Having obtained such high correlations, and considering the fact that all the 12 scales were quantified on the basis of the same common lightness scale, visual differences (i.e., [DELTA][E.suv.v,G], [DELTA][E.sub.v,DOI], and [DELTA][E.sub.v,OP]) and the corresponding instrumental differences (i.e., [DELTA][G.sub.i], [DELTA][DOI.sub.i] and [DELTA][LW.sub.i]) were employed in order to determine the errors involved in implementation of the "principle of additivity." Equations (2) and (3) were used to calculate the "total visual difference" ([DELTA][E.sub.v,T]) values and the corresponding "total instrumental difference" ([DELTA][I.sub.T]) values, respectively, for each level of
achromaticity, and for each group of observers (i.e., expert, inexpert, and all observers).