Figure 3-10. Actual focal spot and effective focal spot.
c. Line Focus. For maximum effectiveness, a diagnostic x-ray tube should
provide an actual focal spot large enough to permit the necessary heatloading and an
effective focal spot small enough to produce optimum detail. This is accomplished in
part by the application of the line focus principle. For the purpose of line focus, the
x-ray tube is designed so that the electrons bombard a rectangular area on the target
surface. A specific target angle will then produce an effective focal spot that will be
approximately square and much smaller than the actual focal spot. Figure 3-11 shows
two different targets with angles of 20 and 40. As you can see, both targets produce
effective focal spots of equal size, but the actual focal spot at 20 is considerably larger.
Consequently, the 20 tube produces a relatively large actual and a relatively small
Figure 3-11. Angle and actual focal spot size in relations to effective
focal spot size.