Zeiss METROTOM 1500 G3, Operating Instructions Manual, Page: 85 / 120

Setting up the workpiece and measurement
61211-8140502 METROTOM 1500 G3 1.4
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Selecting voltage and current
Voltage
The high voltage in an X-ray tube accelerates electrons onto a target
(e.g. tungsten). The electrons are decelerated in the target and excite
the atomic nuclei, thus generating X-ray radiation with different ener-
gies. The complete spectrum of all X-ray photons generated is referred
to the X-ray spectrum.
The voltage, the target material and the prefilters used define the X-ray
spectrum for acquisition of the workpiece.
An increase in voltage produces a high-energy spectrum. Photons are
typically generated with energies between approx. 20 keV and the ap-
plied voltage. For example, with an acceleration voltage of 180 kV X-ray
photons with up to 180 keV are produced. The higher the energy of an
X-ray photon, the better it can penetrate the workpiece. A spectrum
with a higher average energy is referred to as “hard” X-ray radiation.
Due to the varying radiographic capability of the photons, the transmis-
sion of radiation through the object no longer has a linear relationship
to the projected thickness. This leads to artifacts in the reconstruction.
Since the spectrum always becomes harder following the penetration of
objects, this effect also is called beam hardening.
Current
The current of the X-ray tube describes how much light is generated and
must be set totally independent of the voltage.
The X-ray spectrum is defined via the voltage and the prefilter, while the
current controls the light intensity. A higher current generally leads to a
brighter image, i.e. more X-ray photons contribute to the image forma-
tion. The noise in the reconstructed volume can thus be reduced consid-
erably. However, due to the increase in the current more energy also is
transferred to the target. Therefore, the area struck by the electrons, the
so-called “source spot”, must be enlarged in order to prevent melting of
the tungsten. The maximum achievable resolution is thus impaired.
In order to define a good measurement, you can initially concentrate on
the setting of the spectrum and the position of the workpiece. The opti-
mal current is then set via the Magic Wand function in METROTOM OS.
Here, the current is selected so high that the resulting focal spot size
matches the currently set voxel size (as a result of the geometric ar-
rangement).