The influence of colour and surface topography on the measurement of effective refractive index of offset printed coated papers

This paper explores the influence of colour and surface topography on refractive index measurement by reflection, with particular reference to offset lithography printed coated paper. Previous work has studied the influence of coated paper porosity on the distribution of ink components throughout a printed offset ink film, using the technique of gloss goniophotometry and refractive index measurement. Porous papers were shown to have a fast removal of ink oils, which lead to a surface concentration of organic pigment and consequent depletion of resin binder. The resulting surface was optically rough and gave a less glossy ink film. The theory for gloss goniophotometer refractive index measurement is based upon polarised light reflectance at the interface between two smooth, non-absorbing isotropic media. A printed coated paper is not ideally suited to optical studies, as it is coloured, rough, non-homogeneous and contains particles which are capable of scattering light. This work was therefore carried out to determine the influence of colour and roughness on the measurement of effective refractive index. established that the effect of colour on measurement of refractive index is negligible when measured at an angle of 75°. The issue of roughness has not been resolved theoretically, with conflicting references on its effect on the measured RI. However, experimental work in the literature suggests that the effect of roughness on the measured RI is only marginally dependent upon the scale of the roughness.