CIELAB system

The quantitative measurement of pigment or pigmented system deterioration upon exposure to heat or light used to be expressed by visual numerical standards. In modem times color differences are expressed in the CIELAB system which has become the leading method for color characterization (8). 

In the process of testing, color deviations are expressed ia the CIELAB system which projects total color differences either on the axes of the rectangular LAB or the equivalent polar LHC system (26). In either case tested samples must fall within acceptable ranges or limits estabhshed versus a standard by the pigment manufacturer and accepted by the pigment user. 

There are instances in which one manufacturer may designate a bluish red dye as Red 4B and another manufacturer uses Violet 2R for the same dye. To resolve such a problem the manufacturers pattern leaflets should be consulted. These show actual dyed pieces of cloth so the colors of the dyes in question can be compared directly in the actual appHcation. Alternatively, colors can be specified in terms of color space coordinates. The Cielab system is becoming the standard in this system the color of a dye is defined by three numbers, the L, a, and b coordinates (see Color). 

The CIELAB system (1976) strictly standardizes the light source and the observer. CIE recommends three standard sources, A is an incandescent lamp, and B and C are lamps provided with different two-cell Davis-Gibson liquid hlters that simulate noon daylight and average daylight, respectively. Since the main object of the system is to obtain colorimetric results for normal tri-chromats (people with normal color vision), the standard observer must represent the human population with normal... 

CIE color system, 7 304. See also Commission Internationale de l clairage (CIE) chromacity diagrams, 7 313-315 standard illuminants, 7 315-316 CIEDE200 color difference equation, 7 322 CIELAB, 7 304 CIELAB Color Space, 7 320 CIELAB color values, of flax fiber, 11 614t Cielab dye nomenclature system, 9 244 CIELAB Metric Color Spaces, 7 320 CIELAB system, 19 428, 451 CIELUV Color Space, 7 319-320 CIE Standard Observer, 7 311-312 Cigarette filters... 

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. 

After all differences in tinctorial strength are accounted for, any remaining deviation is due to the inherent difference in shade between the two pigments and cannot be eliminated by simply adapting the amounts. It is useful to describe the remaining color difference by referring to the CIELAB system. 

The system used by colourists in the dyes and pigments industry is the so-called CIELAB system. [Note CIELUV is the one used in the display industries (Chapter 3).] In this system the three-dimensional nature of colour space is taken into account namely hue (the colour attributes), saturation (the absence of white grey of black) and lightness (also called brightness). In CIELAB colour space the coordinate L is a measure of hghtness-darkness, where a perfect black is 0 i.e. zero whiteness) and a perfect white is 100 a correlates with red (-Ea ) and green (-a ) and b with... 

An important advantage of the CIELAB system is that the resulting color difference can be split up into component contributions, namely lightness, saturation, and hue, corresponding to the arrangement of the color space ... 

After the color of the sample and reference pigments has been measured, color differences are usually calculated by transformation of the X, Y, and Z values into the CIELAB system to calculate color difference. Color measurement results of black and white pigments can be expressed more simply because they only amount to a determination of the relative color undertone. For this, the environment of the reference pigment is divided into eight sectors, these being filled with color names from red to violet . The octant in which the CIELAB color position of the sample is located is found by calculation. 

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 ... 

D1N99, trying a different approach In contrast to the formulas stated above this formula tries to keep the calculation of the color difference as simple as in the CIELAB system while changing the color space to give a better conformance to the visual impression. 

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. 

Eq. 3-3), a concept introduced in Section 3.1.5 in the context of cluster analysis. This technique, introduced in 1976, uses the tristimulus values X, Y, and Z as inputs and converts these to the corresponding L, a, and b coordinates in the uniform color space. The distance between two points in the CIELAB system can be calculated as ... 

The values Xn, Yn, and Z are a reference white color and vary with the illumination selected. This color space also has some inherent distortion/ but far less than that of Figure 11.8. The CIELAB system is widely used in forensic analyses to describe color and color differences. 

Solid state absorption spectra are usefiil for evaluating relative tinctorial strength by comparison with reference standards. The absorption at Amm as well as the absorption sum over the visible range of the spectrum, must be taken in consideration when determining relative strength from absorption spectra. After accounting for tinctorial differences based on the absorption spectra, other color differences are best determined by using the CIELAB system for color measurement. 

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