ASTM E2214-02

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Designation: E 2214 – 02
Standard Practice for
Specifying and Verifying the Performance of Color-
Measuring Instruments
This standard is issued under the fixed designation E2214; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Recent advances in optics, electronics and documentary standard have resulted in a proliferation of
instruments for the measurement of color and appearance of materials and objects.These instruments
possessverygoodperformancebuttherehasbeenlittleprogresstowardstandardizingtheterminology
and procedures to quantify that performance. Therefore, the commercial literature and even some
documentary standards are a mass of confusing terms, numbers and specifications that are impossible
to compare or interpret.
Two recent papers in the literature, have proposed terms and procedures to standardize the
specification, comparison and verification of the level of performance of a color-measuring
,
instrument. Followingthoseprocedures,thosespecificationscanbecomparedtoproducttolerances.
Thisbecomesimportantsothatinstrument users andinstrument makers can agree on howto compare
or verify, or both, that their instruments are performing in the field as they were designed and tested
in the factory.
1. Scope priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
1.1 This practice provides standard terms and procedures
for describing and characterizing the performance of spectral
2. Referenced Documents
and filter based instruments designed to measure and compute
2.1 ASTM Standards:
the colorimetric properties of materials and objects. It does not
D2244 Practice for Calculation of Color Tolerances and
setthespecificationsbutrathergivestheformatandprocessby
Color Differences from Instrumentally Measured Color
which specifications can be determined, communicated and
Coordinates
verified.
E284 Terminology of Appearance
1.2 This practice does not describe methods that are gener-
E1164 Practice for Obtaining Spectrophotometric Data for
ally applicable to visible-range spectroscopic instruments used
Object-Color Evaluation
for analytical chemistry (UV-VIS spectrophotometers).ASTM
2.2 Other Documents:
Committee E13 on Molecular Spectroscopy and Chromatog-
ISO InternationalVocabularyofBasicandGeneralTermsin
raphy includes such procedures in standards under their juris-
Metrology (VIM)
diction.
NIST Technical Note 1297 Guidelines for Evaluating and
1.3 This standard does not purport to address all of the
Expressing the Uncertainty of NIST Measurement Re-
safety concerns, if any, associated with its use. It is the
sults
responsibility of the user of this standard to establish appro-
DIN55600 BestimmungderSignifikanzvonFarbabständen
This practice is under the jurisdiction of ASTM Committee E12 on Color and
Appearance and is the direct responsibility of Subcommittee E12.04 on Color and
Appearance Analysis. Annual Book of ASTM Standards, Vol 06.01.
Current edition approved June 10, 2002. Published August 2002. ISO/IDE/OIML/BIPM, International Vocabulary of Basic and General Terms
Ladson, J., “Colorimetric Data Comparison of Bench-Top and Portable in Metrology, International Organization for Standardization, Geneva, Switzerland,
Instruments,” AIC Interim Meeting, Colorimetry, Berlin, 1995. 1984.
3 6
Rich, D., “Standardized Terminology and Procedures for Specifying and Taylor, Barry N., and Kuyatt, Chris E., Guidelines for Evaluating and
Verifying the Performance of Spectrocolorimeters,” AIC Color 97 Kyoto, Kyoto Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note
1997. 1297, U. S. Government Printing Office, Washington, D. C., 1984.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2214–02
bei Körperfarben nach der CIELAB-Formel 4. Summary of Practice
4.1 This practice defines standardized terms for the most
3. Terminology
common instrument measurement performance parameters
3.1 Definitions of appearance terms in Terminology E284
(repeatability, reproducibility, inter-instrument agreement,
are applicable to this pracitce.
inter-model instrument agreement, accuracy) and describes a
3.2 Definitions of metrology terms in ISO, International
set of measurements and artifacts, with which both the produc-
Vocabulary of Basic and General Terms in Metrology (VIM)
ers and users of color-measuring instruments verify or certify
are applicable to this practice.
the specification and performance of color-measuring instru-
3.3 Definitions of Terms Specific to This Standard:
ments. Following this practice can improve communications
3.3.1 colorimetric spectrometer, n—spectrometer,onecom-
between instrument manufacturers and instrument users and
ponent of which is a dispersive element (such as a prism,
between suppliers and purchasers of colored materials.
