How do I measure the effective focal spot?

There are many special applications of radiography in NDT. This chapter describes a limited number of different examples to illustrate this diversity. Apart from the use of radiation in image forming radiography, it is also used in, for instan- ce, measuring instruments such as metal alloy analysing instruments (Positive Material Identification, PMI). This type of non-image forming instruments and applications are outside the sphere of this book.

Measuring the effective focal spot

The effective focal spot is an important feature of an X-ray tube and is specified by the manufacturer. In general it can be said; “the smaller the better”. As focal spot size is also a critical exposure parameter (see section), the accuracy of the manufacturer’s information is of vital importance. Until recently, the film or pinhole method was commonly used. Since 1999, EN 12543-1 prescribes a standardised method which, however, does not have the general support of suppliers, as it requires expensive instrumentation and is time-consuming. The EN-method, suitable for effective foci <0.2 mm, involves scanning the X-ray tube radi- ation beam with a scintillation counter through a double collimator with an extremely small opening of 10 μm. The resulting intensity values are then represented in a three- dimensional (isometric) diagram from which the effective focal spot can be deduced. The film or pinhole method is still used by X-ray tube operators, to verify the equipment- manufacturer’s data. Following the “camera obscura” principle, the X-ray tube projects its focus through a very small hole in a lead plate onto a film. The lead plate is positioned exactly halfway between focus and film. To prevent scattered radiation, sometimes the hole is made in a tungsten plug which forms part of the lead plate. After development, the effective focal spot size can be measured on the film, with the aid of a magnifying glass. The latter method, still allowed and accepted by EN, results in marginally smaller effective focal spot sizes. Establishing the effective focal spot size of a panoramic X-ray tube is con- siderably more complicated. It is therefore recommended to just make a radiograph of the object with the right IQIs and check the results for compliance with the quality requirements specified.