Uniform colour space

Stephen Westland continues the topic of human perception of colour in Chapter 4. He describes the retinal image and the effects of the surroundings on the perception and appearance of colour, especially the phenomena of colour constancy and colour contrast. The importance of white as a reference point in the field of human vision and its role in adaptation is presented. The development of colour appearance models is outhned and the relationship of these uniform colour space models to their use in the formation of digital image... 

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. 

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. ...

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. 

Uniform color scale n. A scale wherein the units of color difference that are judged to be equal have equal scale differences. McDonald R (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. Uniform color space Three-dimensional space wherein all pairs of colors, judged to be equally different, are separated by nearly equal distances. 

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 Dutch edition of Physical Aspects of Colour was completed by the late P. J. Bouma in 1945. It has been translated into English with some additions by de Groot (Bouma, 1947). The concept of color space is extremely well developed, together with the problem of transformation from one system to another, as, for example, the U.C.S. system (uniform chromaticity scale), in which ideally the colors would be uniformly distributed over the color plane, as judged by the eye. The historical development of color science is also thoroughly treated. 

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