CMC colour difference
A single number colour difference, as provided by E*, is obviously desirable, but it has been shown to have some weaknesses, due to the fact that it applies equal weighting to all three attributes of the colour. In practice, we are more likely to accept a greater difference in lightness than hue. To overcome some of these limitation, an alternative colour difference formula was published by the British Standard Institution (BSI) in 1988, referred to as the CMC (1:c). This is based on work carried out by the Colour Measurement committee (CMC) of the Society of Dyers and Colourists. It is not, at this stage, widely used in the printing industry. The (1:c) is substituted with two numbers as weighting factor for lightness (1) and chroma (c) to improve the correlation with visual perception. The CMC colour difference is then quoted as CMC (2:1), the numbers being the weighting factors.
COLOUR MEASURING INSTRUMENTS
Up until now we have been concerned with how to define colour using the CIE standards, but to make the actual measurement we require a means of evaluating the reflected light from the sample. To be more specific, we need to measure and determine the tristimulus value (X,Y and Z). This can be done using a spectrophotometer or a tristimulus colorimeter. Although both instruments can provide the tristimulus value, the manner in which they do this is quite different.
The spectrophotometer determines the proportion of light reflected from the sample in discrete wavelengths throughout the visible spectrum, at 5, 10 or 20 nm intervals depending on the spectrophotometer to provide