Radiographic image subtraction: WANTO (Weapons Agencies Nondestructive Testing Organization) '86

Image subtraction is an old but useful technique that needs occasional resurrection to remind us all that it exists. It is useful mainly when small changes must be detected. Here ''small changes'' can refer to physically small components in complex images, or small changes in intensity (density). It is mainly applicable when changes in a particular part are to be detected, in a before-after situation. It can be applied to detect differences between a part and a ''master part'' if the ''production'' parts differ from the master in ways which are small compared with the features one is attempting to detect. Even if the production part differs significantly from the master, it is often useful to make a negative mask as an aid to interpretation of the radiograph.