What is the smoothness of paper?

This property of paper is a measure of the flatness or profile of the paper's surface. The profile or appearance of a paper surface can be described in terms such as "finish" and "pattern". Smoothness and roughness are inversely proportional and depend on many factors involved in the papermaking process such as degree of fiber refining, degree of wet pressing, degree of calendering, supercalendering, use of coatings and desired paper finish. Smoothness is an important parameter for writing, paper printing properties, aesthetic value and also important for paper bags. The smoothness of paper is usually measured using an "air flow meter". These instruments measure on the principle that the void volume between a paper and a flat surface such as glass is proportional to the smoothness or roughness of the paper, and the airflow rate between these surfaces is directly proportional to the void volume presented on the paper. Some instrument types are Bekk, Bendtsen, Sheffield and Print-surf smoothness Testers. Standard protocols for testing vary depending on the instrument used for testing.