© Color. Technol., 117 (2001) 19 Web ref: 20010104 Coloration Technology Society of Dyers and Colourists Performance of lightness difference formulae W Chou, a H Lin, a M R Luo, a, * S Westland, a B Rigg a and J Nobbs b a Colour & Imaging Institute, University of Derby, Kingsway House, Kingsway, Derby DE22 3HL, UK b Department of Colour Chemistry, University of Leeds, Leeds LS2 9JT, UK There are large variations between different previously published lightness difference experimental data sets. Two hundred and eight pairs of matt and glossy paint samples exhibiting mainly lightness differences were accumulated. Each pair was assessed about twenty times by a panel of fourteen observers using the grey scale method. The results were used to derive a new lightness difference formula (CII), and to a large extent, a new CIE lightness difference formula (CMC99). Both formulae were found to be more accurate than the typical deviation of an individual assessment from the mean of a panel of 20 observers, and outperformed the existing formulae using the present data set. The new CMC99 lightness difference formula is integrated into the new CIE colour difference equation CIEDE2000. The results also showed that special attention should be paid to measuring very dark samples. This is caused by poor instrument repeatability and inter-instrument agreement in this colour region. Introduction There are a number of advanced colour difference formulae currently in use that have been extended from CIELAB [1], which include CMC [2], CIE94 [3] and BFD [4,5]. They have been proven to perform much better than CIELAB for predicting the small to medium colour differences typically found in surface colour industries. The CMC and CIE94 colour difference formulae are the current ISO standards for the textile [6] and paint [7] industries, respectively. However, there are large differences between the three advanced formulae (CMC, CIE94 and BFD) in predicting lightness differences [8]. This is mainly caused by a large disagreement between the earlier published data sets [9– 12] which were used for deriving these formulae. Figures 1a–d show the CIELAB colour differences (DE) divided by the visual differences (DV) plotted against the L* values for the BFDF–L [9], RIT–DuPont–L [10], BFDB–L [11] and BFDL–L [12] data sets, respectively. For each data set, sample pairs were chosen from the main set (BFDF, RIT–DuPont, BFDB and BFDL) to exhibit mainly lightness Figure 1 A plot of DE/DV values against L* scale for (a) BFDF–L, (b) RIT–DuPont–L, (c) BFDB–L and (d) BFDL–L data sets; and a plot of the lightness weighting functions of the CMC99, CMC, BFD and CIE94 equations 0 20 40 60 80 100 0.0 1.0 2.0 3.0 L* DE/DV 0 20 40 60 80 100 0.0 1.0 2.0 3.0 L* DE/DV 0 20 40 60 80 100 0.0 1.0 2.0 3.0 L* DE/DV 0 20 40 60 80 100 0.0 1.0 2.0 3.0 L* DE/DV (a) (b) (c) (d) CMC99 CMC BFD CIE94 DE/DV DE/DV DE/DV DE/DV