Small and Moderate Colour Differences

In nineteen regions of colour space groups of about 30 samples, differing by small to moderate amounts from one designated as ‘the standard’ were generated by offset printing. Twenty observers estimated the perceived size of each of these differences twice, using cateogry scaling ‐ the judgements were not based on acceptability. The samples were placed, in turn, adjacent to the standard on a middle gray surround, subtended 4d̀ on the retina, were illuminated with semidiffuse fluorescent light, simulating daylight at about 45d̀ incidence, and viewed along the normal. The visual data were converted from an ordinal to an interval scale. The samples were measured by abridged spectro‐photometry, and CIE coordinates, obtained by integration using the spectral‐power distribution of the illumination source and the 1931 2d̀ standard observer. Colour differences were calculated by eleven formulae and correlated with the visual interval‐scale results. The usual low correlation coefficients were found. The data are analysed colour by colour and significant differences are noted. Collectively, the Adams Chromatic Value (ANL AB 40), Saunderson‐Milner, and CIE 1976 L*a*b* formulae gave better performances.