Four soil color charts compared in CIELAB color space

AIC 2004 Color and Paints, Interim Meeting of the International Color Association, ProceedingsOur goal in this study is to evaluate the regularity in CIELAB color space of the chips fromfour standard soil color charts from two different manufacturers, having different use degrees.2.1 Soil Color Charts2. MATERIAL AND METHODSTwo standard American editions of the Munsell Soil Color Charts (Munsell Color Co. 1975,2000) and two standard Japanese editions of the Revised Standard Soil Color Charts (FujiharaIndustry Co. 1970, 2001) were analyzed spectrophotometrically. The American charts of 1975(USA1975) and 2000 (USA2000) have 180 and 238 chips, respectively, arranged on sevenhue charts: 10R, 2.5YR, 5YR, 7.5YR, 10YR, 2.5Y, and 5Y. Both the Japanese charts of 1970(JAPAN1970) and 2001 (JAPAN2001) have 258 chips, including an additional hue chart(7.5R). The USA2000 and JAPAN2001 charts were completely new, whereas JAPAN1970and USA1975 had been previously used for soil color measurements in the field andlaboratory. Before spectrophotometric color measurements, the chips of JAPAN1970 andUSA1975 were cleaned by wiping off the dirt lightly with a clean wet cloth.2.2 Color measurementsColor measurements of all chips were performed using a Minolta CM-2600dspectrophotometer, operating with specular component excluded, and a bandwidth of 10 nm.Nine measurements (3 zones × 3 replications) were made for each color chip, and the averageresult was adopted for further computations. The D65 illuminant and CIE 1964Supplementary Standard Observer were assumed for the calculation of colorimetric data inCIELAB system (CIE 1986). The repeatability (standard deviation of ten successivemeasurements) of our instrument for the CIELAB coordinates was lower than 0.048 units.Lightness, chroma and hue-differences between neighboring chips within each color chartwere analyzed from ∆L*, ∆C*, and ∆H*, the later using the Sève’s equation (Sève 1991).3. RESULTS AND DISCUSSIONWe plotted in CIELAB the Munsell loci of constant hue and chroma (Figure 1). Each point onthe a*b* plane was the average of the chromatic coordinates measured for the color chips withthe same Munsell hue and chroma notations. So, these loci refer to the average Munsell valueof the charts. According to Berns (2000), in a perfect match between the Munsell variablesand CIELAB, these loci of constant hue and chroma would be a symmetrical and circular‘spider-web’ plot. The chart USA2000 approaches this condition rather well (Figure 1a). Thehue lines, which join dots of constant Munsell hue, are nearly straight and evenly spaced. Thechroma contours, which join dots of constant Munsell chroma, are nearly circular on the a*b*diagram.There was very little difference in how well the color chips were visually spaced in hue inUSA2000 (Figure 1a) and JAPAN2001 (Figure 1b). The CIELAB hue-differences (∆H*)between neighboring chips at chroma /1, /2, /3, /4, /6, and /8 were 0.93, 1.68, 2.44, 3.05, 4.42,and 5.72 CIELAB units, respectively, in USA2000, and 0.88, 1.68, 2.40, 3.15, 4.51, and 5.72CIELAB units in JAPAN2001. However, the values of ∆L* and ∆C* (with some exceptionsat 2.5Y) were greater in USA2000 than in JAPAN2001 (Table 1). This means that the152

