In 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 1
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 thesample, but ignore other important features such assurface texture, gloss and sheen. Thus a gloss paint sample and a matt paint sample might have the same tristimulusvalues, 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.
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 beingmeasured (typically a 2 cm diameter circle).In judging a colour visually some sort of averaging takes place, but the observer isalways conscious of any non-uniformity over the area viewed.Thus a matt paint surface, a woven textile surface and a pile fabric will alwayslook different from one another, but their measured tristimulus values could be the same.
Ignoring 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 primarieswhich if additively mixed will give1. the same colour as a surface2. illuminated by a standard source3. and viewed by a standard observer4. using one of the standard geometries5. (assuming that the instrument does correspond to the standard conditions).It follows that the mixture of the CIE primaries would be unlikely to match thesurface ifit was illuminated by a different sourceor if the ‘match’ was viewed by an individual observeror if different illuminating or viewing geometry was used.
Obviously a degree of control can be exercisedover the source and the geometry.If these are important, we must try to ensure that the instrumental conditionscorrespond as closely as possible to those to be usedwhen 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 maywell correspond reasonably closely with the average judgement of realobservers,bearing in mind that in many applicationsthe product is mass-producedand will be seen by many different observers.
Usefullness of the CIE System standard observer usefulness A full specification of a colour requires X, Y and Z values (or equivalent sets such as x, y and Y or L*, a* and b*) for several different illuminants. The results are still valid only 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 acceptable to 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.
Illuminant limitations Strictly speaking, the tristimulus values tell us nothing about the colour of a sample although, as discussed above, with experience we can make a reasonable estimate of the 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 = 0.314 and 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.
Matches to target limitations In many applications the aim is to match a particular target, which might be defined by 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 viewing geometries, 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 different standard 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.
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 the same 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 close to 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.