Two further sets of results, including data from measurements made with several tristimulus colorimeters (Colormaster and Color‐Eye) and visual assessments by a relatively large number of observers, have been obtained for (a) a series of matt paint surfaces and (b) a series of gloss paint surfaces in approximately the same region of colour‐space as the wool and viscose rayon samples discussed previously (l). All samples in any set have been assessed by a ranking method, with reference to a standard, and a statistical approach has been used to provide a quantitative estimate of the visual spacing. The number of colour‐difference equations employed has been increased to include the four equations (1964 CIE; Glasser Cube Root; MacAdam Modified Friele; Munsell Renotation) recommended by the CIE for further study. In all cases an estimate has been made of the degree of correlation between instrumental and visual results for all samples (wool, viscose rayon and matt and gloss paints). The established equations all show poor correlation with the visual results, whereas an equation similar to that reported (l) in Part II of this series, which places much less emphasis on lightness weighting, provides a reasonable fit for all samples. There is little evidence that any significant differences exist in the assessment by colourists of paint and textile surfaces, although the reproducibility of instrumental measurements is much better for the former. The results from a single instrument and, in particular, the Colormaster are at least as reliable as the assessments of a single observer and are more reproducible. These and other relevant points are discussed in some detail.
The precision of measurements obtainable with several types of colorimeter (Colormaster Marks IV and V, Small Sphere Color‐Eye DI, Colorcord Mk IIA, Harrison 61 and Harrison 70) for several substrates (ceramic tiles, fabrics, yarns and threads, loose stock, slubbing and plain carpet) has been studied. Several methods of preparation and presentation of the sample to an instrument are considered and the results are compared in terms of standard deviations in 1964 (CIE) units. For textile samples the most generally satisfactory technique is to present the prepared sample behind glass in a rotating head. Under these conditions a wide range of fabrics can be measured with approximately the same precision (0<1 CIE) as ceramics. For yarns and threads, carefully wound on formers, the precision is approximately 0<3 CIE, and a slightly better precision can be obtained for carpets. Values of < 0<3 have been obtained for loose stock using a special rotating head, but values for slubbing are somewhat worse (0<8–1<0). Except for the Colorcord, good between‐instrument reproducibility (< 0<3 CIE) can be obtained for measurements of colour differences. For a given sample the absolute values (X, Y, Z) vary considerably with the instrument, the differences between pairs of instruments being equivalent to several CIE units and up to 19 when the Harrison 70 is used, even for measurements on a ceramic tile.
Methods of describing the extent of agreement between the ΔE values calculated from a colour‐difference equation and the corresponding visual estimates of colour difference are discussed. Lack of agreement will be due to a combination of errors in the visual assessments, in the measurements, and in the colour‐difference equation itself. If there were no error in the instrumental measurements, the equation error for a particular colour difference would be the difference between the calculated ΔE value and the mean visual assessment of a very large number of observers (ΔVtrue). Experimental data in the form of acceptances (%) can be converted to ΔV values directly proportional to the observed colour differences. The overall equation error for n colour differences can be calculated from Eqn 1. The Davidson and Friede data are considered to be the most satisfactory of those presently available for testing the suitability of equations for industrial colour‐tolerance work and have been used to assess the accuracy of several well‐known equations. After allowing for errors in the visual assessment and in the instrumental measurements, σ(log ΔE) for the 1964 CIE equation was 0.22. Similar values (0.16‐0.23) were found for other equations. A lower a‐value was found for a very simple empirical equation essentially based on the x, y chromaticity diagram rather than on any transformation of it. The usual transformations tend to be based on data covering the whole chromaticity gamut, whereas real surface colours cover only a fraction of the possible area. This fact, together with the knowledge that most equations are based on data corresponding to small fields of view or large colour differences, could account for the relative failure of the standard equations.
A set of gloss‐paint samples exhibiting large (10–25 CIE units) colour differences has been prepared. Visual assessments have been made under different viewing conditions. The results substantiate the view that visual assessments vary markedly with the size of sample and, together with earlier work (2), show that assessments also vary with the size of the colour difference. Thus, colour‐difference equations can correlate well with visual assessments only for given conditions. Existing equations apply best to conditions very different from those encountered in industry. It should not be too difficult to develop a more suitable equation for industrial use, given sufficient experimental data obtained under appropriate viewing conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.