A study of the secondary source phenomenon due to the presence of off-focal radiation in industrial micro-focus cone-beam X-ray computed tomography (XCT) systems and its influence on dimensional measurements is presented. Although off-focal radiation production within X-ray tubes has been studied for medical X-ray imaging systems, its properties and its effects on dimensional measurements in industrial XCT systems have not been discussed prior to this research. The study comprises: evaluation of the geometrical properties of off-focal radiation through a theoretical and experimental study of two-dimensional projection images of the scanned objects, verification of these properties using computer simulations and evaluation of the impact of the secondary source on dimensional measurements using experimental and simulation approaches. In addition, ways to minimise the effects of off-focal radiation are discussed, and reference samples for characterising the properties of off-focal radiation are proposed. The authors would like to thank all of their colleagues at the National Physical Laboratory for their assistance, knowledge and encouragement that was often encountered. In particular, the authors would like to thank, amongst others, David Williams, for his undivided attention, John Boone for X-ray source spectrum data. This project was funded by the UK Government's Department for Business, Energy and Industrial Strategy (BEIS) through the UK's National Measurement System programmes.

With Regards,
Sara Giselle
Associate Managing Editor
Journal of Medical Physics and Applied Sciences