Zeiss METROTOM 1500 G3, Operating Instructions Manual, Page: 116 / 120

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Attachment
Information on computed tomography
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61211-8140502 METROTOM 1500 G3 1.4
Factors for magnification
1 Detector
2 Rotary table axis
3 X-ray source
Focus (focal spot)
If high voltage is applied between the cathode and the anode in the X-
ray tube and current is flowing through the tube, the electrons are ac-
celerated in the direction of the anode. They hit the anode at high
speed. This point is called “focal spot”. At this point, the kinetic energy
of the electrons is converted into X-ray radiation.
The focus represents the smallest diameter of the electron beam bun-
dled in the X-ray tube. When the electron beam is focused, the focus lies
on the target surface.
NOTE
The two designations focus and focal spot are generally used as syn-
onyms. Strictly speaking, this is only valid if the focus lies in the same
plane as the focal spot. This is not the case when defocusing takes
place at higher power. The focus is shifted and the focal spot is thus
magnified. See illustration below.
Defocusing
If the radiation intensity must be increased, the focal spot needs to be
magnified to avoid melting of the target material. This is achieved by de-
focusing the electron beam. Then the focus no longer lies on the target
surface.