Smoothness or its inverse roughness is a property that influences both the appearance and the functionality of the paper. From the point of view of paper printing, it refers to the perfection of a paper's surface and the degree to which its uniformity resembles the surface of flat glass. Paper is said to have a smooth or rough texture, meaning that the irregularities on its surface are small or large. In the paper industry, the quality of the paper's surface or smoothness is often referred to as finishing or satin.

Papers are very different in their relative smoothness, because smoothness depends on other properties of the paper. Short fibers produce a smoother paper than long fibers. The preparation of the pulp and the way in which the fibers are distributed as the paper is formed on the machine fabric, have a great influence on the formation and the smoothness or roughness.

Bendtsen surface roughness is calculated from the air flow at the contact surface between a flat surface (glass plate substrate), circular measurement head and a paper or cardboard sample resting on the glass plate.

The air passes through the space between the ground crown of the head and the contact surface of the paper or cardboard sample. Airflow, measured in ml / min, is a sample measurement of surface roughness.

Air porosity is defined as the ability of paper to allow a flow of air under pressure to pass through it. It is a property related to the structure of the paper that depends on the number, size, shape and distribution of the pores in a sheet

Paper is a very porous material, as can be seen from its low specific weight, compared to that of cellulose, its main component. The specific weight of paper, from 0.5 to 0.8 g/cm³, is considerably less than that of cellulose, of 1.5. Porosity can be defined as the ratio between the volume of the space occupied by air in a paper and its total volume.

The air content, in common papers is usually 50% and can reach up to 70%. This air is found in paper, in 3 ways:

1. Real pores that are openings through the sheet,

2. Superficial pores that are only connected to one of their surfaces

3. Holes that contain air inside the blade..