Residual ink in recycled pulp can be determined from a wide variety of pads and sheets using either a constant or measured near infrared (NIR) scattering coefficient. The method is usually chosen on the basis of the opaqueness of the prepared test media. Although both methods are regularly used, it is unclear whether NIR scattering properties of pulp vary due to changing proportions of fibers and fines and whether the changing NIR scattering coefficient affects the residual ink values. We investigated the effect of varying scattering coefficient on residual ink results obtained with unknown (constant) and known (measured) NIR scattering coefficients. We measured the NIR scattering coefficients and residual ink values (using the wavelength of 700 nm) from low-grammage sheets with deliberately varied filler content. By varying the filler content, changes were detected in the NIR scattering properties of pulp; therefore, the residual ink values were biased when a constant scattering coefficient was used. However, when the scattering coefficient was measured during the determination of residual ink, no deviation was observed when compared with the values calculated according to mass proportions. The measured NIR scattering coefficient should always be used during the determination of residual ink values.
