Focal spot

Focal spot is the area of the anode surface which receives the beam of electrons from the cathode.

Basic concept

Size and shape of the focal spot is determined by the size and shape of the electron beam when it strikes the anode 1.

Size and shape of the electron beam is determined by:

  • dimensions of the filament tungsten coil
  • construction of the focusing cup
  • position of the filament in the focusing cup

The focal spot sizes commonly employed are:

  • 0.3mm and 0.6mm, usually for mammography
  • 1.0mm and 1.2mm, usually for general radiography

The effective focal length of a focal spot can be calculated using:

Effective focal length = Actual focal length x sin θ 

where θ is the anode angle

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Article information

rID: 29771
Section: Physics
Tag: shetty, refs
Synonyms or Alternate Spellings:
  • Focal spots

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