ISO 12641:1997

IS0 12641:1997(E)
Contents
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
1 Scope .
......................................... 1
2 Normative references
................................................. 1
3 Definitions
...............................................
4 Requirements
4.1 Targetdesign .
......................................... 3
4.2 Transmission targets
............................................
4.3 Reflection targets
...........................
4.4 Allowable tolerances on patch values
................ 11
4.5 Spectral measurement and calorimetric calculation
............................................. 11
4.6 Data reporting
............................................
4.7 Datafileformat
.......................................... 12
4.8 Useable target life
Annexes
....................... 13
A Gamut mapping - computational reference
........................................... 14
B Application notes
............................................ 17
C Data file format
0 IS0 1997
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 the publisher.
International Organization for Standardization
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Printed in Switzerland
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@ IS0
lSO12641:1997( E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide federation of
national standards bodies (IS0 member bodies). The work of preparing
International Standards is normally carried out through IS0 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. IS0 collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 12641 was prepared by Technical Committee
ISO/TC 130, Graphic technology.
Annexes A, B, and C of this International Standard are for information only.
0.”
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IS0 12641 :1997(E) @ IS0
Introduction
The technical requirements of this International Standard are identical to the
American National Standards IT8.7/1- 1993 and IT8.7/2- 1993. These documents
resulted from the joint efforts of an international industry group that included
participants representing a broad range of prepress vendors, film manufacturers, and
users. This group, initially identified as the DDES (Digital Data Exchange
Standards) Committee, later became the founders of the ANSI IT8 (Image
Technology) accredited standards committee which is responsible for electronic data
exchange standards in graphic arts prepress.
Purpose of this International Standard
Colour input scanners do not all analyze colour the same way the human eye does.
These devices are designed to optimize the signal generated when typical materials
are scanned. Colour reflection and transparency products use various combinations
of proprietary dye sets to achieve visual responses that simulate the colour appea-
rance of natural scene elements. The ability to achieve the same colour appearance
from different combinations of dyes is referred to as metamerism. Because both
photographic dyes and input scanner sensitivities vary from product to product, there
is a variability in the input scanner response to metameric colours produced by the
various materials. The intent of this International Standard is to define an input test
target that will allow any colour input scanner to be calibrated with any film or paper
dye set used to create the target. This International Standard is intended to address
the colour reflection and transparency products which are generally used for input to
the preparatory process for printing and publishing.
The target was designed to be useable for calibration by visual comparison and as a
numerical data target for electronic systems and future development. The target
design made use of a uniform colour space to optimize the spacing of target patches.
The tolerances developed for individual coloured patches meet the values needed for
both numerical and visual analysis.
Design of the target
The CIE 1976 (L*a*b*) or CIELAB colour space was chosen as the space to be used
for the design of the colour calibration target. Uniform spacing in hue angle,
lightness and chroma, and tolerancing in terms of differences in these parameters
(AEzb) is believed to provide a reasonable distribution of coloured patches in the
most effective manner. Although CIELAB was defined with reference to reflection
viewing conditions, tolerancing in terms of vector differences (AE,*& does provide a
reasonable error estimate for transmission materials as well, although the uniformity
of the space is dependent upon the conditions of viewing.
The design goal was to define a target that would have, as its main part, as many
common coloured patches as was practical, regardless of the dye set used. It was
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IS0 12641:1997(E)
determined that the remainder of the target should define the unique colour
characteristics of the particular dye set used to create a specific target; the values for
each target patch should be established using a common procedure.
To provide a reasonable measure of the colour gamut that is within the capability of
modern colour papers and films, all manufacturers of these products were invited to
provide colour dye data along with the necessary minimum and maximum density
data for each of their image forming colour dye sets. Data were provided by Agfa
Company, Eastman Kodak Company, Fuji Photo Film Company, and Konica
Corporation. These data were then used to estimate the CIELAB colour gamut that
each paper and film dye set could produce. This estimate was achieved by
mathematical modeling (by several of the participating companies) using methods
which were different but gave very similar results. Annex A provides additional
reference material concerning the method used in selecting aim values.
The following documents provide reference information on the computational
methods used in gamut determination:
1. N. Ohta, “The Color Gamut Obtainable by the Combination of Subtractive Color
Dyes. V. Optimum Absorption Bands as Defined by Nonlinear Optimization
Technique.” Journal of Imaging Science, 30[ 11, 9- 12( 1986).
M. Inui, “Fast Algorithm for Computing Color Gamuts,” Color Research and
2.
Application, 18[5], 341-348 (1993).
All computations were based upon the use of the CIE 2 degree observer and D,,
illuminant. All transmission measurements were made using diffuse/normal or
normal/diffuse geometry as defined for total transmittance. All reflection
measurements were made using 0”/45 O or 45 O/O” geometry as defined in IS0
13655. The reference white was assumed to be a perfect diffuser. The use of an
absolute reference allows all colours on similar media (reflection or transmission)
that have the same calorimetric definition to also look the same when viewed at the
same time.
The gamut plots developed were then used to determine the colour gamuts for film
and paper that were common to all of the provided dye families. The limiting values
of chroma were then reduced to 80% of their computed values to create a “common
gamut” for purposes of target design.
The goal was to have all coloured patches defined in the same way (regardless of the
product used) and to have as many patches as practical. The defined colour gamut
therefore required a pattern with a consistent reference. An existing colour input
target provided by Eastman Kodak Company under the designation of “Kodak
Colour Reproduction Guides, Q-6O’rM” was used as a guide in the development of the
target. The Q-60 TM target used 12 approximately uniformly spaced hue angles in
CIELAB. These were sampled at three chroma values at each of three lightness
levels. Although this pattern does not provide equal spacing in terms of AEZ,,, it
does provide an easily understandable and defined patch arrangement. It was
adopted for these targets with the addition of a fourth product-specific chroma value
at each hue angle/lightness combination.
