obtaining raw DSC colour characterization data, a spectral method and a target
method. The method that is most applicable in any particular situation depends on a variety of factors
including, but not limited to, the following:
⎯ the extent of one's prior knowledge about the spectral content of the scenes or originals to be captured;
⎯ the equipment available;
⎯ the accuracy required.
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ISO 17321-1:2006(E)
The spectral method requires elaborate equipment in a laboratory environment, but can be used to produce
characterization data for samples with arbitrary spectral distributions. The target method is suitable for studio
and field use, but can only provide accurate characterization data to the extent that the target spectral
characteristics match those of the scene or original to be photographed.
4.2 Spectral sensitivity-based characterization — Method A
4.2.1 Equipment
4.2.1.1 General
Spectral sensitivity-based characterization measurements shall be obtained by using a light source and
monochromator to evenly illuminate a diffuse transmissive or reflective surface with electromagnetic radiation
(light) containing a limited range of wavelengths centred on selected wavelengths, as specified in 4.2.3.
Integrated relative radiance measurements of the illuminated surface shall be obtained for each selected
wavelength using a radiance or irradiance meter with a spectral sensitivity calibration accurate to within 0,1 %
and traceable to a national standards laboratory.
4.2.1.2 Light source
The light source shall output radiation where the power is a smooth function of the wavelength, such as that
obtained from a quartz-halogen source. Light sources that have strong emission lines shall not be used.
NOTE A fluorescent lamp is a typical light source with strong emission lines.
4.2.1.3 Monochromator spectral sampling and band pass
The bandpass of the illuminating instrument (monochrometer) shall be 5 nm or narrower. The sampling
interval shall not be greater than the bandpass. The monochromator should exhibit an approximately
triangular band pass, with the full width at half-maximum wavelength range approximately equal to the sample
spacing. The integrated radiance at all wavelengths more than 10 nm from the peak wavelength on which the
monochromator is set shall be less than 1/1 000, and should be less than 1/10 000, of the integrated radiance
within 10 nm of the peak radiance. Interference filters or a double monochromator may be used to meet this
requirement.
4.2.1.4 Illuminated surface
The illuminated surface should be the interior of an integrating sphere. It is recommended to obtain an
integrating sphere with three ports close together. A transmissive diffuser is placed over one port, and
illuminated by the monochromator. This produces an even illumination on the interior of the sphere. The
second port is for the DSC, and the third port is for the radiance or irradiance meter. Other evenly illuminated
surfaces may be used, but it is the responsibility of the user to ensure such surfaces do not have
characteristics that could influence the measurements. In all cases, stray light shall be prevented from
entering the integrating sphere or camera.
NOTE 1 This can be achieved by carefully enclosing the integrating sphere with the camera attached with an opaque
black fabric or plastic.
NOTE 2 The radiances produced for this measurement, and for the OECF measurement described in 4.2.3, need to be
comparable to those encountered in the normal operation of the digital camera.
4.2.2 Camera settings
Fixed exposure settings shall be selected to provide peak output levels between 50 % and 90 % of saturation.
Any automatic gain or adaptive tone reproduction (analog or digital) shall be disabled, compression shall be
minimized, and all user settings shall be recorded. White balancing (analog or digital) shall be fixed, so that
variations in white balance do not influence the measurements. Flash should be disabled to reduce the
possibility of stray light.
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ISO 17321-1:2006(E)
4.2.3 Capturing of raw images of the output of the monochromator
The procedure for capturing raw image data using Method A shall be as follows.
a) Use a monochromator to illuminate a diffuse transmitting or reflecting surface with light centred on
selected wavelengths so the illuminated area is large enough to fill the field of view of the DSC. The
radiance fall-off at the DSC focal plane should be even, constant as the monochromator peak wavelength
is changed, and circularly symmetric with the radiance at the edge no less than 70 % of the radiance at
the centre.
b) Use a radiance or irradiance meter to measure the relative radiance of the illuminated surface as a
function of wavelength.
