We introduce a method to measure liquid absorption into paper based on
measuring white light reflected from it. The method was used with 49.8–51.7
g/m2 pilot papers, whose hydrophobicity was tuned by their fiber
content and furnish. In addition, one of the samples was internally sized.
Impact wetting was used where a droplet of mineral oil, isopropanol or
deionized water was applied to the paper surface opposite to the one facing
the monitor. The volume of liquid per droplet was (0.9±0.2) μl to
(1.3±0.1) μl. The sample was illuminated with an ordinary 20 W light
bulb from above. The light intensity reflected off the sample was recorded
with a fast charge-coupled camera during the liquid penetration. Optical
changes in the paper sample were studied by calculating the average
intensity of the reflected light from a selected area. The results showed
that the speed of pore wetting, mainly in the z-direction, could be measured
with each liquid-sample combination. Hence a digital camera based light
reflection measurement can provide information about thickness direction
liquid transport in paper.