AIC 2004 Color and Paints, Interim Meeting of the International Color Association, Proceedingslightness and chroma distances between new color chips depended on the manufacturer. Onthe average, in the Japanese charts, ∆L* values were 8.98 and ∆C* 5.36 CIELAB units, whilethe average values in the American charts were 9.90 and 5.86 CIELAB units, respectively.b*6050403020100-10Hue linesChroma contours-10 0 10 20 30 40 50 60a*(a)Color chips with the same MunsellHue and Chroma in USA2000Figure 1. Loci of constant Munsell hue and chroma for American (a) and Japanese soil-colorcharts (b) plotted in CIELAB.As illustrated in Figure 1b, the old charts have greater irregularities in the hue and chromaspacing. With respect to the new charts (JAPAN2001), the lines appeared deformed in the oldcharts (JAPAN1970). For the old charts, Munsell hue lines are unevenly spaced and Munsellchroma contours have ups and downs. The chroma contours in JAPAN1970 are below thoseof JAPAN2001, and the hue lines in JAPAN1970 are on the right or left of the hue lines ofJAPAN2001. More specifically, it seems that yellow charts such as 5Y, 2.5Y or 10YRreddened, while red charts as 7.5R or 10R underwent a certain yellowing, and all charts fadedin chroma as a consequence of the field and laboratory use.Table 1. Average CIELAB lightness (∆L*) and chroma (∆C*) differences for one step ofMunsell value and chroma, respectively, in hue charts of different editions.Charts 7.5R 10R 2.5YR 5YR 7.5YR 10YR 2.5Y 5YJAPAN1970 ∆L* 7.45 7.19 7.07 7.02 7.62 7.41 7.57 7.73JAPAN1970 ∆C* 4.28 4.58 4.30 4.11 4.23 4.94 5.10 5.66USA1975 ∆L* 9.51 8.35 9.13 8.98 9.45 9.37 9.41USA1975 ∆C* 4.89 4.90 5.43 5.31 5.80 6.31 6.14USA2000 ∆L* 9.93 10.15 9.83 9.88 10.01 9.95 9.55USA2000 ∆C* 5.27 5.46 5.80 5.85 6.33 5.95 6.33JAPAN2001 ∆L* 8.64 8.95 8.82 8.90 9.20 9.06 9.12 9.14JAPAN2001 ∆C* 4.60 4.78 4.90 5.09 5.49 5.63 6.09 6.33Color fading over time seems to have homogenized the color gamut represented by theJapanese charts, reducing the hue, lightness, and chroma distances between neighboringchips. With respect to the new charts, hue differences (∆H*) decreased in old charts between0.24 (at chroma /1) and 0.80 (at chroma /6) CIELAB units, with a mean reduction of 21%. Onthe average, lightness and chroma distances (∆L* and ∆C* in Table 1) also change from 8.98b*6050403020100-10Hue 5YHue 2.5YHue 10YRHue 7.5YRHue 5YRHue 2.5YRChroma /3Chroma /2Chroma /1Chroma /8Chroma /6Chroma /4-10 0 10 20 30 40 50 60a*Hue 10RHue 7.5R(b)JAPAN2001JAPAN1970153

AIC 2004 Color and Paints, Interim Meeting of the International Color Association, Proceedingsand 5.36 CIELAB units in JAPAN2001 to 7.38 and 4.65, respectively, in JAPAN1970. Thisindicates a reduction of lightness distance by 18% and chroma distance by 13%.Although we do not know the specific causes, color fading in American used charts wasless pronounced. The average values in USA2000 were 9.90 for ∆L* and 5.86 for ∆C*,whereas the average values in USA1975 were 9.17 for ∆L* and 5.54 for ∆C*. A reduction ofonly 0.73 units (7%) for ∆L*, and 0.32 units (5%) for ∆C* occurred in USA1975.4. CONCLUSIONSThis study shows that the steps between color chips of standard soil color charts havedifferent size depending on their manufacturer and degree of use. For new charts, averagelightness and chroma distances between nearest neighboring chips in American charts aregreater than in Japanese charts by 0.92 and 0.50 CIELAB units, respectively. Comparing theaverage step of old and new charts, lower hue (0.80 to 0.24 CIELAB units), lightness (1.60 to0.73 CIELAB units), and chroma (0.71 to 0.31 CIELAB units) differences were found.We believe that the original quality of printing and the color fading over time lead torelevant color changes in standard soil color charts.ACKNOWLEDGMENTSResearch Projects CGL2004-02282/BTE and FIS2004-05537, Ministerio de Educación yCiencia (Spain) and FEDER. Acción Coordinada BOJA N° 17 de 27.01.2004.REFERENCESBerns, R. S. 2000. Billmeyer and Saltzman’s Principles of color technology. New York: John Wileyand Sons.Bigham, J. M., and E. J. Ciolkosz. 1993. Soil color. Madison: Soil Science Society of America.CIE. 1986. Colorimetry, Publication 15.2. Vienna: Commission Internationale de l’Éclairage.Dobos, R. R., E. J. Ciolkosz, and W. J. Waltman. 1990. The effect of organic carbon, temperature,time, and redox conditions on soil color. Soil Science 150: 506-512.FAO. 1998. World reference base for soil resources, World Soil Resources Reports 84. Rome: Foodand Agriculture Organization of the United Nations.Fujihara Industry Co. 1970. Revised standard soil color charts. Tokyo: Fujihara Industry Company.——. 2001. Revised standard soil color charts. Tokyo: Fujihara Industry Company.Melville, M. D., and G. Atkinson. 1985. Soil color: Its measurements and its designation in models ofuniform color space. Journal of soil Science 36: 495-512.Munsell Color Co. 1975. Munsell soil color charts. Baltimore: Munsell Color Company.——. 2000. Munsell soil color charts. Baltimore: Munsell Color Company.Sève, R. 1991. New formula for the computation of CIE 1976 hue difference. Color Research andApplication 16: 217-218.Soil Survey Staff. 1993. Soil survey manual, Handbook 18. Washington: US Government PrintingOffice.——. 1999. Soil taxonomy: A basic system of soil classification for making and interpreting soilsurveys, Handbook 436. Washington: US Government Printing Office.Address: Manuel Sánchez Marañón, Departamento de EdafologíaFacultad de Ciencias, Universidad de Granada, 18071 Granada, SpainE-mails:154