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Measuring 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 E1165.

  1. Put a small pinhole in a lead sheet of sufficiently thickness to stop all of the radiation. The smaller the pinhole, the sharper the image.

  2. 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 properly aligned.

  3. 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.

  4. Expose and develop the film.

  5. Measure the diameter of the focal spot image on the film.

Teaching Tips

Signal Interpretation
DAC Curve

Eddy Current
Depth of Penetration
Lenz Law

Backscatter Radiation
Film Handling Artifacts
Focal Spot Size
Lead Screens
Source/Object Orientation