The JPC79 colour–difference formula has represented a substantial improvement over earlier formulae and is being applied successfully in industrial shade passing. Modifications are described which overcome certain problems and, to some extent, simplify the formula. With available experimental data, the modified version performed even better than the original formula. Perceptibility data are fitted better by increasing the JPC79 lightness weighting by a factor of two. The new formula, designated CMC (: c), has the best overall performance of any formula so far published.
The Commission lnternationale de I'Eclairage (CIE) has recently published a new colour-difference formula, called CIE94, for use in industrial pass/fail colour-difference work. It is based in CIELAB colour space but on the CMC(/:c) colour-difference formula. In this paper the history behind the development of the new formula is outlined, before the formula itself is descibed and compared with the CMC(/:c) formula. The role of future work in this area is briefly reviewed and the attitude of the Society's Colour Measurement Committee to CIE94 is outlined.
A survey has been made of the use and methods of Instrumental colour difference measurement in quality control of colour within Europe. Several colour differnce formulae are in use. The performances of the most used formulae, namely CIELAB, CMC, BFD, M&S, and Datacolor, are compared. The degree of implementation of colour difference measurement in the textile, leather, automobile, and coatings industries is summarised. The use of new computer graphics to ensure continuity of colour from batch to batch and the alternative methods of colour sorting to minimise colour differences between batches are described.
Systematic variation in AN LAB matching tolerance was found in the padfail judgements of a single dyehouse observer assessing 8454 standard/sample pairs and 599 different colours. The matching formula derived to fit the variations gives improved correlation with visual decisions on several sets of independent published matching data. In subsequent dyehouse trials the formula was found to be a viable alternative to visual pass/fail matching.
Several investigations have been carried out in recent years into variables affecting the accuracy of instrumental colour passing. lo the U.K. there have been notable investigations by McLaren [ I ] , Jaeckel [2], sand Coates and Rigg [3]. As a result, the Adams---Nickerson colour-difference formula with a scale factor of 40 (originally 42) has now been adopted by the Society of Dyers and Colourists for use with textiles in the U.K. under the name of ANLAB(40) colour space and by IS0 as a Tentative Test for colour-difference work in textiles. It has also been adopted as a British Standard for plastics and is under consideration by IS0 for this purpose.There are, however, even in this recommended space, highly significant variations in the visual equivalence of numerical colour differences from different areas of colour space. A numerical colour difference of 1 .O AN unit in a grey is easily seen, whereas with a yellow a colo.ur difference of 2.0 AN units is hardly detectable, Considerable caution must therefore be exercised in the interpretation of numerical colour-difference values from different areas of colour space.A practical solution to this variation is to adjust individual numerical tolerances as necessary to agree with visual observation.McLaren has reported attempts to improve the correlation of the formula with visual assessments, using multiple-regression techniques [4]. The Davidson and Friede (51 visual samples chosen for his investigation consisted of a large number of colour pairs which were assessed for match acceptability for dyeing carpet yarns [4]. McLaren has shown that the matching panel tolerance in the Davidson and Friede investigation (5% acceptable level) is about 1.5 AN(42) units. The investigation therefore relates to the application of a fairly tight matching tolerance and not to the larger tolerances which can also be applied in the textile industry. (For example, in the sewing-thread industry tolerances of around 3.0 AN(42) units are acceptable for certain articles.)The purpose of the current investigation was to investigate, by means of a colour-matching grey scale, how numerical colour values vary for different grades of match throughout ANLAB colour space, to determine whether the irregularity could be specified in the form of an equation, and to try to reduce the 'non-uniformi ty' to less significant proportions. Experim ntalThe investigations relate to ANLAB space with a scale factor of SO [ANLAB(SO)], in which L=50(0.23 V y ) , a=50(Vx -Z/ZMgo = f(V,) and f(V) = 1.22191" -0.23111p t Over the years each skein dyed in our normal laboratory vv), b=50[0.4(vy -V Z ) ] , x / X~g o = f(vx), Y = f(vy>, 0.23951 V3 0 . 0 2 1 0 0 9~ t 0 . 0 0 0 8 4 0 4~.work has been wound into a 6 cm X 6 cm card and each card has been measured and filed under its colour co-ordinates. From this file of some 60,000 cards a minimum of five cards was selected from each of 16 colour centres and 11 grey centres* The selected centres corresponded to the following visual description:Bright colours:Dull colours:...
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