Colour systems/scales

When the sample has been processed, the result exists as a patch (or stain) on a 5 pm (usually ungridded) membrane, and this is then matched to a manbrane filter colour rating scale, based on quantitative and/or gravimetric levels of contamination. Accuracy levels by this method are lower than those obtained in methods such as the one previously described. The reason for the higher accuracy of the Conpar method is almost certainly the closer control of the membrane (0.8 pm) coupled with a viewing system that is, at its lowest point, 50-100 times better than the naked eye. 

In soil and other geosciences, colour is commonly measured using the Munsell colour classification system. This system defines colour in terms of hue H (position of colour in the spectrum), chroma C (the purity of the hue going from the grey to the pure colour) and value V (the lightness of the colour on a scale ranging from black to white). The reflectance measurements can be converted into the characteristic parameters or coordinates of the different measurement systems. 

Mineral. Chemical composition. Colour. Density Hardness (Mohs scale). Crystallo- graphic system. 

Luminex is also a bead-based, non-separation technology using the Luminex colour-coded beads and detection systems (Luminex 100 IS or Luminex HT ). The readers used for this assay format are based on the principle of flow cytometry. The system enables assays to be multiplexed, i.e. allowing different analytes to be monitored simultaneously. The Luminex HT system is compatible with 96- and 384-well microplates14 but throughput of the reader is still a limiting factor for large-scale HTS. 

It crystallises in tabular hexagonal scales belonging to the hexagonal system, and possesses a steel-like lustre. The edges are ruby-red in colour, and give a red streak. Density, 5-187 to 5-193.4... 

The MacAdam optical sensitivity chart shown in Fig. 26.21 does not take luminence into account and Silberstein (Phil. Mug., 1946, 37, 126) showed that when these ellipses were transferred to the three-dimensional version of the ehromatieity chart they would be ellipsoid as shown in Fig. 26.23. In order that a really satisfactory universally applieable speei-fieation for tolerances in colour matching can be established it is desirable that these ellipsoids should be spherical. This, unfortunately, is not possible and the nearest approach is the Simon and Goodwin system whieh takes the form of about 100 different charts in which, by alteration of the angle of the eo-ordinates of the ehromatieity chart the ellipses are COn-verted to eireles. The scale is such that each half-inch is equivalent to a threshold value of discrimination Fig. 26.24 (Shiot and Goodnih, Amer. Dyes Rep., 19.58, 105). 

This coloured scale can be compared to codification systems for water quality mapping. 

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. 

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. 

Cochineal extract (Colour Index 75470, E 120)is the final alcohol-free material obtained after aqueous ethanolic extraction of the dried bodies of the female scale insect Dactylopius coccus Costa which lives on cacti such as Opuntia or Nopalea coccinellifera a species indigenous to Peru and Mexico, although also found in the Canary Islands. In practice a simpler procedure consists of aqueous alkaline extraction. The colourant principle is carminic acid which is more well-known in the form of carmine, an aluminium chelate of carminic acid, a material insoluble in water and stable on the acidic side. A uv spectrophotometric study has been made (ref. 21). Thin layer chromatography of cochineal has been examined on acetylated cellulose with the solvent system, ethyl acetate-tetrahydrofuran-water (6 35 47) in which the Rf was 0.94 (refs.22,23). The history of the chemistry of cochineal has been discussed (ref.24). 

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