Smoothness of medical packaging substrates may be measured using a modern Gurley tester.

The Sheffield smoothness of a substrate is calculated by measuring the rate of air flow over the surface of the sheet. The test measures the amount of air that escapes when a flat surface is pressed onto the sheet. The smoother the sheet the less air escapes. The instrument is also capable of measuring smoothness and reporting the result according to the Bekk method. Smoothness is closely related to porosity. Smoothness or sheet flatness can affect ink receptivity. The smoother the sheet the sharper the printed image.

TAPPI T 538 Smoothness of Paper and Paperboard (Sheffield Method)

Gurley Smoothness Tester