Micro-focus X-ray units are those with a focal spot size of less than 100 µm, with advanced micro-focus units offering focal spot sizes as low as 5 µm, which is achieved by focussing an electron beam at the target in combination with sufficient cooling to avoid overheating and damage to the system.
As a non-destructive testing (NDT) technique, micro-focus radiography allows for the observation of minute details within an object through the production of magnified X-ray images. Because micro-focus X-ray tubes produce X-rays from a small focal point, they enable high-magnification, high-resolution imaging, enhancing flaw detection capability and improving reliability in comparison to conventional radiography. These tubes come in a range of designs, including rod anode micro-focus X-ray tubes, which feature a long, thin, pencil-shaped anode (up to 1.5m) designed to reach inside narrow components.
The X-ray source generates a continuous beam of X-rays from the very small focal point. These focal points can be transmission targets, where a thin (1–10 µm thick), disc-shaped layer of metal (typically tungsten) is applied to the substrate – usually of beryllium or diamond – through which the beam passes to cast an X-ray projection onto the detector sensor. Alternatively, reflection targets can be used for higher power applications, due to the better cooling, although these offer a lower level of magnification.
As the beam passes through the sample, which is placed on a manipulator turntable, it casts an X-ray projection onto the detector sensor. The sensor converts the transmitted X-ray projection into a visible greyscale image, which is displayed on a monitor.
The magnification of the sample depends on its position between the micro-focus X-ray source and the detector. Moving the sample towards the X-ray source increases the magnification and improves the spatial resolution by increasing the number of pixels over which the X-ray image is projected. Even when operating at maximum magnification, the micro-focus X-ray spot ensures the image is always sharp. The associated software enables contrast enhancement and image integration.
Micro-focus X-ray produces high-resolution 2D images of a sample. These images, when taken in large quantities and from different angles, can be built into a computer-generated 3D digital model of a part in a process known as micro-focus X-ray computed tomography (CT).
Micro-focus X-ray systems are able to obtain very high resolution images due to the negligible penumbra effect associated with them. The penumbra effect is the blurry, partial shadow zone surrounding the dark, central shadow (umbra) of an object, which occurs when a light or radiation source is not from a single point. It causes hazy edges in photography, shadows, and radiographic images, with size determined by the source size and distance.