Tanveer Ahmed 1

USEFULNESS AND LIMITATIONS OF

THE CIE SYSTEMIn many ways the CIE system of colour specification has been remarkably successful.Almost all important applications of colour measurement use it, and thebasic system has survived unchanged for over 60 years. The additions made since 1931Have led to improvements in some respects, but have not changed the basic principles of the system in any way.

Tanveer Ahmed 2

the system has certain limitationsignore Texture , Gloss

These stem basically from the limited objectives of the system, rather than from a failure to meet those objectives.

The CIE tristimulus values for a sample are related to the colour of the sample, but ignore other important features such as surface texture, gloss and sheen.

Thus a gloss paint sample and a matt paint sample might have the same tristimulus values, but obviously will not look the same.

Whether the colours of the two samples will look the same dependscritically on the geometrical arrangements for illuminating and viewingthem.

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the system has certain limitationsSimilar geometry for illuminatin and viewing

Only if the instrumental geometry of illumination and viewing conditions issimilar to that used for visual matching will the colours be seen to be close.

An instrument will always average out the light reflected from the area being measured

(typically a 2 cm diameter circle).

In judging a colour visually some sort of averaging takes place, but the observer is always conscious of any non-uniformity over the area viewed.

Thus a matt paint surface, a woven textile surface and a pile fabric will always lookdifferent from one another, but their measured tristimulus values could be the same.

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Unlikely to match--- limitationsIgnoring all features other than colour, the tristimulus values for a sample give onlya limited amount of information.

The tristimulus values tell us the amounts of threeimaginary primaries which if additively mixed will give1. the same colour as a surface2. illuminated by a standard source 3. and viewed by a standard observer 4. using one of the standard geometries 5. (assuming that the instrument does correspond to the standard conditions).

It follows that the mixture of the CIE primaries would be unlikely to match the surface if it was illuminated by a different source or if the ‘match’ was viewed by an individual observer or if different illuminating or viewing geometry was used.

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Only choice for standard observer-limitations

Obviously a degree of control can be exercised over the source and the geometry.

If these are important, we must try to ensure that the instrumental conditions correspond as closely as possible to those to be used when viewing the object visually.

The only choice as far as the standard observer is concerned is whether to use the 1931 (2°)observer or the 1964 (10°) observer.

Neither is likely to correspond closely to any individual observer, but either may well correspond reasonably closely with the average judgement of real observers,

bearing in mind that in many applications the product is mass-produced and will be seen by many different observers.

Tanveer Ahmed 6

Usefullness of the CIE System standard observer usefulness

A full specification of a colour requires X, Y and Z values (or equivalent sets such asx, y and Y or L*, a* and b*) for several different illuminants.

The results are still validonly for the standard observer and could be unsatisfactory for a real observer.

This should not be a problem in practice since the need is usually for the colours to be acceptableto a large number of potential customers;

the standard observer is probably a better guide to the population in general than any one observer. It often happens that one particular individual (the head dyer or senior buyer for a chain store, for instance) inspects fabrics, and problems may arise if that individual’s colour vision is appreciably different from that of the standard observer. Problems will be most severe for highly metameric pairs of samples.

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Illuminant limitations

Strictly speaking, the tristimulus values tell us nothing about the colour of a samplealthough, as discussed above, with experience we can make a reasonable estimate ofthe colour from either X, Y and Z or x, y and Y values. It is then essential that the illuminant used for the measurements is known.

Chromaticity coordinates of x = 0.314and y = 0.331 correspond to a neutral colour if derived from measurements under illuminant D65, but to a blue colour if derived from measurements under illuminant A.

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Matches to target limitations In many applications the aim is to match a particular target, which might be definedby a set of tristimulus values.

If we produce a sample and wish to test whether this matches the target, the sample and target measurements must correspond to exactly the same conditions (illuminant, standard observer, illuminating and viewinggeometries, and, in practice, the same instrument).

If, for example, the sample really is a good match to the target but the tristimulus values are measured using differentstandard observers for the sample and target, the resulting tristimulus values and chromaticityncoordinates would be appreciably different.

Again the sample and target might have different surface structures, such as those of matt paints, gloss paints or pile carpets: the tristimulus values could then be identical but the surfaces would look appreciably different. Whenever possible sample and target should have the same surface structure.

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Same Material & same instrument / observer / illuminant limitations It is usually important that the colours within one batch of fabric, and between

repeat batches, should match closely. In these cases the samples will of course be of thesame material and the same dyes or pigments will have been used. It would be natural to measure all the samples using the same instrument and (if a spectrophotometer was used) to calculate the tristimulus values for the same standard observer and standard illuminant. Under these conditions, if the tristimulus values for a sample are very closeto those for the standard, then

the sample will be a close visual match to the standard for any normal observer viewing under a light source roughly equivalent to the standard illuminant used to

calculate the tristimulus values. (Exceptions are known: for example, the appearance of metallic paints depends very much on the illuminating and viewing geometries.) If the variation of appearance with, say, viewing angle is different for the sample and the standard, they may not match visually even though the instrumental results (obtained with a different viewing angle) indicate that they should.