A narrowly focused X-ray beam (1) penetrates the cross section of the moving panel (2), preferably at an angle of 45º.

A scanning detector system (3) records the density-dependent scattered radiation along the plane of the panel and a stationary detector (4) records the transmitted radiation.

By combining both signals the density of any part of the cross section can be directly calculated, independent of the surrounding structure. Thus, a scan of the full thickness as well as a sub scan of selected areas can be made.