APPLICABILITY OF STANDARD GREY SCALE FOR REPORTING PERCEIVED COLOR DIFFERENCE OF PRINTS ENHANCED WITH PEARLESCENT PIGMENTS

Printed materials that exhibit goniochromism pose a challenge not only for colour measurement but also for estimating colour differences. A common procedure for determining perceived colour difference implies assessing colour pairs by means of a standard grey scale. This method was proven to be adequate for estimating, among others, perceived differences of low chroma goniochromatic samples found in the automotive industry, as well as for assessing colour differences of conventional prints. In this work, we were interested in the applicability of the same approach for estimating colour differences of printed samples enhanced with pearlescent pigments. Such prints are regarded as goniochromatic and depending on the type of the pigment, can exhibit very high chroma in angles close to specular reflection. This may pose a problem in evaluating colour differences because grey scale only shows a change in lightness. In addition, in some illumination/viewing geometries the differences between the two samples can be higher than those presented in the grey scale. Nine colour pairs (low-chroma digitally printed samples enhanced with three types of pearlescent pigments) were assessed in six viewing/illumination angles by a panel of ten observers. Our results show that chroma of the samples did not influence significantly interand intra-observer agreement, implying that grey scale references can be used for determining the perceived differences between chromatic samples. SDC grey scale provided sufficient range of colour differences only for the limited set of printed samples, leading to the conclusion that for assessing the differences of printed samples enhanced with pearlescent pigments it is necessary to use reference scale with higher difference range.