grating or interference filter or wedge or tunable or discrete
5. Significance and Use
series of monochromatic sources), that is normally capable of
producing as output colorimetric data (such as tristimulus
5.1 Intoday’scommerce,instrumentmakersandinstrument
values and derived color coordinates or indices of appearance
users must deal with a large array of bench-top and portable
attributes) as well as the underlying spectral data from which
color-measuring instruments, many with different geometric
the colorimetric data are derived.
andspectralcharacteristics.Atthesametime,manufacturersof
3.3.1.1 Discussion—At one time, UV-VIS analytical spec-
colored goods are adopting quality management systems that
trophotometers were used for colorimetric measurements. To-
require periodic verification of the performance of the instru-
day, while instruments intended for use in color measurements
ments that are critical to the quality of the final product. The
share many common components with UV-VIS analytical
technologyinvolvedinopticsandelectro-opticshasprogressed
spectrometers, there are two distinct classes of instruments.
greatlyoverthelastdecade.Theresulthasbeenagenerationof
UV-VISanalyticalspectrometersaredesignedtooptimizetheir
instruments that are both more affordable and higher in
use in chemometric quantitative analysis, which requires very
performance. What had been a tool for the research laboratory
precisespectralpositionandverynarrowspectralwindowsand
is now available to the retail point of sale, to manufacturing, to
moderate baseline stability. Colorimetric spectrometers are
design and to corporate communications. New documentary
designed to optimize their use as simulations of the visual
standards have been published that encourage the use of
colorimeter or as the source of spectral and colorimetric
colorimeters, spectrocolorimeters, and colorimetric spetrom-
information for computer-assisted color matching systems.
eters in applications previously dominated by visual expertise
They allow more tolerance on the spectral scale and spectral
or by filter densitometers. Therefore, it is necessary to
window width but demand much more stability in the radio-
determine if an instrument is suitable to the application and to
metric scale.
verifythataninstrumentorinstrumentsareworkingwithinthe
3.3.2 inter-instrument agreement, n—a form of reproduc-
required operating parameters.
ibility in which two or more instruments from the same
5.2 This practice provides descriptions of some common
manufacturer and model are compared.
instrumental parameters that relate to the way an instrument
3.3.3 inter-model agreement, n—a form of reproducibility
will contribute to the quality and consistency of the production
in which the measurements of two or more instruments from
of colored goods. It also describes some of the material
differentmanufacturersorofdifferentbutequivalentdesignare
standards required to assess the performance of a color-
compared.
measuring instrument and suggests some tests and test reports
3.3.3.1 Discussion—Modern instruments have such high
toaidinverifyingtheperformanceoftheinstrumentrelativeto
precision that small differences in geometric and spectral
its intended application.
design can result in significant differences in the performance
6. Instrument Performance Parameters
of two instruments. This can occur even though both instru-
ments exhibit design and performance bias which are well
6.1 Repeatability is generally the most important specifica-
within the expected combined uncertainty of the instrument
tion in a color-measuring instrument. Colorimetry is primarily
and within the requirements of any international standard.
a relative or differential measurement, not an absolute mea-
3.3.4 mean color difference from the mean, MCDM, n—a
surement. In colorimetry, there is always a standard and a trial
measure of expectation value of the performance of a color-
specimen. The standard may be a real physical specimen or it
measuring instrument.
may be a set of theoretical target values. The trial is usually
3.3.4.1 Discussion—MCDM calculates the average color
similar to the standard in both appearance and spectral nature.
difference between a set of readings and the average of that set
Thus, industrial colorimetry is generally a test of how well the
of readings. MCDM = average(DE(average(Lab)− Lab)), for
i i instrument repeats its readings of the same or nearly the same
i=1to N readings. Any standard color difference or color
specimen over a period of minutes, hours, days, and weeks.
tolerance equation can be used as long as the report clearly
6.1.1 TheISOVIMdefinesrepeatabilityasameasureofthe
identifies the equation being used (see Practice D2244).
randomerrorofareadingandassumesthatthesamplestandard
7 8
DIN,DeutschesInstitutfürNormung,-Taschenbuch49,Farbmittel1,Pgimente, ISO 13655 Spectral Measurement and Colorimetric Computation for Graphic
Füllstoffe,Farbstoffe,DIN5033Teil1bisDIN55929,BeuthVerlagGmbH,Berlin. Arts Images, International Organization for Standardization, Geneva, Switzerland.
