This standard primarily specifies the test method for determining the smoothness of paper and board using the Bekk principle. The core of the method lies in creating a specific vacuum environment on both sides of the sample and measuring the time required for a certain volume of air to flow through the contact surface between the sample and the glass anvil, thereby indirectly quantifying the surface smoothness of the material. The specialized instrument required for this test is called a Bekk smoothness tester or board smoothness tester. This method is particularly suitable for relatively smooth paper and board, but it is not applicable to materials with rough surfaces, excessive thickness, or high air permeability, as excessive air passage can affect the accuracy of the test results.