Abstract: Method and apparatus for detecting differences in the wall thickness of a hollow object at various locations around its perimeter. Radiant electromagnetic waves are directed through one side of the object. The intensity of the waves is sensed at two adjacent locations where they emerge from the other side of the object after the waves have passed through two wall portions of the object at different sides thereof. The intensity of said waves at said locations is compared to ascertain any difference in wall thickness at said locations. This comparison is repeated as the waves scan the perimeter of the object and the successive comparisons indicate any trend of wall thickness departure from an average thickness. Corrective measures can then be taken at the apparatus which fabricates the object to restore wall thickness to the desired amount. The invention has particular utility in monitoring and controlling wall thickness of extruded plastic tubing in the course of fabricating plastic bags.