ASTM D2244-93(2000)

Designation:D2244–93 (Reapproved 2000)
Standard Test Method for
Calculation of Color Differences From Instrumentally
Measured Color Coordinates
This standard is issued under the fixed designation D2244; 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.
This standard has been approved for use by agencies of the Department of Defense.
INTRODUCTION
This test method originally resulted from the consolidation of a number of separately published
methodsfortheinstrumentalevaluationofcolordifferences.Asrevisedin1979,itincludedfourcolor
spaces in which color-scale values could be measured by instruments, many of which were obsolete,
and the color differences calculated by ten equations for different color scales. The sections on
apparatus,calibrationstandardsandmethods,andmeasurementproceduresservedlittlepurposeinthe
light of modern color-measurement technology. The present revision omits these sections, and limits
the color spaces and color-difference equations considered, to the three most widely used in the paint
and related coatings industry.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 Thistestmethodcoversthecalculation,frominstrumen-
bility of regulatory limitations prior to use.
tally measured color coordinates based on daylight illumina-
tion, of small color differences between nonfluorescent, non-
2. Referenced Documents
metameric, opaque specimens such as painted panels. (Where
2.1 ASTM Standards:
it is suspected that the specimens may be metameric, that is,
D1729 Practice for Visual Evaluation of Color Differences
possess different spectral curves though visually alike in color,
of Opaque Materials
Practices D1729 and D4086 should be used to verify instru-
D3964 Practice for Selection of Coating Specimens for
mental results.) The color differences determined by these
Appearance Measurements
procedures are expressed in terms of approximately uniform
D4086 Practice for Visual Evaluation of Metamerism
visual color perception in CIE 1976 CIELAB opponent-color
2 E179 Guide for Selection of Geometric Conditions for
space (1), Hunter L , a , b opponent-color space (2), and
H H H
Measurement of Reflection andTransmission Properties of
the Friele-MacAdam-Chickering (FMC-2) color space (3).
Materials
1.2 For product specification, the permissible color differ-
E284 Terminology of Appearance
ence between test specimen and reference and the procedure
E308 Practice for Computing the Colors of Objects by
for calculating the color difference shall be agreed upon by the
Using the CIE System
purchaser and the seller. Specific color tolerances may be
required for each material and condition of use since other
3. Terminology
appearance factors (for example, proximity, gloss, and texture)
3.1 Definitions—For the following definitions as well as for
may affect the correlation between the magnitude of a mea-
other definitions of terms used in this test method, see
sured color difference and its commercial acceptability.
DefinitionsE284:tristimulusvalues,chromaticitycoordinates,
1.3 This standard does not purport to address all of the
and opponent-color scales.
safety concerns, if any, associated with its use. It is the
3.2 Definitions of Terms Specific to This Standard:
3.2.1 color difference:
3.2.1.1 color difference (perceived)—the magnitude and
This test method is under the jurisdiction ofASTM Committee E 12 on Color
character of the difference between two colors described by
and Appearance and is the direct responsibility of Subcommittee E12.04 on Color
such terms as redder, bluer, lighter, darker, grayer, or cleaner.
and Appearance Analysis.
Current edition approved Sept. 15, 1993. Published November 1993. Originally
e1
published as D2244–64T. Last previous edition D2244–89 .
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this method. Annual Book of ASTM Standards, Vol 06.01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D2244–93 (2000)
3.2.1.2 color difference (computed)—the magnitude and formed by three rectangular coordinates representing the light-
direction of the difference between two psychophysical color ness scale Y and chromaticity scales x and y, where:
stimuli defined by tristimulus values, or by chromaticity
X
x 5 (1)
coordinates and luminance factor, as computed by means of a
X 1 Y 1 Z
specified set of color-difference equations.
Y
y 5
X 1 Y 1 Z
4. Summary of Test Method
Y 5 Y
4.1 The differences in color between a reference and a test
specimenaredeterminedfrommeasurementsmadebyuseofa where X, Y, and Z are tristimulus values for either the 1931
spectrophotometer or a colorimeter. Reflectance readings from
CIE standard observer (2° observer) or the 1964 CIE supple-
such instruments are converted by computations to color-scale mentary standard observer (10° observer) and standard illumi-
values, or these color-scale values may be read directly from
nant C, D , or another of daylight quality.These scales do not
instruments that automatically make the computations. Color- provide a perceptually uniform color space. Consequently,
difference magnitudes are computed, from differences in these color differences are seldom if ever computed directly from
color-scale values, that represent the perceived color differ-
differences in x, y, and Y.
ences between the reference and the test specimen.
