Digital radiography for the inspection of small defects

Digital radiography offers several advantages over conventional film-based radiography for NDT applications; the digital detectors typically require much less radiation to create an image. The disadvantage of using digital detectors is the relatively poor resolution in comparison to fine-grain film. The manufacturers of such detectors counter this objection by pointing out that projection magnification can be used to increase the image size, thereby, increasing the effective resolution of the final image. This is correct, if a suitable mini- or micro-focus generator is used. Unfortunately, these generators tend to be significantly lower in power, so it is essential that the detector be extremely sensitive. Many manufacturers of digital radiographic inspection units claim that the systems can use projection magnification up to a factor of 100 or more. This may be the case, but the geometric unsharpness (calculated from their own published specifications) would be many times larger than the pixel-pitch, making such high values of projection magnification redundant. TWI has been working with a consortium of digital radiographic users from across Europe. Our intention is to develop an inspection unit that uses a highly sensitive digital detector that is capable of rivalling fine-grain film in sensitivity, contrast and resolution. The samples used range from 1 mm-thick magnesium castings to 10 mm-thick steel welds.