CIELAB colour space

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... 

The CIE is an international body which coordinates worldwide cooperation and exchange of information on matters relating to illumination, colour, vision and image technology. It is recogtused as an international standardisation body, and has defined and standardised several colour spaces. For more information on the CIE and the CIELAB colour space (an internet search on the term CIELAB will produce thousands of additional links) http /Avww.cie.co.at/ [accessed July 2011] ISO 11664-4 2008(E), CIE Colorimetry — Part 4 1976 L a b Colour Space, CIE, Vienna. 

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. 

The experiments reported here make use of the Thin Film Colorimetric Interferometry technique for film thickness measurement. In this technique, 3 X 8 bit SXGA still pictures are after transformation from RGB to CIELAB colour space converted to the film thickness map using appropriate colour/film thickness calibration curves. They are obtained from Newton rings for static contact formed between the steel ball and the glass disc coated with the chromium layer only. During several last years the accuracy and resolution of TFCI has been improved and it is now able to measure film thickness to within 1 nm [13]. 

The original 1931 CIE Y, x, y system of colour measurement is not visually uniform (Fig. 3.4a). Constant hue and chroma are distorted and equal visual distances increase several-fold fi om purple-red to green. Improved spacing has been accomplished by both linear and non-linear transformations of Y, x, y (Bems 2000). Near uniform colour spaces of practical importance are the Hunter and the CIELUV and CIELAB spaces. In the Hunter (1958) Z, a, b colour space the lightness co-ordinate L is the square root of the tristimulus value T, and a, and b are the red/green and yellow/blue opponent co-ordinates. The 1976... 

The co-ordinates of L, a and b in CIELAB serve to define the location of any colour in the uniform colour space. However, in most industrial applications the object of measuring products is usually to determine how far they may be divergent from a set standard, both in colorimetric terms and in acceptability of visual match. The determination of uniform colour differences by CIELAB is not the same as the recognition of acceptability. CIELAB is based on the perception of just noticeable colour differences in the cyhndrical co-ordinates of the system. However, acceptability differences are based on the perception of colour tolerance differences of real materials of industrial interest, e.g., textiles. 

The objective, i.e., the psychophysical are related to the stimulus and are evaluated from spectral power distributions, the reflectance or transmittance of the object and observer response. They provide the basis for the psychometric qualities which correspond more nearly to those perceived. For CIELAB space the terms are lightness L, hue h = iwr (b /a ) and chroma C = a + b Y. CIELAB total colour differences AE can be expressed either as the co-ordinates of colour space or as the correlates of lightness, chroma and hue. Hence... 

In principle, the higher the order of polynomial used, the more accmate the colour space transformation. However, there are some important parameters to be considered the material of the test target, the number of colours used for deriving the transform coefficients and their distribution throughout the colour space. The predicted error between the measured and predicted tristimulus values can be calculated using a colour difference formula such as CIELAB (CIE 15.2, 1986). 

There are other minor nonequivalencies of ISO test procedures compared to ASTM standards, but for color difference the ISO Test Procedure No. 105 is unique. Those who use Colour Measurement Committee (CMC) procedures—particularly CMC 2 1 Lightness to Color ratio—claim that it facilitates a uniform description for acceptability decisions that is better than any other system in existence. These equations permit the use of a single number tolerance, DEcmc, in a nearly uniform color space. The CMC formula is a modification to the perceptibility CIELAB formula. It is fully described elsewhere in this book, but it deserves some brief notice here because, after all, it is an ISO procedure. The CMC developed the basic British Standard No. 6923, Calculation of Small Color Differences. Soon afterward, in 1989, the American Association of Textile Chemists and Colorists (AATCC) adopted AATCC Test Method 173, CMC Calculation of Small Color Differences for Acceptability. Ford Motor Company indicated a preference for using CMC 2 1 ratio color difference for plastics weathering data for plastics interior trim materials. 

Fig. 3.4 Colour diagrams (a) CIE 1931 chromaticity diagram showing non-uniformity of spacing of red, yellow and blue unique hues (b) CIELAB uniform diagram showing relationship of red/green ( +/—) and yellow/blue (b +l—) opponent eo-ordinates to lightness L, ehroma C and hue angle h. ...

Fig. 3.4b), known as CIELAB, has generally replaced the Hunter space for industrial applications although this has been somewhat slower in parts of the food industry where methods established on the Hunter system have economic reasons for its continued use. The improvements in CIELAB are due to the nonlinear cube root transformation of the 1931 tristimulus values, which more approximate the visual spacing of the coloured samples in the Munsell system. The formulae are... 

The aim of the more popular of the colom scales is to represent colours in a three-dimensional colom space, such that similar visual colom difieiences are represented by approximately similar distances in the colom space. Such colour scales are known as uniform scales. The original CIE scales, represented diagrammatically in Fig. 5.3, were not intended for identifying the colours of objects and are certainly not uniform in spacing colours according to their visual differences. Over the last forty years the Commission Internationale de I Eclairage (CIE) in particular have worked to try to stem the confusion caused by the presence of the many formulae in use and standardise colom space. They managed to combine the opponent chromatic attributes used by Hunter and Schofield, with the MacAdam cube-root simplification of the earlier Judd polynomial. The result was the publication of CIELAB (or ClEL a b ) in 1976 (Smith 1997). At this point, the asterisk snperscript was introduced in order to differentiate the CIELAB parameters from other similar ones still in use. 

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