Perception and measurement of the whiteness of papers with different gloss and FWA amount

The effect of mean gloss level on perceived whiteness was evaluated by magnitude estimation and pair wise comparison of papers with varying shades and gloss levels. The samples were printed on substrates of different gloss to target L*a*b* values measured under a light booth illumination having a 5000 K correlated colour temperature. Observers were able to rate the whiteness of the samples with large mean gloss differences in the 5000 K illumination and in the same illumination with an additional UV lamp. The CIE whiteness equation predicted well the perceived whiteness in both illuminations and the mean gloss level had no significant effect on perceived whiteness. This means that the CIE whiteness equation can be used to compare the perceived whiteness of papers having very different mean gloss levels. However, due to the different amount of fluorescent whiteness agents (FWA) in the papers, the perceived whiteness prediction was only valid when the measurement was performed under the same illumination as for the visual evaluations. Typical indoor illumination with florescent light tubes contains much less UV than the D50 or C illuminants. The instrumental whiteness differences due to mean gloss and instrument geometry were negligible compared to the effect of the UV content of the illumination.