New requirements for digital radiographic testing of welds according to ISO standards

Users of EN 14784-2 (general principles for computed radiography with phosphor imaging plates) reported about difficulties to achieve testing class B in weld testing with imaging plates. One of the reasons is the insufficient consideration of the inherent detector unsharpness (ui) in the minimum requirements. Digital detectors have a higher inherent unsharpness compared to film, which can even exceed the geometrical unsharpness (ug) of the typical contact technique. In EN 444 and ISO 5579 (general principles for film radiography) ui is neglected for the calculation of the minimum source-to-object distance (SOD), because it is small compared to the geometric unsharpness (ug). Considering ui for digital detectors results in a new equation for SOD (see ISO/FDIS 17636-2). Therefore, the increase in total image unsharpness requires the compensation by a larger SOD to reduce ug. This contribution discusses the need for change of the SOD for different setups (detectors, focal spots, etc.) and explains the difference in image quality, achieved on basis of the extended equation of ISO/FDIS 17636-2. Furthermore, the detection of image quality indicators depends on the achieved Contrast-to-Noise ratio (CNR) and total image unsharpness. Both of them are essential parameters, which influence the contrast sensitivity. Additionally, new compensation principles (e.g. compensation of missing spatial resolution by enhanced contrast sensitivity) allow to widen the application range of digital detectors for radiographic weld testing.