A large number of methods are available for evaluating the topography of paper surfaces. The Bekk smoothness is determined according to ISO 5627 (2002) as follows. At a defined pressure difference, the time in seconds is which a specified amount of air requires to flow radially inwards between the paper surface and a ring-shaped glass plate and on into a vacuum chamber is measured.
The smoother the surface of the paper, the longer the pressure equalization takes. The determi¬nation of roughness according to Bendtsen (ISO 8791/2–90) is related to the Bekk method for the determination of smoothness. In this case, however, the direction of the air flow is reversed and the magnitude of the air flow is measured. The air escapes under defined conditions between the measuring ring of the measuring head and the paper surface under constant overpressure. For a general description of roughness or smoothness, see ISO 8791–1.
Sometimes the topography of the paper surface is described using profile meas¬urements laser scanning. The scanning profilometer uses point sensors in con¬junction with high precision x- and y-stages to capture profiles and 3D data. The stages move the sample under the sensor and the sensor passes the captured height data to the computer for evaluation.
Another surface property of paper is the abrasion resistance. The mechanical abrasion resistance of surfaces is determined in the friction wheel process (DIN 53 109–93). In this process, the amount of abrasion which is obtained by abrading the conditioned or wet sample with an abrasion wheel of defined quality under defined conditions is measured.