TWI Frequently asked questions
by Davide Kleiner
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
| Factor | Effect | Explanation |
| 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
| Factor | Effect | Explanation |
| 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 |