6.2 CIE 1976 L* a* b* Uniform Color Space and Color-
Difference Equation (1, 4)—This approximately uniform color
5. Significance and Use
space is a simplified version of the Adams-Nickerson color-
5.1 The original CIE color scales based on tristimulus
scale system (5-7). It is produced by plotting in rectangular
values X, Y, Z and chromaticity coordinates x, y are visually
coordinates the quantities L*, a*, b*, calculated as follows:
nonuniform. Each subsequent color scale based on CIE values
1/3
L* 5116~Y/Y ! 216 (2)
n
has had weighting factors applied to provide some degree of
1/3 1/3
uniformity so that color differences in various regions of color a* 5500[~X/X ! 2 Y/Y ! ]
n n
spacewillbemorenearlycomparable.Ontheotherhand,color
1/3 1/3
b* 5200[~Y/Y ! 2 Z/Z ! ]
n n
differences obtained for the same specimens evaluated in
X/X ; Y/Y ; Z/Z .0.01
n n n
different color-scale systems are not likely to be identical. To
avoid confusion, color differences among specimens should be The tristimulus values X ,Y,Z define the color of the
n n n
comparedonlywhentheyareobtainedforthesamecolor-scale
normally white object-color stimulus. Usually, the white
system. There is no simple factor that can be used to convert object-color stimulus is given by the spectral radiant power of
accurately color differences in one system to differences in
one of the CIE standard illuminants, for example, C, D or
another system for all colors of specimens.
another of daylight quality, reflected into the observer’s eye by
5.2 For uniformity of practice, the CIE recommended in
the perfect reflecting diffuser. Under these conditions, X , Y ,
n n
1976 the use of two new improved color scales, of which the
Z are the tristimulus values of the standard illuminant with Y
n n
CIELAB scale, with its associated color-difference equation,
equal to 100.
has found wide acceptance in the coatings and related indus-
6.2.1 The total difference DE* between two colors each
ab
tries. However, it has not completely displaced the use of the
given in terms of L*, a*, b* is calculated as follows:
Hunter L , a , b and the FMC-2 scales, which are still
H H H
2 2 2 1/2
DE* 5[~DL*! 1 ~Da*! 1 ~Db*! ]
ab
utilizedtoallowcomparisonwithearlierresultsandbecauseof
(3)
their familiarity to those responsible for interpreting them.
Therefore, all three scales are included in this method as the NOTE 1—The color space defined above is called the CIE 1976 L* a*
b* space and the color-difference equation the CIE 1976 L* a* b*
most widely used in industrial laboratories.
color-difference formula. The abbreviation CIELAB is recommended.
5.3 Users of color differences have found that, in each
system, summation of three vector color-difference compo-
6.2.2 The CIE 1976 (L* a* b*) space fails to approximate
nents into a single scalar value is useful for determining
uniform color spacing when one or more of the ratios X/X ,
n
whether a specimen color is within a specified tolerance from
Y/Y , and Z/Z is less than 0.01. In calculating L*, values of
n n
a standard. However, for control of color in production, it is
Y/Y less than 0.01 may be included if the normal formula is
n
necessary to know not only the magnitude of the departure
used for values of Y/Y greater than 0.008856, and the
n
from standard but also the direction of this departure. Infor-
following modified formula is used for values of Y/Y equal to
n
mation on the direction of a color difference is easily included
or less than 0.008856.
by giving the three instrumentally determined components of
L* 5903.3~Y/Y ! Y/Y #0.008856 (4)
n n
the color difference.
5.4 Selection of color tolerances based on instrumental 6.2.3 In calculating a* and b*, values of X/X ,Y/Y , Z/Z
n n n
values should be carefully correlated with a visual appraisal of less than 0.01, may be included if the normal equations are
theacceptabilityofdifferencesinhue,lightness,andsaturation replaced by the following modified equations for all calcula-
tions of a* and b*:
obtained by using Practice D1729.
a* 5500[f~X/X ! 2 f~Y/Y !] (5)
n n
6. Description of Color-Difference Equations
b* 5200[f~Y/Y ! 2 f~Z/Z !]
n n
6.1 CIE 1931 and 1964 Color Spaces—Thedaylightcolors
of opaque specimens are represented by points in a space where:
D2244–93 (2000)
1/3
produced by plotting in rectangular coordinates the quantities
f~X/X ! 5 ~X/X ! X/X .0.008856 (6)
n n n
L ,a ,b calculated as follows:
H H H
f~X/X ! 57.787~X/X ! 116/116 X/X #0.008856
n n n
1/2
1/3 L 510~Y! (12)
H
f Y/Y 5 Y/Y Y/Y .0.008856
~ ! ~ !
n n n
1/2
a 517.5~1.02X 2 Y!/~Y!
f~Y/Y ! 57.787~Y/Y ! 116/116 Y/Y #0.008856 H
n n n
1/2
1/3
b 57.0~Y 20.847Z!/~Y!
f~Z/Z ! 5 ~Z/Z ! Z/Z .0.008856 H
n n n
f~Z/Z ! 57.787~Z/Z ! 116/116 Z/Z #0.008856
where X, Y, and Z are tristimulus values for the CIE 1931
n n n
standard observer and standard illuminant C.
NOTE 2—To compare values of DE(AN40) calculated by means of the
Adams-Nickerson (AN40) equation with DE* , multiply DE(AN40)
ab
NOTE 3—The subscript L was used to denote Hunter in previous issues
values by the factor 1.1 to facilitate the comparison.
of this method.
