1、BRITISH STANDARD BS 6923:1988 Method for Calculation of small colour differences UDC 535.653.001.24BS6923:1988 This British Standard, having been prepared under the directionof the Multitechnics Council, was published under theauthority of the Board of BSIand comes into effect on 31August1988 BSI 03
2、-2000 The following BSI references relate to the work on this standard: Committee reference M/2 Draft for comment 86/43526 DC ISBN 0 580 16448 9 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Multitechnics Council (M/-) to Technical Com
3、mittee M/2, upon which the following bodies were represented: British Ceramic Research Ltd. British Ceramic Tile Council British Paper and Board Industry Federation (PIF) Institute of Vitreous Enamellers International Wool Secretariat Lighting Industry Federation Ltd. Ministry of Defence Oil and Col
4、our Chemists Association Paintmakers Association of Great Britain Ltd. Royal Institute of British Architects Society of British Printing Ink Manufacturers Society of Dyers and Colourists Society of Leather Technologists and Chemists Textile Research Council Amendments issued since publication Amd. N
5、o. Date of issue CommentsBS6923:1988 BSI 03-2000 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Principle 1 3 Calculation 1 4 Interpretation of results 2 5 Test report 2 Appendix A Test data 4 Appendix B Computer program 5 Figure 1 Derivation of chroma and hue diff
6、erence descriptors 3 Figure 2 Computer program for colour differences 5 Table 1 Tristimulus values for four daylight combinations 1 Table 2 Test data for the CMC (1:1) formula 4 Publications referred to Inside back coverBS6923:1988 ii BSI 03-2000 Foreword This British Standard has been prepared unde
7、r the direction of the Multitechnics Council. In 1976 Commission Internationale dEclairage (CIE) published a colour difference formula known as CIELAB (currently given in CIE Publication No.15.2) which has since become widely used internationally for quantifying the perceived colour difference betwe
8、en surface colours. It has been established as section J01 of BS1006 for textiles and as BS3900-D10 for paints and varnishes. For industry and commerce, however, this formula has a major weakness. Over50000 assessments made by 44 professional shade passers in the textile and paint industries against
9、 reference specimens of262different colours were analysed. It was found that if the decisions had been made using the optimum CIELAB %E* value of the pass/fail boundary for each of the four data sets the number of wrong decisions would have been significantly greater than the average number of wrong
10、 decisions made by the assessors. This weakness has been overcome by dividing CIELAB colour space into an infinite number of ellipsoidal microspaces defined by semi-axes oriented in the directions of lightness, chroma and hue scaled so as to correlate with visual assessment. The relative importance
11、of lightness and chroma differences compared with differences in hue varies in different industries, and to allow for this, relative tolerances I and c are included in the formula. When I = c = 1 the formula quantifies the perceptibility (magnitude) of the colour difference. Different values of I an
12、d/or c allow for variations in the relative importance of lightness and chroma differences, and when these are used the formula quantifies the acceptability of the colour difference. This formula was devised by the Colour Measurement Committee of the Society of Dyers and Colourists and is known as C
13、MC (I:c). The ideas and reasoning behind the modifications made to the JPC79 colour difference formula in developing it into the CMC equation are given by F.J.J. Clarke, R. McDonald and B. Rigg in “Modification to the JPC79 colour-difference formula”, Journal of the Society of Dyers and Colourists,
14、1984, 100, 128132 and 281282. Applying CMC (2:1) to the four data sets already mentioned gives fewer wrong decisions than would be made by the average observer, and in the only data set giving individual assessments(8observers), the number of wrong decisions was no greater than that made by the most
15、 reliable assessor. Currently, optimum values of I and c have only been determined for the textile industry. However, work is proceeding on determining optimum values for I andc for other industries, e.g. ceramics, leather, paper, paint and plastics, which will lead to revisions of this standard. So
16、me test data are given inAppendix A and an example of a simple computer program is given inAppendix B. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standa
17、rd does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated.
