Mittuniversitetet

Whiteness is a commercially important characteristic of paper and board, although its perception depends on many factors that often are neglected by instrumental measurements. High whiteness improves the contrast of printed areas and increases the number of reproducible colours, but few quantitative studies have been published. In this paper, we report just-noticeable image quality difference (JND) from pair wise comparisons of images printed on paper substrate of different shades and whiteness. The JND was estimated to approximately 15 CIE whiteness, for the images and whiteness levels in this study, implying that a large substrate whiteness difference is required to get a significant visual impact on image quality. Unlike previous studies limited to colour rendering issues, the influence of the substrate’s shade as a surrounding frame to the images was also investigated here. It was found that the surrounding frame did not have a significant impact on image quality, when the images had an inherent dim background around the objects in the image. However, floating images in which the image objects are adjacent to the unprinted substrate would need further attention, since their perceived image quality seemed to depend both on the colour reproduction related to whiteness and shade, and on the contrast between the image and the substrate.