ISO 8791-5:2020 "Determination of paper and Cardboard asperity/smoothness (air leak method) Part 5: Okun method" is a standard published by the international standards organization (ISO) that specifies a method for determining the smoothness of paper and Cardboard using the Okun method. The standard applies to all printing papers and Cardboard and aims to unify Measuring methods and ensure accuracy and comparability of test results. The Okun method evaluates the smoothness of paper by measuring the amount of air leakage between the paper surface and the measurement head. This method is important for evaluating paper printability, Spreader performance and surface preparation. It is important to note that ISO 8791-5:2020 is a revised version of ISO 8791-5:2000, which has been repealed. Therefore, ISO 8791-5:2020 is the current valid international standard.