6.2.4 The magnitude, DE* , gives no indication of the
ab
6.3.1 The total difference DE between two colors each
H
characterofthedifferencesinceitdoesnotindicatetherelative
given in terms of L , a , b is calculated as follows:
quantity and direction of hue, saturation, and lightness differ- H H H
2 2 2 1/2
ences.
DE 5[~DL ! 1 ~Da ! 1 ~Db ! ] (13)
H H H H
6.2.5 The direction of the color difference is described by
6.3.2 The magnitude and direction of the color difference
the magnitude and algebraic signs of the components DL*,
are described by considerations similar to those found in 6.2.4
Da*, and Db*:
and 6.2.5, respectively.
DL* 5 L* 2 L* (7)
1 0
6.4 Friele-MacAdam-Chickering Color Space and Color-
Da* 5 a* 2 a*
1 0
Difference Equation (3, 10)—This color space is more nearly
Db* 5 b* 2 b*
1 0 perceptually uniform than the CIE 1931 space in terms of the
MacAdam chromaticity-difference values (11). It is produced
where L* , a* , and b* refer to the reference, and L* , a* ,
1 0 0 1 1
by a linear transformation of CIE 1931 tristimulus values X, Y,
and b* refertothetestspecimen.Thesignsofthecomponents
Z into tristimulus values P, Q, S as follows:
DL*, Da*, and Db* have the following approximate meanings
(8):
P 50.724X 10.382Y 20.098Z, (14)
1DL* 5 lighter (8)
Q520.48X 11.37Y 10.1276Z,
2DL* 5 darker
S 50.686Z.
1Da* 5redder ~lessgreen!
Approximate lightness difference DL and chromatic
FMC-2
2Da* 5greener ~lessred!
differences D C (“yellow-blue”) and D C (“red-green”) are
1Db* 5yellow ~lessblue! calculated as follows:
2Db* 5bluer ~lessyellow! DL 50.279K PDP 1 QDQ /aD (15)
~ !
FMC22 2
6.2.6 For judging the direction of the color difference
DC 5 K S PDP 1 QDQ!/bD 2 K DS/b
~
1 1 1
betweentwocolors,itisusefultocalculatetheirCIE1976hue
DC 5 K ~QDP 2 PDQ!/aD
3 1
angles h and CIE 1976 chromas C* as follows:
ab ab
21 where:
h 5tan ~b*/a*! (9)
ab 2 −6 2 2 2 2 4 4
a = 17.3 310 (P +Q )/[1+2.73P Q /(P +Q )]
2 2 1/2
2 −4 2 2
C* 5[~a*! 1 ~b*! ]
b = 3.098 310 (S +0.2015Y )
ab
2 2 1/2
D =(P +Q )
Differencesinhueangle Dh betweenthetestspecimenand
ab
−3 2
K = 0.55669+0.049434Y−0.82575·10 Y +0.79172 3
reference can be correlated with differences in their visually
−5 3 −7 4
10 Y − 0.30087·10 Y
perceived hue, except for very dark colors (9). Differences in
−3 2
K = 0.17548 + 0.027556Y− 0.57262·10 Y + 0.63893 3
chroma DC* can similarly be correlated with differences in
ab
−5 3 −7 4
10 Y − 0.26731·10 Y
visually perceived chroma.
6.2.7 For judging the relative contributions of differences in
NOTE 4—The correlations between DL and perceived lightness,
FMC-2
lightness,chroma,andhuetothetotalcolordifferencebetween
between DC and perceived yellowness-blueness, and between DC and
1 3
twocolors,itisusefultocalculatetheCIE1976huedifference perceived redness-greenness, are not well established and should not be
used unless confirmed by visual observations.
D H* between them as follows:
ab
NOTE 5—When K = K =1, the FMC-1 equations are obtained.
2 2 2 1/2 1 2
DH* 5[~DE* ! 2 ~DL*! 2 ~DC* ! ] (10)
ab ab ab
6.4.1 The total difference DE between two colors is
FMC-2
where DE* is calculated as in 6.2.1 and C* is calculated
ab ab
calculated as follows:
as in 6.2.6; then the equation
2 2 2 1/2
2 2 2 1/2
DE 5[~DL ! 1 ~DC ! 1 ~DC ! ] (16)
FMC22 FMC22 1 3
DE* 5[~DL*! 1 ~DC* ! 1 ~DH* ! ] (11)
ab ab ab
contains terms showing the relative contributions of light-
7. Test Specimens
ness difference DL*, chroma difference DC* , and hue differ-
ab
ence DH* to the total color difference DE* . 7.1 This method does not cover preparation techniques.
ab ab
6.3 Hunter L ,a ,b Color Space and Color-Difference Unless otherwise specified or agreed, prepare specimens in
H H H
Equation (2)—This approximately uniform color space is accordance with Practice D3964.
D2244–93 (2000)
8. Procedure 10.1.2 For CIELAB color differences, L*,a*,b* for the
0 0 0
reference, DL*, Da*, Db*, and if desired Dh , DC* , and
ab ab
8.1 Select appropriate geometric conditions for color mea-
DH* for each specimen.
surement
...