Lightness levels were chosen for each hue angle to best characterize the gamut at that
hue angle. The three common chroma values were then chosen such that one fell on
the computed 80% chroma limit common to all the products and the others were
equally spaced in chroma between this value and the neutral. The fourth chroma,
which is product-specific, was defined to be the maximum available from each
product at the specific hue angle and lightness level. This provided a consistent
mapping for all products.
IS0 12641:1997(E) 0 IS0
It was also felt to be important to include scales in each of the individual dyes, dye
pairs, and a dye neutral areas to define product minimum and maximum
along with
densi ties.
A “vendor-optional” area was provided so that different target manufacturers could
add unique patches of their own determination beyond those which are required by
this International Standard.
Manufacturing tolerances
In order to permit practical production of these targets, tolerances had to be set
which were capable of being achieved over a significant number of targets.
However, this conflicted with the relatively narrow tolerances required for numerical
colour calibration. Different tolerances were therefore defined for differing
applications, with the objective of minimizing variations as far as was reasonable.
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IS0 12641:1997(E)
INTERNATIONAL STANDARD @ IS0
Graphic technology - Prepress digital data exchange - Colour targets
for input scanner calibration
3 Definitions
1 Scope
This International Standard defines the layout and For the purposes of this International Standard, the
calorimetric values of targets for use in the calibration of a following definitions apply.
photographic product/input scanner combination (as used in
the preparatory process for printing and publishing). One 3.1 CIE tristimulus values: Amounts of the three reference
target is defined for positive colour transparency film and colour stimuli, in the CIE-specified trichromatic system,
another is defined for colour photographic paper. required to match the colour of the stimulus considered.
NOTE 1 In the 193 1 CIE standard calorimetric system, the
tristimulus values are represented by the symbols X, Y, 2.
2 Normative references
3.2 CIELAB colour difference; CIE 1976 L*, a*, b*
The following standards contain provisions which, through
colour difference, AEZ,: Difference between two colour
reference in this text, constitute provisions of this
stimuli defined as the Euclidean distance between the points
International Standard. At the time of publication, the
representing them in L*, a*, b* space. [International
editions indicated were valid. All standards are subject to
Lighting Vocabulary 845-03-551.
revision, and parties to agreements based on this
International Standard are encouraged to investigate the
AE z. = [ ( AL*)2 + ( Aa*) + ( Ab*)2 ] ‘/z
possibility of applying the most recent editions of the
standards indicated below. Members of IEC and IS0
where
maintain registers of currently valid International Standards.
AL*, Aa* and Ab* refer to the difference between
corresponding values for the two stimuli.
ISO/IEC 646: 199 1, Information technology - IS0 7-bit
coded character set for information interchange.
3.3 CIELAB colour space: CIE 1976 L* a* b* colour
space: Three-dimensional, approximately uniform, colour
IS0 1008: 1992, Photography - Paper dimensions -
space produced by plotting in rectangular coordinates the
Pictorial sheets.
quantities L*, a* and b* defined by the equations:
IS0 10 12: 199 1, Photography - Film dimensions -
L* = 116[f(YlY#.,)] - 16
Pictorial sheets .
a* = SOO[f(XlX,,) - f(Y/Y,,)]
b* = 2OO[f(YIY,,) - f(Z/Zn)]
IS0 13655: 1996, Graphic technology - Spectral
measurement and calorimetric computation for graphic
where for:
arts images.
X/X,, > 0,008 856, f(XlX,,) = (XIX,)‘”
Y/Y,, > 0,008 856, f( Y/q,) = (Y/Y,,)“’
CIE 15.2: 1986, Colorimetry (second edition).
Z/Z,, > 0,008 856, f(Z/Z,,) = (ZlZ,,)“3
CIE 17.4: 1987, International Lighting Vocabulary (fourth
and for:
edition).
XIX,, I 0,008 856, f(X/X,,) = 7,786 7(X/X,,)+16/116
7( Y/Y,,)+1 6/l 16
r/Y,, I 0,008 856, f( Y/Y,,) = 7,786
7(Z/Z,,)+16/116
Z/z,, I 0,008 856, f(Z/Z,,) = 7,786

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IS0 12641:1997(E)
confused with minimum neutral density.
and X,,
= 96,422,
= 100,000 and
K,
3.14 minimum neutral density: Minimum density that a
= 82,521, for the conditions of IS0 13655.
zn
photographic product can achieve (maximum transmittance
or reflectance factors) and maintain a C,*, = 0.
Further:
Czb = (a*’ + b*2)s
NOTE 3 It should not be confused with minimum density
(Dmin).
and
h ab = tan“(b*la*)
3.15 maximum density (Dmax): Density corresponding
to the minimum transmittance or reflectance factor that a
where:
photographic product can achieve.
if a*>0
0” < h,, < 90”
b* 2 0
NOTE 4 It is not necessarily neutral in colour and should not be
90” < 180” if a* 5 0
s huh
confused with maximum neutral density.
b*>O
a*<0
180” I h, < 270” if
3.16 maximum neutral density: Density corresponding to
b* I 0
the maximum density that a photographic product can
270” I h,, < 360” if a* 2 0
achieve (minimum transmittance or reflectance factors) and
b* maintain a C,*,) = 0,
[CIE Publication 15.21
NOTE 5 It should not be confused with maximum density
3.4 transmittance factor: Ratio of the measured flux
(Dmax).
transmitted by the sample material to the measured flux
when the sample material is removed from the sampling
3.17 input scanne
...