CIELAB coordinates, color difference

In colorimetric terms, color differences are generally characterized by the distance between two colors in one of the color notation systems, most commonly the CIELAB system. The difference determined in this way represents the total color difference. Since a color is characterized by three quantities, a color difference can also be expressed in terms of three difference components (that is, differences in the color coordinates) and thus described in greater detail. Visually equal color differences between almost equal colors may, upon colorimetric measurement, be considered distinct if the colors of one pair differ strongly from those of the other pair. The formulas for color differences are further elaborated in order to remedy this defect. 

Colorimetric analyses performed in the reflection mode have been used to determine colors of various flowers" as well as the colors and color development of fruits and berries. In an interesting study, the cyclamen red (or pink) colors of some carnation cultivars have been found to be based on the macrocyclic anthocyanin, pelargoni-din 3,5-diglucoside (6",6" -malyl diester).The CIELAB coordinates revealed that these flowers showed similar colors as some rose cultivars, which, however, were mainly based on a very different pigment, cyanidin 3,5-diglucoside. 

Reflectometer coordinates CIELAB color differences Undertone ... 

Next the CIELAB coordinates are converted to differences in metric lightness, metric chroma, and metric hue (not hue angle), AL, AC, AH, respectively. By themselves, these parameters define a spherical color difference volume in the rectangular coordinates (L, C, H) with a radius of AEab ... 

Instrumental Color Measurements. When treated samples showed AATCC ratings of 4 or less, they were measured against the untreated samples on the Sargent-Welch SP8-200 UV-vis spectrophotometer with a diffuse reflectance sphere attachment. The instrument is interfaced with a Hewlett-Packard HP-85 computer and run by color measurement software. Spectral reflectance data at 10-nm intervals, tristimulus values, chromaticity coordinates, and CIELAB color difference were determined for these samples (21). 

Institute) and later worldwide by the CIE ( CIELAB for standards, see Table 1.1, Color differences ). The three coordinates are denoted by a (the red-green axis), b (the yellow-blue axis), and I (the lightness axis). See figure 1.6 for a simple representation of the CIELAB system. To calculate the CIELAB coordinates, X, Y, and Z are first converted into the functions X, Y, and Z by using a relationship that approximately takes account of the physiologically equidistant lightness steps ... 

The CIELAB and CIELUV systems can be used advantageously to calculate color differences AE where L, a, b and L, u, v are the chromaticity coordinates of the color points ... 

Beside visual assessment, the color of an object can be characterized more precisely in terms of mathematical models, which are based on the remission and transmission spectra in the visible light area [6]. The most common CIELAB system, defines a color space and allows one to describe a specific color in terms of L (lightness) and the color coordinates a and b, and color differences or tolerances by related AL, A Ab and a combined AE value. L, a, b, values can be transformed into the probably more convenient terms of L (lightness), h (hue) and c (chroma). Today, colorimetry software is used for color matching and allows very precise calculations of pigment formulations. 

UV/VIS The topic of colorants and color analysis was introduced in the previous chapter textiles represent one of the most important forensic applications of colorant chemistry and analysis. Colorant analysis and comparison and the use of chromaticity coordinates were discussed in detail in the previous chapter all are applicable to fibers. The plotting of CIELAB coordinates is useful in comparing colors and shades. Some fibers are metameiic, meaning that their color will appear different under different Uluminants. Textiles are often designed to have this property. Metameric fibers perceived as being the same color are distinguished by different UV/VIS spectra. 

AE (CIELab) is a widely used measure of color difference. However, because AE (CIELab) is derived from a rectangular coordinate system, and visual color perception is elliptical in nature, it was recognized that there were areas in which the setting of tolerances for visual color difference were unacceptable. CIELCh color space, a cylindrical coordinate system, addresses some of these problems and gives better correlation with the elliptical response of the eye. Further enhancement of this correlation has been made using the CMC tolerancing modification to CIELCh to obtain AE (CMC). CMC refers to the Color Measurement Committee of the Society of Dyers and Colourists. As an introduction to color difference, color tolerancing, and other aspects of color measurement, some of the publications available from spectrophotometer manufacturers are quite helpful. Additional information on color difference can be found in any of the color measurement references cited, particularly. Refs. 6-8 and 14. 

Work within the color community continued for the ideal, uniform color space. In 1976, the CIE recommended a more nearly uniform color space known as L a b with the official designation CIELAB. L a b is an opponent-type color space with rectangular coordinates similar to Hunter L,a,b. The L indicates lightness (0 to 100), the a indicates redness (+) and greenness (-), and the b indicates yellowness (+) and blueness (-). The limits for a and b values are around -80 and +80. The two color scales do not correlate and a color located in L,a,b space will be in a different location in L a b. ... 

Standard practice for multiangle color measurement of metal flake pigmented materials, ASTM Standard, Book of Standards, Vol. 06.01 ASTM E2194-03, ASTM International, West Conshohocken, PA, 2009. Standard test method for evaluation of color for thermoplastic traffic marking materials, ASTM Standard, Book of Standards, Vol. 06.02 ASTM D4960-08, ASTM International, West Conshohocken, PA, 2008. Colorimetric evaluation of colour coordinates and colour differences according to the approximately uniform CIELAB colour space, DIN Standard DIN 6174, DIN, Berlin, 2007. 

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