A regular check of the accuracy of X-ray inspection systems by means of standard test pieces (STP) has been established in the EU by the decision of the EU Commission (2010) 774 and the European Civil Aviation Conference (ECAC). Thus, all EU countries and some non-EU countries are legally obliged to regularly check and ensure the image quality and performance of their X-ray inspection systems. For this purpose, test kits contain samples of various materials with different densities.