18、This will be indicated in the amendment table on the inside front cover.BS6923:1988 BSI 03-2000 1 1 Scope This British Standard provides a method of calculating the colour difference between two specimens of the same material, measured under the same conditions, such that the numerical value DE, the
19、 total colour difference, quantifies the perceptibility of the colour difference or the acceptability of the colour match. The latter permits the specification of a maximum value which depends only on the closeness of match required for a given end-use and not on the colour involved nor on the natur
20、e of the colour difference. NOTE 1Appendix A gives test data for use in checking computer programs, andAppendix B contains a computer program for determining colour difference. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Principle The magn
21、itude of the colour difference between two specimens is calculated by means of a modified version of the internationally recognized CIELAB colour difference formula. 3 Calculation 3.1 CIELAB values Calculate the CIELAB L*, C and h abvalues from the X, Y, Z tristimulus values of each specimen as foll
22、ows: For these equations X n , Y nand Z nare the tristimulus values of the illuminant, with those for CIE standard illuminant D 65and the 10 observer, the preferred combination, being as given in Table 1, which also states the values for other daylight combinations. Table 1 Tristimulus values for fo
23、ur daylightcombinations C* = (a* 2+ b* 2 ) 0.5 h ab= arctan b*/a* expressed on a 0 to 360 scale with the a* + axis being at 0 and the b* + axis at90 . Calculate the CIELAB colour differences as follows, where subscripts T and R refer to the test specimen and reference specimen respectively: 3.2 CMC
24、(I:c) values Calculate the ellipsoid semi-axes from the L*, C and h abvalues of the reference specimen as follows: unless L* 0.008856 but L* = 903.3(Y/Y n ) if Y/Y nu 0.008856 a* = 500 f(X/X n ) f(Y/Y n ) b* = 200f(Y/Y n ) f(Z/Z n ) where f(X/X n ) = (X/X n ) 1/3 if X/X n 0.008856 but f(X/X n ) = 7.
25、787 (X/X n ) + 16/116 if X/X nu 0.008856; f(Y/Y n ) = (Y/Y n ) 1/3 if Y/Y n 0.008856 but f(Y/Y n ) = 7.787(Y/Y n ) + 16/116 if Y/Y nu 0.008856; f(Z/Z n ) = (Z/Z n ) 1/3 if Z/Z n 0.008856 but f(Z/Z n ) = 7.787 (Z/Z n ) + 16/116 if Z/Z nu 0.008856. * ab Combination Tristimulus value X n Y n Z n D 65/1
26、0 94.811 100.000 107.304 D 65/2 95.047 100.000 108.883 C/10 97.285 100.000 116.145 C/2 98.074 100.000 118.232 * ab * abBS6923:1988 2 BSI 03-2000 Calculate the colour differences in CMC (I:c) units using the following equation: NOTEWhen I = c = 1 the formula quantifies the perceptibility of colour di
27、fferences. Optimum values of I and c for quantifying the acceptability of a colour match have so far only been determined for the textile industry, and these values are I = 2 andc = 1. 4 Interpretation of results 4.1 General The colour difference between two specimens can be partitioned into compone
28、nts of lightness, chroma and hue, which are discussed in4.2 to 4.4. 4.2 Lightness component The lightness component is calculated for use in the equation for DE (see3.2) as %L*/IS L , and is the difference in lightness between the test specimen and the reference specimen; it is designated DL. The na
29、ture of any lightness difference can be derived from the following equation (see3.1): If %L* is positive, the test specimen is lighter than the reference specimen. If %L* is negative, the test specimen is darker than the reference specimen. If %L* is zero, there is no difference in lightness. 4.3 Ch
30、roma component (seeFigure 1) The chroma component is calculated for use in the equation for DE (see3.2) as %C /cS cand is the difference in chroma between the test specimen and the reference specimen; it is designated DC. The nature of any chroma difference can be derived from the following equation
31、 (see3.1): If %C is positive, the test specimen is of more chroma than the reference specimen. If %C is negative, the test specimen is of less chroma than the reference specimen. If %C is zero, i.e.the test specimen lies on the isochroma line passing through the position of the reference specimen, t
32、here is no difference in chromatic strength. 4.4 Hue component The hue component is calculated for use in the equation for DE (see3.2) as %H /S Hand is the difference in hue between the test specimen and the reference specimen; it is designated DH. The nature of any hue difference can be derived in
33、the following way. Rotate the chroma axis passing through the a*b* coordinates of the reference specimen so that it passes through the a*b* coordinates of the test specimen, minimizing angular displacement. The hue associated with the first major axis to be crossed during rotation, continuing if nec
