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Instrumental Colour Comparison
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Scope and Field of Application
These three British Standards (and their ISO equivalents) are intended to be used together by those wishing to carry out an instrumental comparison of the colour of paint films or related products with a standard colour.
Instrumental colour matching has the advantage that it can provide reproducible data regarding the closeness of a colour match. The disadvantage is that the data generated may not always correlate with the way that the human eye perceives colour.
In such cases you should consider visual colour matching. This has the advantage that it provides information on how the colour is perceived which, arguably, is the only thing that really matters. The disadvantage of visual colour matching however is that it relies on the opinion of an observer and may well result in disagreement between observers.
In our opinion there is no need to choose between instrumental and visual colour matching. The most satisfactory way to compare colours is to use both methods.
Colour matching is a complex subject and one that is liable to give rise to disputes. This aspect is dealt with in greater detail on our colour laboratory page.
There are no pass/fail criteria defined in the standards. This is a matter of agreement between the parties concerned.
The actual standards are copyright-protected documents and we are not able to provide you with copies. If required however, you can easily obtain copies from the British Standards Institution.
Summary of Methods
The three standards deal with different aspects of instrumental colour matching:-
BS 3900-D8: ISO 7724/1
This standard describes the fundamental principles and requirements necessary for instrumental colour matching. In particular it defines the colour coordinate system recommended by the Commission Internationale de l'Eclairage (CIE). There have been a number of important developments since this standard was drafted, notably in both the hardware and software capabilities of colour measuring instruments and we will be happy to advise clients who are concerned with the latest developments in the theoretical aspects of colour science.
BS 3900-D9: ISO 7724/2
The standard describes how instruments are used to measure the colour of paint films. The instruments described include spectrophotometers and tristimulus colorimeters. Procedures are described for textured and non-textured paints and for calibrating the equipment. Obviously this standard has to be used in conjunction with the operating instructions for the actual instrument used.
We carry out instrumental colour comparisons using a GretagMacbeth Color-Eye 7000A Spectrophotometer. This enables us to measure both in transmission and reflectance and to report Delta E colour differences using the L a b and C M C systems. The latter is reported to give a better correlation with visual perception of colour.
BS 3900-D10: ISO 7724/3
The greater part of this standard is taken up with the mathematical formulae used to calculate colour differences in terms of lightness, chroma and hue. Methods are described for comparing samples which differ in surface reflectivity and those where the colour is very dark or very light. Although nowadays these calculations are carried out using software supplied with the colour measuring instruments, the standard provides important information regarding the equations that are used.
Sample Requirements
We need to be able to press the sample against the port of the spectrophotometer in such a way that no light can leak past the sample and interfere with the measurement. Ideally the test sample and the reference colour standard should be flat panels about 150 x 100 mm. The paint surfaces should be smooth or lightly textured and the substrates should not be perforated. Provided the other requirements apply we can work with samples as small as 10 x 10 mm and as large as 300 x 300 mm
If you wish, we can prepare the test panels. The quantity of liquid paint we needs depends on the method of application. We will pleased to advise you regarding our requirements.
Accreditation
PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS) to carry out testing to BS 3900-D9: ISO 7724/2. We are not accredited to test according to the other methods listed on this page.
Please contact Peter Collins for further details.
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