Operating Manual

4.3 Tube voltage and tube current
The voltage across the X-ray tube determines the energy spectrum and so the hardness of the
radiation, see figure 3-4. The intensity is proportional to the tube current, see figure 4-4.
This graph shows that, contrary to a change in tube voltage, a change in tube current does
not shift the spectrum (in other words: the hardness does not change).
The energy spectrum is also influenced by the characteristics of the high voltage applied to
the tube. When the spectrum of one X-ray tube on constant voltage is compared with that
of another with a current of pulsating voltage, of the same kV value, both spectra will be
slightly different. With a current of pulsating voltage there are, during each cycle, moments
of relatively low voltage, during which there will be a greater proportion of “soft” X- rays,
with their side-effects. This means that a set working on a constant voltage will provide a
higher intensity of hard radiation than one on a pulsating voltage; although both working
at the same nominal kV value.
However, even identical X-ray tubes may also show differences in generated energy. The
energy generated by one 200 kV X-ray tube will not be true identical to the energy genera-
ted by another X-ray tube with the same applied voltage, not even if they are the same type
of tube. This behaviour impedes individual calibration in kV of X-ray sets. Another reason
why it is hard to calibrate an X-ray tube within a small tolerance band is, that the absolute
level and wave characteristics of the supplied high voltage are difficult to measure.
It follows that it is difficult to standardise and calibrate X-ray equipment as far as spectra
and kV-settings is concerned, which precludes the exchange of exposure charts, see secti-
on 9.1. Each X-ray set therefore requires its own specific exposure chart. Even the exchan-
ge of a similar control panel or another (length) of cable between control panel and X-ray
tube can influence the level of energy and its spectrum. Usually after exchange of parts or
repair the exposure chart for that particular type of X-ray set is normalised (curve-fitting)
for the new combination of components. In practice adjusting the zero point of the expo-
sure graph is sufficient.
3534
Effective focal spot size
The projections of the focal spot on a surface perpendicular to the axis of the beam of
X-rays is termed the “ effective focal spot size” or “ focus size”, see figure 2-4. The effective
focus size is one of the parameters in radiography, see section 11-1. The effective focus size,
principally determining the sharpness in the radiographic image, has to be as small as pos-
sible in order to achieve maximum sharpness. The dimensions of the focus are governed by:
1. The size of the focal spot, and
2. The value of angle α, see figure 2-4.
It should be noted that when in radiography we speak of the “size of the focus” without spe-
cifying this more exactly, it is normally the effective focal spot size which is meant.
Conventional X-ray tubes have effective focal spot sizes in the range 4 x 4 mm to 1 x 1 mm.
There are fine-focus tubes with focal spots from 0.5 x 0.5 mm ad microfocus tubes down to
50 μm diameter or even much less, known as nanofocus tubes.
The effective focal spot size can be determined in accordance with the procedures descri-
bed in EN 12543 replacing the old IEC 336 which however is still in use. For more infor-
mation on focal spot measurement see section 18.1.
1. Dimension of the electron beam
2. Focal spot
3. Effective focal spot size
4. Anode target
5. True focus size
Fig 3-4. Energy spectra at varying tube voltages and con-
stant tube current (here 10mA)
Fig. 4-4. Energy spectra at varying values for tube current
and constant high voltage (here 200 kV)
Fig. 2-4. Effective focal spot size
KeV
KeV
relative intensity relative intensity