Radiography in Modern Industry - Kodak

A filter reduces excessive subject contrast (and hence radiographic contrast) by hardening theradiation. The longer wavelengths do not penetrate the filter to as great an extent as do theshorter wavelengths. Therefore, the beam emerging from the filter contains a higher proportion ofthe more penetrating wavelengths. Figure 39 illustrates this graphically. In the sense that a morepenetrating beam is produced, filtering is analogous to increasing the kilovoltage. However, itrequires a comparatively large change in kilovoltage to change the hardness of an x-ray beam tothe same extent as will result from adding a small amount of filtration.Figure 39: Curves illustrating the effect of a filter on the composition and intensity of an x-ray beam.Although filtering reduces the total quantity of radiation, most of the wavelengths removed arethose that would not penetrate the thicker portions of the specimen in any case. The radiationremoved would only result in a high intensity in the regions around the specimen and under itsthinner sections, with the attendant scattering undercut and overexposure. The harder radiationobtained by filtering the x-ray beam produces a radiograph of lower contrast, thus permitting awider range of specimen thicknesses to be recorded on a single film than would otherwise bepossible.Thus, a filter can act either to increase or to decrease the net contrast. The contrast andpenetrameter visibility (See "Film Graininess, Screen Mottle") are increased by the removal of thescatter that undercuts the specimen (See Figure 40) and decreased by the hardening of theoriginal beam. The nature of the individual specimen will determine which of these effects willpredominate or whether both will occur in different parts of the same specimen.Radiography in Modern Industry 55