34、essary, will describe the nature of the hue difference unless the reference specimen is of the same hue: to describe a test specimen as being redder than a reference specimen which is red, for example, is meaningless. To overcome this limitation the rotation of the chroma axis is continued until the
35、 next major axis is crossed. The hue associated with this axis is also quoted, expressed in brackets. One of the two hue difference terms will be valid, the other meaningless. If desired the value of DH may be designated + if the chroma axis rotation is anticlockwise and if clockwise. Examples of th
36、e derivation of chroma and hue difference descriptors are given inFigure 1. 4.5 Colour difference of white, grey and black (achromatic) specimens Although the total colour difference in CMC (I:c) units is valid for achromatic specimens the method of partitioning this difference is valid for lightnes
37、s differences only when C u 4.0, since the chroma and hue difference components often do not correspond with visual assessments. 5 Test report The total colour difference (see3.2) and the component colour differences (see clause4) shall be described as being in CMC (I:c) units and the values of I an
38、d c shall be stated. If any combination of illuminant and colour matching functions other than D 65/10 have been used in the calculations it shall be stated. In the case of achromatic specimens (see4.5), which are defined as having C u 4.0, only the total colour difference DE and the lightness diffe
39、rence shall be reported. * ab * ab * ab * ab * ab * ab * abBS6923:1988 BSI 03-2000 3 NOTE 1The following information can be deduced from the figure. a) Compared with reference specimen R A , test specimen T Ais equal in chroma and yellower (redder). b) Compared with reference specimen R B , test spe
40、cimen T Bis of more chroma and equal in hue. c) Compared with reference specimen R C , test specimen T Cis of less chroma and bluer (greener). d) Compared with reference specimen R D , test specimen T Dis of more chroma and bluer (redder). e) Compared with reference specimen R E , test specimen T Ei
41、s of less chroma and greener (yellower). NOTE 2See also K. McLaren and P.F. Taylor “The derivation of hue-difference terms from CIELAB coordinates”, Color Research and Application, 1981, 6, 76. Figure 1 Derivation of chroma and hue difference descriptorsBS6923:1988 4 BSI 03-2000 Appendix A Test data
42、 To help check computer programs giving DE values from the CMC formula some test data are given inTable 2. The data are for illuminant D 65 and the10 observer, using X n= 94.811, Y n= 100.000 andZ n =107.304. An attempt has been made to include differences covering the various possible paths in the
43、program. Table 2 Test data for the CMC (1 : 1) formula Pair Tristimulus values Lightness component, %L*/IS L Chroma component, %C ab /cS c Hue component, %H ab /S H Total colour difference, DE CMC (1:1) X Y Z 1 19.41 19.5525 28.41 28.64 11.5766 10.5791 0.1715 0.9638 1.04 1.4282 2 22.48 22.5833 31.6
44、31.37 38.48 36.7901 0.1572 0.7608 0.9855 1.2549 3 28.995 28.7704 29.58 29.74 35.75 35.6045 0.1153 1.0770 1.3981 1.7686 4 4.14 4.4129 8.54 8.51 8.03 8.6453 0.0675 1.7214 1.0659 2.0258 5 4.96 4.6651 3.72 3.81 19.59 17.7848 0.4662 2.2106 2.1041 3.0872 6 15.60 15.9148 9.25 9.15 5.02 4.3872 0.2087 1.3552
45、 1.0856 1.7489 7 73.0 73.9351 78.05 78.82 81.80 84.5156 0.2449 1.1188 1.5171 1.9009 8 73.995 69.1762 78.32 73.4 85.306 79.713 1.5994 0.1221 0.5709 1.7026 9 0.704 0.613873 0.75 0.65 0.972 0.851025 1.7677 0.2679 0.2344 1.8032 10 0.22 0.093262 0.23 0.1 0.325 0.145292 2.2980 0.8213 0.2092 2.4493 NOTEThe
46、 test data use values of I = c = 1.BS6923:1988 BSI 03-2000 5 Appendix B Computer program An example of a simple computer program, written in BASIC, that has been used to calculate colour differences is given inFigure 2. Figure 2 Computer program for colour differences6 blankBS6923:1988 BSI 03-2000 P
47、ublications referred to BS 1006, Methods of test for colour fastness of textiles and leather 1) . BS 3900, Methods of test for paints 1) . BS 3900-D10, Determination of colour and colour difference: calculation. CIE Publication No. 15.2 Colorimetry, 2nd edition, 1986 2) . F.J.J. Clarke, R. McDonald
48、and B. Rigg, “Modification to the JPC79 colour difference formula”, Journal of the Society of Dyers and Colourists, 1984, 100, 128132 and 281282. K. McLaren and P.F. Taylor, “The derivation of hue-difference terms from CIELAB coordinates”, Color Research and Application, 1981, 6, 76. 1) Referred to
49、in the foreword only. 2) Available from the Hon. Librarian (National Illumination Committee), c/o Thorn Lighting Limited, Great Cambridge Road, Enfield, Middlesex EN1 1UL.BS 6923:1988 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for preparing BritishStandards. It presents the UK view on standards in Europe and at the international level. It is incorporated by Royal Charter. Revisions BritishStandards are updated by amendme
