Figure 3-9. Rotating anode used to dissipate heat.
3-11. FOCAL SPOT
a. Actual Focal Spot. Electrons are focused to a specific area on the target by
means of a focusing cup. The focusing cup is merely a depression in the cathode
structure that partially surrounds the filament. That area of the target bombarded by the
electrons is known as the actual focal spot. The actual focal spot and the focusing cup
are illustrated in figure 3-10. The size of the actual focal spot significantly affects the
heat loading capacity of the tube. With larger focal spots, greater heat loading is
possible. The size of the focal spot is determined by a combination of three factors: (a)
the size and shape of the filament, (b) the size and shape of the focusing cup, and (c)
the angle of the target surface.
b. Effective Focal Spot. The focal spot, as it appears from directly beneath the
tube at right angles to the electron stream, is called the effective focal spot and is also
shown in figure 3-10. The size of the effective focal spot is a very important factor in a
diagnostic x-ray tube because it affects the detail on a radiograph; the smaller it is, the
better the detail. The size of the effective focal spot is determined by the size of the
actual focal spot and the angle of the target.