The ISO 8791-4:2021 standard specifies a method for determining the asperity/smoothness of paper and Cardboard using the printing surface method (Print-surf method). This method is suitable for all printing paper and Cardboard capable of forming a substantially airtight sealing of the protected area of the measurement head. Assess the asperity or smoothness of the paper by measuring the amount of air that leaks through the surface of the paper. This method is particularly suitable for printing paper and helps to optimize printability and ensure the performance of the paper during printing.