c) Capture images of the illuminated surface at wavelengths ranging from 360 nm to at least 830 nm, and
preferably to 1 100 nm in 10 nm or smaller increments. The DSC shall be set up as described in
ISO 14524 for alternative focal plane OECF measurements. The images shall be captured with the DSC
lens and any filters used for general picture taking (such as an infrared blocking filter) in place. The data
output by each colour analysis channel of the DSC shall remain independent, i.e. not be matrixed. The
relative radiance of the surface shall also be recorded for each image. Where the DSC under test can be
shown to have essentially no sensitivity at wavelengths within the above wavelength ranges, these
ranges may be truncated appropriately.
d) Determine the alternative focal plane OECF of the DSC in accordance with ISO 14524, except that the
measurement may be performed at the peak sensitivity wavelength for each colour analysis channel.
4.2.4 Post-processing of the data
Use the inverse alternative focal plane OECF to linearize the raw DSC responses at each wavelength.
Average a 64 × 64 pixel block of values at the centre of each image to determine the linearized DSC response
at each wavelength.
4.2.5 Calculation of the relative spectral sensitivities of the DSC
Calculate the relative spectral sensitivities at each wavelength for each colour analysis channel by dividing the
linearized DSC response by the relative surface radiance. Optionally, DSC relative response values for
various scene spectral radiances may be calculated by taking the scalar product of the spectral radiance and
the spectral sensitivity vectors for each DSC colour channel (see Annex D for more information).
Normalize the spectral sensitivities so the sum of the green channel sensitivities is unity. A different channel
may be normalized to unity if so reported.
NOTE If desired, the OECF of each channel, as measured in accordance with ISO 14524, can be used to determine
absolute spectral sensitivities from the relative spectral sensitivities.
4.2.6 Data reporting
The data shall be reported in tabular form, with the relative spectral sensitivity reported for each channel at
each selected wavelength.
4.3 Target-based characterization — Method B
4.3.1 General
The method to be used for the collection of target-based characterization data consists of imaging a reflective
or transmissive colour target of known spectral and colorimetric characteristics, under specified illumination,
recording the output of the DSC for each patch, and providing these data for subsequent processing.
NOTE When target-based characterization is used, the resultant characterization data is only applicable for similar
geometric and spectral illumination characteristics.
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ISO 17321-1:2006(E)
4.3.2 Test target
The choice of test target to be used shall be a decision of the individual doing the characterization and may be
a commercially available target or a custom target developed for the purpose of digital still camera
characterization. Annex C provides a listing of some of the characteristics that should be considered in
developing an ideal target for DSC characterization.
Regardless of the target used it shall have available a tabulation of the spectral reflectance factor or spectral
transmittance factor for each patch. This data shall be from at least 380 nm to 730 nm at least at every 10 nm
and should be from 360 nm to 830 nm at least at every 10 nm. In addition, colorimetric values for all of the
colour patches should be included. Measurements and computation of colorimetric parameters shall be in
accordance with ISO 13655. The use of telephoto-spectrometer from the identical position where the camera
is set is preferred.
Where the DSC under test can be shown to have essentially no sensitivity at wavelengths within the above
wavelength ranges, the target measurement requirements may be truncated appropriately.
NOTE 1 Two commonly available commercial targets that many have used for this application are the traditional
24 patch ColorChecker and the 237 patch ColorChecker DC Digital Camera Color Reference Chart. (ColorChecker is the
trade name of a product supplied by GretagMacbeth. This information is given for the convenience of users of this part of
ISO 17321 and does not constitute an endorsement by ISO of the product named. Equivalent products can be used if they
can be shown to lead to the same results.)
NOTE 2 If measurements are not performed to 830 nm, there is a possibility that unwanted IR sensitivity will not be
identified during subsequent testing. One approach is to separately look at the transmittance of any UV and IR blocking
filters, the basic DSC filters, and the response of the detector itself. Another approach is illustrated by the example in
ISO 14524 which provides a method for evaluating the sensitivity of a DSC to IR radiation.
