Colour measurement instrumentation

This chapter has attempted to present the basics of colour measurement as applied to foods. It is important to realise that the wide variability in the nature of foods and food products, from both their structure and pigmentation, may limit any colour measurement technique applicable only to that particular food. This is because the numeracy of the data is unlikely to match the visual experience of equivalent reference atlas colours. In some cases this discrepancy may be large and has to be recognised as an intrinsic property of the food. These differences may arise from the difference of the visual experience of the product when viewed under normal lighting conditions as opposed to the limitations of its optical properties when presented to the particular colour-measuring instrument. This is... 

Little AF, Christie RM. Textile applications of photochromic dyes. Part 1 establishment of a methodology for evaluation of photochromic textiles using traditional colour measurement instrumentation. Colouration Technol 2010 126(3) 157—63. 

The use of colour-measuring instruments allows specification and communication of colour by means of international standard terms such as CIE L a b C h°. Colour difference values can also be determined instrumentally and, if optimized colour difference equations are used, can out-perform a single trained colourist. However, the visual appreciation of colour and colour difference is still a subjective response, affected by many factors. Care must be taken to include measurement conditions in the interpretation of colour measurement data. The gold standard answer must always be that which agrees with the majority of a group of human observers. Whether this can be achieved by optimized equations based on CIE L a b colour space or by colour appearance models remains to be established. 

Certain commercial developments in the last twenty years or so have had important effects on the pattern of pigments production. One is the rapid growth of textile production in the Asia Pacific region and another is the development of automated dyeing methods, coupled with instrumental methods of colour measurement and of computerised colour recipe prediction. 

Fabric hand is defined as the estimated quality of a fabric, evaluated as reaction of the sense of touch, which is integrated in our brain to a total value. Important components of the sensory perception are the smoothness, compressibility and elasticity of the textile sample. As the hand of fabrics is primarily a subjective feeling - similar to the impression of colour - there have been many efforts to find methods for objective evaluation of the fabric hand. While instrumental colour measurement is very common in textile quality control, the automated measurement of fabric hand is still a topic of discussion and subjective manual hand evaluation is still the most important method. Simple methods to measure some aspects of fabric hand were described by Dawes and Owen. Many factors such as the colour of the textile, the hght and the surroundings, influence the manual evaluation of fabric hand. The American Association of Textile Chemists and Colorists has published guidelines for subjective hand evaluation. ... 

On-line use of colour measurement is likely to increase especially where the colour values obtained by the instrument can be incorporated automatically into... 

The two commonly used instruments discussed allow both laboratory and nearline colour measurements to be made. With the advent of almost fully automated lines, true on-line colour measurement would be of great value in many factories. McFarlane (1988) discusses the economic benefits of on-line and closed-loop systems. His paper highlights closer adherence to specifications, minimisation of waste during start-up and product change, as well as an increase in process efficiency and plant throughput as key issues. 

Finally, an exhaustive study of some of the variables affecting the measurement of tomato paste colour has been carried out as part of the COST 90 programme by Brimelow (1987). The effects of such factors as paste dilution, test duration, instrument aperture size and illumination area, sample temperature, standardisation hitching-post tile temperature, and the presence or absence of extraneous light were all examined. A standard colour measurement procedure was advocated for this important food commodity. As a parallel effort in this work, a new European standard tomato red tile was specified by the COST 90 colour group participants, to act as a hitching post in the measurement of tomato pastes and purees. 

Instrumental colour measurements prior to testing and after 2000 hours of UV exposure... 

Mathematical approaches that make use of concepts associated with colour space for colour measurement and specification based on reflectance spectroscopy have become well-established. Indeed the calculations involved are normally implemented in the software that is provided within an instrument used for colour measurement. The basis of all colour measurement systems is the CIE system of colour specification, which was agreed in 1931. The Commission Internationale de I Eclairage (CIE) is an international organisation promoting cooperation and information exchange on matters relating to the science and art of lighting. An important approach used for the... 

Although the original method was developed using a Lovibond tintometer for colour measurement, a more precise instrumental method could be employed. The colour absorbs at a maximum at 525 m/ and a 1-cm layer of a 0-008 per cent furfural solution under the given conditions has. an extinction of 0-70. 

Application of Beer s Law. Consider the case of two solutions of a coloured substance with concentrations c, and c2. These are placed in an instrument in which the thickness of the layers can be altered and measured easily, and which also allows a comparison of the transmitted light (e.g. a Duboscq colorimeter, Section 17.5). When the two layers have the same colour intensity ... 

E. Photoelectric photometer method (Section 17.6). In this method the human eye is replaced by a suitable photoelectric cell the latter is employed to afford a direct measure of the light intensity, and hence of the absorption. Instruments incorporating photoelectric cells measure the light absorption and not the colour of the substance for this reason the term photoelectric colorimeters is a misnomer better names are photoelectric comparators, photometers, or, best, absorptiometers. 

The colour filters used with absorptiometers should be examined from time to time by measuring the absorbance in a spectrophotometer if the results depart markedly from those expected for the filter, it should be replaced. Normally a filter as supplied with the instrument will be used, but if necessary, filters from the Wratten range supplied by Kodak Ltd or from the Spectrum Filter or Bright Spectrum Filter series supplied by Ilford Ltd may be used. 

Evaluation of these optical properties may be done by simple observation this approach is useful clinically (Knibbs, Plant Pearson, 1986), since acceptability of the colour match to the surrounding tooth material can be readily seen without the need for instrumental measurement. On the other hand, for quantitative evaluation of optical properties, some kind of instrumental measurement is necessary, and the property usually evaluated is opacity. 

Experimentally, several precautions must be taken if reliable Raman data are to be obtained from solution studies. Firstly, the instrumental slit-width should be appreciably smaller than the half-width of the band to be studied. This means that slits wider than 2 cm-1 are to be avoided. Secondly, photolytic decomposition of the sample and local boiling of the solvent have also to be avoided. Careful choice of laser frequencies, use of a low incident power and, if necessary, sample spinning are indicated. The need for a relatively high solute concentration usually means that there is little choice of solvent. Particularly for coloured samples the presence of a vestigal resonance Raman effect must be tested by measurements with a variety of... 

The RQ flex test kit (Merck) which uses specific test strips is useful for the semi-quantitative determination of several analytes. D(+) ascorbic acid can be determined in fortified food products with an accuracy of 85-115% (unpublished data), however the procedure cannot be applied to coloured food products. Added iron salts may be extracted from food products with dilute sulphuric acid and adjusted to pH2 with NaOH solution. Fe3+ is reduced to Fe2+ with ascorbic acid. Fe2+ reacts with Ferrospectral to form a red-violet complex. An internal calibration is provided on a barcode which is read by the RQ-flex reflectometer prior to any measurements. This avoids the need to calibrate the instrument with standard solutions. 

The simplest types of photometric instrument are designed for measurements in the visible region of the spectrum only and rely on coloured filters and simple photoelectric detectors. The name colorimeter is often used to describe such instruments although this is not necessarily correct and the word should probably be reserved for visual comparators rather than photoelectric instalments. 

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