Zeiss METROTOM 1500 G3, Руководство по эксплуатации, Страница: 117 / 120

Attachment
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Information on computed tomography
61211-8140502 METROTOM 1500 G3 1.4
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Defocusing
1 Focusing unit
2 Electron beam
3 Target
4 Focus with 10 W; the focus lies exactly on the surface of the target
5 Limit of the focal spot in the case of a shifted focus at medium power, e.g. 100
W
6 Limit of the focal spot in the case of a shifted focus at high power, e.g. 200 W
NOTE
Defocusing starts at a power of 10 W. If the power is increased, the fo-
cal spot must be magnified by about 1 µm per watt to avoid melting of
the target.
Focal spot size The focal spot size depends on the power and lies in the µm range. The
focal spot size is relative to the blurring of the X-ray image: the smaller
the focal spot, the clearer the image.
Target material The physical loading capacity of the target material influences the focal
spot size. For this reason, tungsten is used as the anode material. It has
the highest melting point of all metals and a high ordinal number.
Effect of larger workpieces
Higher radiation intensities are required for the penetration of larger
workpieces. The focal spot is magnified and blurring increases. As larger
workpieces can only be represented slightly magnified, the blurring is
not very significant.
NOTE
To avoid increased blurring of the voxel matrix due to the larger focal
spot, the focal spot size must be correlated with the voxel size prior to
the measurement. Note: The focal spot should be smaller than the voxel
size.