Colour measurement

Citation

(1998), "Colour measurement", Pigment & Resin Technology, Vol. 27 No. 2. https://doi.org/10.1108/prt.1998.12927baf.004

Colour measurement

Colour measurement

Industry is keen to maintain its use of traditional, established colour standards, but there is a shift towards automatic, objective methods of obtaining the colour data.

Demand for colour data is often based on differential measurements from fixed colour standards. The standards commonly used were typically established and agreed long ago in response to the fact that few materials are sufficiently permanent in colour to risk keeping a sample as a check on future production. They usually take the form of specified dilutions of stock solutions (for example, the Platinum-Cobalt solutions used to specify the widely used Hazen Scale for grading clear liquids) or calibrated coloured glass filters (as used to specify the Gardner Scale for oils, resins and varnishes). In both cases measurement relies on a visual comparison of the standard with the sample under test. As such, the results are inherently subjective, being influenced by the discriminatory power of operators, their interpretation of a colour match and physiological factors such as age, eye fatigue and colour vision. Furthermore, when using liquid colour standards, these solutions must be prepared regularly and with considerable care to avoid variations which give rise to inaccuracies.

The traditional colour scales (see Table I) have become well established and form the basis for internationally recognised standards test methods. As a result, industry is keen to maintain their usage. However, in recent years there has been a shift in emphasis away from the standard test solutions and visual comparator instruments which are still often specified in the test methods, towards automatic objective methods of obtaining the required colour data. The recognition of this trend was the motivating force behind Tintometer's development of the PFX190 series of spectrophotometric colorimeters which includes the established colour scales for resins, varnishes and industrial oils (Plate 1). A popular version of the PFX190 for the coatings industry is the PFX190/1 which will automatically measure the colour of liquid chemicals and industrial oils according to the Platinum-Cobalt/Hazen Colour scale for light coloured samples and the Gardner or Iodine Colour scales for darker products. For laboratory research applications, colour can also be displayed in terms of internationally recognised CIE values and spectral data. In response to the growing demand for consistent and reliable colour data, the US Naval Stores Rosin Scale specified in ASTM D 509 is also available as an upgrade kit for the Lovibond PFX190. The scale is based on the CIE co-ordinates of a master set of Rosin Standards stored at Tintometer in Salisbury.

Plate 1 The PFX 190 spectrophotometric colorimeters

It is believed that the application of spectrophotometry to colour measurement guarantees the objectivity of colour data and gives improved reproducibility and repeatability of results. The PFX190 gives companies confidence in the colour data they use to make important quality assurance decisions regarding valuable batches or consignments; it also allows them to work to tighter colour specifications. These factors are particularly important where colour discrimination is relatively difficult, for instance where there is not much colour in the sample, with dark samples where relatively little light is transmitted or where there is not a great difference between the colour of different samples.

According to Tintometer the PFX190 also incorporates a range of advanced features which appeal to modern test laboratories:

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  It allows automatic data handling, processing and recording, including monitoring trends in data to highlight colour deviations so corrective action can be taken; the calculation of colour differences from reference colour values for instant pass/reject evaluations; the generation of CIE chromaticity diagrams, transmission curves.

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  The inclusion of colour scales which are based on spectrophotometric measurements such as Hess-Ives Colour Units for products such as chemicals and suffactant liquids and Yellowness Index for the determination of the degree of yellowness under daylight illumination of light transmitting materials.

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  An interface with barcode readers for automatic sample identification, instrument set-up, testing and printing.

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  The facility for users to conduct automatic diagnostics test routines that allow the manufacturer to determine remotely the instrument's performance and any problem areas.

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  The facility for remote password protection which ensures that operating parameters cannot be modified by those not authorised to do so.

The traditional colour scales

When developing the early colour scales, it was often seen as adequate to assess the colour of a sample, without actually measuring it, by comparing it against a colour scale of a single hue but with varying intensities of that hue. These scales, based on single number colour values, make it relatively easy to communicate colour specifications and it is obvious from the value whether the product is pale, intermediate or dark in colour. As shown in Table I, they have been widely adopted as standards in the coatings industry to simplify colour grading activities.

For further details contact The Tintometer Ltd, Waterloo Road, Salisbury SP1 2JY, UK. Tel: +44 (0) 1722 327242; Fax: +44 (0) 1722 412322.