4.3.3 Test procedure
4.3.3.1 Test target illumination
4.3.3.1.1 For laboratory characterization using a reflection target
The spectral power distribution for illuminating the test target shall be photographic daylight D55, as defined in
ISO 7589. The illuminance at the target plane should be between 2 000 lx and 4 000 lx and have a maximum
variation of 1 % over the area being imaged. Annex A outlines a recommended laboratory set-up for
photographing a colour reflection test target.
The primary axis of the incident illumination should be approximately 45° to the normal to the centre of the
target area being imaged. Two or more illumination sources, which are equally spaced around the normal to
the centre of the area of the target that is being measured, should be used.
NOTE With the optical axis of the DSC normal to the test target, this will help to minimize the probability of specular
reflections entering the field of view of the DSC.
In qualifying the illumination source, particular attention should be paid to the rolloff in red response. The
spectral distribution index (SDI) described in ISO 7589 assumes a rolloff in red response which is normal with
silver halide films, but does not naturally occur with typical DSC sensors. If a DSC has a long wavelength red
response that is significantly different from that assumed in ISO 7589, the SDI criterion is possibly not
sufficient for qualifying the illumination source. Annex B of ISO 14524 also contains information about the
relevancy of SDI calculations to the qualification of illumination sources for DSCs. If there is some question
about the relevancy of the SDI, the illumination source used should be chosen so that its spectral power
distribution matches that of the desired source as closely as possible, in addition to meeting the SDI criterion.
If it is determined that an IR blocking filter is required for OECF determination, in accordance with ISO 14524,
the long wavelength red and infrared response of the DSC should be checked to determine if the response is
being appropriately dealt with by the DSC’s filters. If the DSC shows abnormally high long wavelength red
response (see the standard red rolloff in ISO 7589), or significant near infrared response, additional filters can
be used with the DSC at all times. It is also possible that the illumination source can be emitting excessive
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ISO 17321-1:2006(E)
amounts of infrared radiation, in which case the IR blocking filter should be placed on the source, and the
source requalified. If measurements of the DSC spectral response to wavelengths from 840 nm to 1 100 nm
were obtained, these spectral response values can be used to further qualify the infrared rejection of the DSC.
The ratio of the sum of spectral responses from 360 nm to 730 nm to the sum of spectral responses from
740 nm to 1 100 nm should be greater than the ISO DSC luminance dynamic range, as measured in
[3]
accordance with ISO 15739 .
4.3.3.1.2 For in situ characterization
When the DSC is to be characterized in situ, the illumination source to be used for actual imaging shall be
used. This can be the digital still camera's own illumination, studio illumination, backlighting for transparent
targets, or natural light (either artificial or daylight).
Where possible, the primary axis of the incident illumination for a reflection target should be approximately 45°
to the normal to the centre of the target area being imaged. Two or more illumination sources, which are
equally spaced around the normal to the centre of the area of the target that is being measured, should be
used.
NOTE Placing the optical axis of the DSC normal to the surface of a reflection test target will help to minimize the
probability of specular reflections entering the field of view of the DSC.
4.3.3.2 Camera focusing
The target distance is dictated by the size of the target, the field of view of the camera, and the focal length(s)
of the camera lens. These should be chosen such that the DSC is in sharp focus for the resulting target
distance.
4.3.3.3 Camera settings
For DSC characterizations performed in laboratory settings, the flash shall be turned off, any automatic gain
control shall be disabled, compression shall be minimized, and all user settings shall be recorded. If possible,
any digital white balancing should be turned off and any analog white balancing should be fixed, so that
variations in the analog white balance do not confound the white balance of the raw DSC data revealed by the
OECFs.
For DSC characterizations performed in situ, compression shall be minimized, and all user settings shall be
recorded. While local conditions can require the use of automatic gain control, if possible it should be disabled.
If possible, any digital white balancing s
...