Image sharpness in
a radiograph depends greatly upon the size of the focal size of the X-ray tube.
The focal spot is the spot on the target where the X-rays strike. Upon
colliding with the target, the electrons are abruptly decelerated and, due to
certain processes in the shells of the target atoms, electromagnetic rays,
X-rays, are released. As a consequence, the source of radiation is as big as
the diameter F of the focal spot itself. As
the size of the focal spot decreases, the image sharpness increases because
penumbra is minimized. An
ideally punctiform X-ray source makes for an ideally sharp image, whereas a radiation
source larger in size increases penumbra, causing the image to be blurred.
The width of the
penumbra is defined as B=F*(M-1).
Click on the image to see how focal spot size affects image sharpness. (Here: Bond wire inside a diode.)