E2214–02
deviationisanestimateofrepeatability.Repeatabilityisfurther repeatability is the most common and important type of
defined as the standard deviation of a set of measurements reproducibility. Repeatability and reproducibility have tradi-
taken over a specified time period by a single operator, on a tionally been evaluated in colorimetry by comparing the color
single instrument with a single specimen. This definition is differences of a set of readings to a single reading or to the
similar to that in Terminology E284, except that the ISO
average of the set of readings.
explicitlydefinesthemetricof“closenessofagreement”asthe
6.3 Inter-Instrument Agreement, as defined in 3.3.2, de-
sample standard deviation. Since color is a multidimensional
scribesthereproducibilitybetweentwoormoreinstruments,of
propertyofamaterial,repeatabilityshouldbereportedinterms
identical design. The ISO has no definition or description of
of the multidimensional standard deviations, derived from the
such a concept. This is because in most test results, a method
square root of the absolute value of the variance–covariance
or instrument dependent bias can be assessed. In this situation,
matrix.
such a test measures the consistency of the design and
6.1.2 The time period over which the readings are collected
manufacturing process. Within the technical description of the
must be specified and is often qualitatively described as
standard geometric and spectral parameters for the measure-
“short,” “medium,” or “long.” The definitions of these time
ment of diffuse reflectance factor and color, a significant
frames do not overlap. This is intentional, providing clearly
amount of latitude exists. This latitude results in a random
defined milestones in the temporal stability of test results.
amountofbias.Foragivendesign,amanufacturermayreduce
6.1.2.1 For the purposes of colorimetry, “short” is normally
the random bias, often to a level less than the stability of
the time required to collect a set of 30 readings, taken as fast
reference materials. The most common form of test for
as the instrument will allow. The actual time will vary as a
inter-model instrument agreement is pairwise color difference
function of lamp and power supply characteristics but should
assessment of a series of specimens. Various parameters are
be less than one hour.
reportedintheliteratureincludingtheaveragecolordifference,
6.1.2.2 “Medium” term is normally defined as, at least the
the maximum color difference, the typical color difference, the
period of one work shift (8 h) but less than three work shifts
RMS color difference or the MCDM mean color difference
(one day).
from the mean, taking the average of all instruments as the
6.1.2.3 “Long” term is open ended but is often described as
standard and the other as the test instrument. Using pairs of
any set readings taken over a period of at least 4 to 8 weeks.
instruments and materials one can derive a multivariate confi-
The longest known reported study described readings taken
dence interval against the value 0.0 difference and then test
over a period of 3 ⁄4 years.
individual components to determine which attribute (lightness,
6.2 Reproducibility is the second most important specifica-
chroma, hue) are the significant contributors to the differences
tion in a color-measuring instrument. According to Terminol-
betweeninstruments.Ifagroupofinstrumentsarebeingtested
ogy E284, reproducibility is a form of repeatability in which
then a multivariate analysis of variance (MANOVA) can be
one or more of the measurement parameters have been
performedtotesttheagreementofthemeansoftheinstrument.
systematically changed. Thus the sample is different, the
6.4 Inter-Model Agreement, as defined in 3.3.3, describes
proceduresorinstrumentaredifferent,orthetimeframeisvery
the reproducibility between two or more instruments of differ-
long. The increase of disorder over a very long time changes
ing design. The latitude within the standard geometric and
the instrument systematically and the set of readings really
spectral parameters described in the preceding paragraph is at
compares a “young” instrument with an “old” instrument.
a maximum when the designs differ. The systematic bias may
6.2.1 The ISO VIM defines reproducibility as a type of
increase by factors of from 5 to 10 because of the increased
repeatability in which either the time frame is very long, in
latitude. Standardizing laboratories will report either the alge-
which the operator cha
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