Penetrameter cassette calibration to 400 kV and effects of extra-focal radiation when measuring tube filtration.

The penetrameter cassette, when used with suitable filters and calibrated against the same average wave form, can detect changes of less than 1 kV in the range 20-50 kV, 1 kV from 50-100 and from 1-4 Kv from 100-400 kV. Problems of calibration are discussed together with the necessity to use a suitable shutter with equipment which has a significant build-up and decay time during which the radiation quality may be varying. Extra-focal radiation does not normally present a significant problem in the measurement of kV, but when the penetrameter is used to measure inherent or total filtration extra-focal radiation can be a problem and may be similar in degree to the effects of varying target angles. When used to determine filtration, the generator used for calibration of the penetrameter and the one being measured should be adjusted to have the same target angle and similar degrees of extra-focal radiation by altering the size of the tube aperture as close to the focal spot as practicable. The results presented confirm the accuracy and convenience of the penetrameter for the measurement of kV and filtration over the extended range of energies.