Colour space

When colour is assessed on the basis of reflectance measurements, it is common to consider the three relevant attributes of perception of colour as hue, chroma (or saturation) which is the colourfulness or richness of the colour, and lightness, which refers to the amount of reflected light. These three attributes may be described using the concept of colour space, which shows the relationships of colours to one another and which illustrates the three-dimensional nature of colour, as portrayed in Figure... 

Figure 2.5 (a )The concept of colour space (b) LAB colour space... 

Many whiteness (W) formulae have been proposed. All are based on CIE colour space and the X,Y,Z tristimulus values. Three of these equations are those of Berger [23] (Equation 11.1), Stensby [24] (Equation 11.2) and the CIE 1982 formula (Equation 11.3). 

Information on the hue of whiteness is provided by dividing CIE colour space, in the neighbourhood of the D65 achromatic point, into a series of parallel strips corresponding to variations in hue of whiteness. The principle is illustrated in Figure 11.4. According to this... 

Fig. 6.10 Left Placement of various synthetic goethites (G), lepidocrocites (L) and hematites (H) in CIE L a b colour space. Right Development of a and b in the CIE L a b colour space during the transformation of ferrihydrite (common starting point) to goethite or hematite, respectively (Nagano et al., 1994, with permission).

Fig. 15.2 Position of 16 red beds in the CIE L a b colour space as compared with 8 synthetic hematites of different colour between yellow-red and purple in a 3% hematite - 97% kaolinite mixture (Torrent Schwertmann, 1987, with permission).

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... 

Barnard K, Martin L and Funt B 2000 Colour by correlation in a three dimensional colour space In Proceedings of the 6th European Conference on Computer Vision, Dublin, Ireland (ed. Vernon D), pp. 375-389. Springer-Verlag, Berlin. 

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. 

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... 

SMITH, K. J. (1997) Colour-order systems, colour spaces, colour difference and colonr scales. In Colour Physics for Industry, 2nd edn, ed. R. McDonald, 121-208, Society of Dyers and Colorists, Bradford. 

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). 

Four different shades of the silicone, within the colour space of human subjects, were obtained, using 7 different pigmenting species in minute concentrations not exceeding 0.2 mass % (and often in much lower concentrations) with master batches that were blended into the elastomer [41]. 

This CVS method uses CIE (L-a-b) colour space. The reason is that CIE space can control colour and intensity information more independently than RGB colour space. This colour space is especially efficient in the measurement of small colour changes, as a result direct colour comparison can be performed based on geometric separation within this colour space [12]. 

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