a schematic lateral block diagram view of a fusing medium 1 for fusing toner on print material 3 which is placed in a printing machine that is not depicted here fusing medium 1 is configured here as a fusing roller by way of example fusing medium 1 may assume additional configurations fusing medium 1 is placed above print material 3 and compresses it with a certain force beneath print material 3 a counter rotating pressure medium 2 is placed which from below exerts a counterforce corresponding to the force of fusing medium 1 acting from above the counter pressure medium 2 is configured here as a counterpressure roller by way of example fusing medium 1 and counterpressure medium 2 move in the directions shown by the curved arrows print material 3 is fed between fusing medium 1 and counterpressure roller 2 in the direction of the straight arrow by action of heat and pressure from fusing medium 1 and counterpressure medium 2 toner is securely fused to print material 3 behind fusing medium 1 and counterpressure medium 2 viewed in the direction of transport toner is fixed to print material 3 adjacent to fusing medium 1 is a measuring device 5 which records the properties of fusing medium 1 for one measuring device 5 records the surface roughness as a property of fusing medium 1 by determining the radius of fusing medium 1 at various locations on the surface of fusing medium 1 in the microscopic.