the Focal Spot Size
To develop an understanding of the effective focal spot size of the
X-ray generator , have students determine its size. The measurement
can be made from a radiograph produced using a flat piece of lead with
a small pinhole in it. The image is produce in a mode similar to that
of an old box camera. The following is a general procedure for making
the measurement, and a more detailed procedure can be found in ASTM
- Put a small pinhole in a lead sheet of sufficiently thickness to
stop all of the radiation. The smaller the pinhole, the sharper the
- Position the lead sheet directly perpendicular to X-ray beam and
exactly half way between the X-ray tube target and the film. Center
the pinhole in X-ray beam. This step is very important so use a string,
level, plumb bob or other tools to make sure the sheet and hole are
- Set the X-ray generator amperage (mA) relatively high and the energy
(kV) relatively low to produce an exposure where few make it past
the lead sheet except through the hole. Try not to saturate the radiograph
in the region of the hole.
- Expose and develop the film.
- Measure the diameter of the focal spot image on the film.