The targets of X-ray tubes are fundamentally classified as reflection targets and transmission targets. The difference when using reflection or transmission targets is in the available radiation power, and therefore the measurement time in conjunction with the available minimum focal spot size. In a reflection target (also referred to as a direct emitter), the X-rays are reflected by the target. This design provides greater heat dissipation allowing higher power and thus shorter measurement times, but at the cost of lower resolution. The minimum focal spot size that can be achieved with a reflection tar- get tube is a few microns. This is sufficient, in principle, for many measurement tasks, where a structural resolution in the micron range is seldom required. With sub-voxeling, nevertheless, measurement errors of less than 1 µm can be obtained. The decisive disadvantage of reflection targets, however, is that small focal spot sizes can be achieved only with very low power and thus longer measurement times (typically 5µm at 5W). Transmission targets are penetrated by the X-rays, which are propagated in the direction of the electron beam. X-ray tubes with transmission targets have the advantage that the thin target produces a smaller beam diameter (focal spot) and thus a higher resolution is achievable. Also using transmission targets, the focal spot size depends on the power setting. In recent years, however, transmission target tubes have been developed that greatly reduce this effect in comparison with reflection target tubes. It is thus possible to achieve a small focal spot even at medium power levels (typically 5µm at 25W). Very high requirements for resolution can thereby be met even for larger work pieces and relatively short measurement times.

For lower resolution requirements and higher power levels, deliberate defocusing is used to limit the power density at the target and thus to in- crease its service life. X-ray tubes are available in both open and closed designs. In a closed X-ray tube, the vacuum is generated once, by the manufacturer, and is maintained by hermetically sealing the vacuum chamber. Closed X-ray tubes are available as reflection target tubes for voltages up to about 150 kV. These tubes can be used over a service life of several years without maintenance. After this period, the complete X-ray tube must be replaced. Microfocus X-ray tubes with voltages above 150 kV and transmission target tubes are built as open systems, because the wear on the electrodes is higher and thus regular maintenance is required. For open X-ray tubes, the vacuum is generated by a separate vacuum pump during operation. This makes it possible to open the X-ray tubes for servicing. Considering the maintenance costs of open systems and replacement costs for closed systems, the cost to operate both types of tubes is similar. Newer systems with transmission target and monoblock design (tube, vacuum pump, and high voltage generator in a single unit) provide the advantages of both designs and allow good resolution at high power with low maintenance costs.

See in the image above: Principle of X-ray generation: The heated cathode emits electrons in a vacuum. They are accelerated by the electric field between the cathode and anode. When they impact the target, the electron radiation is converted to X-rays. a) Reflection target b) Transmission target