Colour tristimulus

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). 

The different instruments (Hunter, Gardner, Instrumental Colour Systems, etc.) process absorbance and reflectance data in slightly different ways, which means that the values obtained can differ slightly from one instrument manufacturer to another. Therefore, a product s defined colour has to be qualified with a statement indicating the instrument used. Notwithstanding this limitation, it is not uncommon to find a tristimulus colour meter in a manufacturer s quality assurance laboratory so that routine quantitative assessment of a product s colour can be made. This is particularly tine for tomato-based products, whose nature makes conventional spectrophotometric assessment meaningless. 

To plot colour values on a two-dimensional system it is necessary to have only two co-ordinates. With a tristimulus combination this can only be achieved when the values add up to unity so that if X and Y are defined the value of Zis 1 — (.iV-l- T). The proportions of the imaginary stimuli in a colour can be made to add up to one by converting them into the so-called chromaticity co-ordinates ( , y, and z) by the following treatment ... 

One of the simplest tristimulus colorimeters is the Lovibond Tintometer. The apparatus consists essentially of a viewing tube and light reflected from a standard white surface. One half of the field is illuminated by the light reflected by, or transmitted through, the specimen under examination. The other half is illuminated directly by a beam of standard light. Into the path of the latter beam coloured glass filters of the subtractive primaries are inserted and moved by mechanical means. 

Colour can be useful when describing different batches of drug substance, since it can sometimes be used as an indicator of solvent presence or, more importantly, of degradation. In addition, subtle differences in colour may be due to variations in the particle size distribution. Usually colour is subjective and is based on individual perception however, more quantitative measurements can be obtained by using, e.g., tristimulus colorimetry (Nyqvist et al. 1980 Ve-muri et al. 1985 Nyqvist and Wadsten 1986 Stock 1993). 

Tables 3 and 4 present the tristimulus coordinates and the corresponding absorption coefficients of the different samples presented in Figs. 2 and 3. Some observations can be drawn. Except for the industrial product, the corresponding colour characteristics are rather close, even if the visible spectra show some differences. This is particularly true for the tristimulus method which gives an integrated response more adapted for dyes than for water or wastewater. The ISO method seems to discriminate more efficiently the colours but needs a more complete comparative study for each absorption coefficient.

Sample Tristimulus coordinates Dorn, l Purity Colour hue... 

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... 

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 original CIE system of colorimetry was designed for colour specification and has only limited use as a colour-appearance model. It can be easily shown that the CIE tristimulus values that can be used effectively to specify a surface colour do not correlate well with the appearance of that surface. This chapter introduces the distinction between colour specification and colour appearance and then reviews how simple adaptive processes that pool information at different spatial positions in an image can be used as a basis for colour-appearance models. A typical colour-appearance model, CIECAM97S, is then described. Finally, some further uses of colour-appearance models are discussed. 

A further limitation of the original CIE system is illustrated by Fig. 4.1 where the two grey patches ate physically identical, create the same local rate of photopigment absorption, and give the same cone excitations according to Equation 4.1. Yet, they appear to be veiy different in lightness because of the difference in their excitations relative to nearby areas. Thus whereas colour constancy demonstrates that patches of colour that have different tristimulus defiiutions can have the same colonr appearance, simultaneous colour contrast demonstrates that patches of colonr that have the same tristimulus definitions... 

WASSEF EGT (1959) Linearity of the relationship between the tristimulus values of corresponding colours seen under different conditions of chromatic adaptation. Optica Acta, 6, 378-393. 

There are a bewildering variety of methods and instruments available to the food technologist in the field of colorrr measurement. When one is approaching the subject for the first time or when attempting to devise a method for a material outside the normal experience, the wealth of possibilities available sometimes makes the choice difficrrlt. It is the purpose of this chapter to attempt to identify a systematic approach in order to ease the task. The approach is concerned primarily with the use of tristimulus colorimetry and reflectance spectrophotometry. These two techniques will be discussed in detail along with colour scales and formulae commonly in use. The key points which may influence measured results will also be described followed by advice on the selection, preparation and presentation of samples. Finally, detailed references to work carried out in the field of food colour measirrement and where to access further information will be provided. 

In a tristimulus colorimeter, three or four filters duplicate the response of the standard observer. The filters, which correspond to the three primary colours in the spectrum (red, green and blue), can be combined to match most colours. The more sophisticated instruments carry a fourth filter to simulate the blue part of the CIE [[X bar]] function shown in Fig. 5.1. The other essential parts of a tristimulus colorimeter are a white light source, an array of photometers and, nowadays, a computer or an interface to one, as shown in Fig. 5.2a. The computer can collect responses as well as carry out data transformations between CIE and other colour scale systems or between different standard white light sources or white diffusers. Hunter and Harold (1987) give a good summary of the transformation formulae employed. Data from a colorimeter is given as a three-point output, commonly CIELAB, HunterLab or 7, x, y. 

Fig. 13.3 Conventional and encapsulated turmeric colour dissolved in a soft drink media (30 mg curcumin/litre soft drink media (10% sucrose, pH3)) and exposed to light in a Heraeus accelerated lighting unit (480 W/m, 300-800 nm). Colour change is measured using a Minolta Tristimulus CT310 calibrated with demineralised water.

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). 

Tristimulus colorimeter n. Instrument used to measure quantities, which can be used to obtain an approximation of tristimulus values. They are normally equipped with three (or four) special filters to obtain R, G, and B values which must then be normalized to CIE magnitude. They were designed and are properly used only for measuring the color difference between two similar, non-metameric samples. McDonald R (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. 

X n. Special color matching functions of the CIE standard observer used for calculating the X tristimulus value. McDonald R (ed) (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. 

Reflectance meters such as tristimulus colorimeters measure colour on the basis of the amount of light reflected from the surface of the fruit (Kader 1985). Several types of tristimulus colorimeters are available that measure the colour of fruit depending on lightness, red to blue tone and yellow to green tones. Two common types are the Hunter Colorimeter system and the various Minolta Chroma Meters (CR-300 series), both types use the L, a, b coordinates (Brown and Walker 1990 McGuire 1992). 

---