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In radiographic non-destructive testing, what factors affect definition and contrast?

   

Frequently Asked Questions

The following tables illustrate the main factors and how these affect radiographic definition and contrast.

Abbreviations: ffd focus-to-film distance
  ofd object-to-film distance
  s or f focal spot or source size
  Ug definition

Factors affecting definition

FactorEffectExplanation
Film speed (grain size) Slower film (finer grain) = better definition Boundaries are defined by grains: smaller dots = sharper boundary
Energy (kV, kVp, keV, MeV) Lower energy = better definition Lower energy = smaller area affected by a photon/electron strike
Screens Close contact = better definition Affected area is enlarged by any gap between screens and emulsion
Vibration Any vibration reduces definition Applicable to any radiograph
Geometry of image formation Longer ffd, smaller s or f, smaller ofd = better definition Interaction of ffd, f or s, ofd in determining Ug = (f x ofd) / (ffd-ofd)
Development Optimum time is required Under-development is uneven, developing only part of a boundary; over-development leads to irregular growth along a boundary of black metallic silver into the less dense area

Factors affecting contrast

FactorEffectExplanation
Photographic density Higher density = higher contrast Derived from the slope of the characteristic (sensitometric) curve: minimum density determined from contrast, maximum from adequate transparency
Film speed (grain size) Slower film (finer grain) = better contrast Making grain size smaller greatly slows the film: to reduce the slowing, more silver salt is added to the emulsion giving higher contrast
Energy (kV, kVp, keV, MeV) Lower energy = better contrast Energy affects subject (object) contrast: at lower energies, a given thickness increase (or physical density increase) results in greater absorption
Screens Lead screens = less scatter Scatter has a longer path length through the lead, and scatter is more readily absorbed per mm of path length, than the primary beam
Variations in object thickness or physical density Large difference in thickness or physical density = higher contrast Subject (object) contrast is the difference in the amount of radiation penetrating adjacent areas of the object
Development Optimum time is required Under-development gives low density and unevenness of density; over-development results in the preferential darkening of areas that should be light

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