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

It is 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.