Radiography in Modern Industry - Kodak

Duplicating RadiographsSimultaneous RadiographyIf it is known beforehand that a duplicate radiograph will be required, the easiest and mosteconomical way of obtaining it is to expose two x-ray films of the same type simultaneously in theoriginal exposure. Thus two essentially identical radiographs are produced at little or no extra costin exposure time.If lead-foil screen techniques are used, it is usually advisable to put both films between a singlepair of screens. If each film is placed between a pair of screens, the absorption of the back andfront screens, which separate the two films, results in a lower density on the back film. When twofilms are used between a single pair of screens, an increase in exposure is normally requiredbecause each film receives the intensification from only a single screen instead of from two. Theexposure increase required must in most cases be determined by tests because it depends onboth the kilovoltage and the specimen.If envelope-packed film with integral lead oxide screens is used, two envelopes can besuperimposed since the absorption of the lead oxide layers within the package is relatively low.With direct exposure techniques, both films should be put into the same exposure holder, eitherwithout interleaving paper or with both sheets in the same interleaving folder. With directexposure techniques it may sometimes be found, especially at the higher kilovoltages, thatexposure time must be slightly decreased to achieve the same density as obtained on a singlefilm. This is because each film acts as an intensifying screen for the other. The effect is small, butmay be puzzling if unexpected.Two or more films in factory-sealed envelopes may be exposed simultaneously. No adjustment ofexposure is required because the intensifying radiation from the films is largely absorbed in thematerial of the envelopes.In direct exposure techniques, if it should be necessary to use separate cardboard or plasticexposure holders for each film, care should be taken to remove any lead backing from the frontexposure holder. The presence of the lead will cause a marked difference in density betweenfilms.In general, observance of these rules results in two identical radiographs of equal density.Copying RadiographsWhen copies of existing radiographs are needed, they can be produced by contact printing onspecial duplicating film. This is a direct-positive film, which produces a duplicate-tone facsimile.The characteristic curve of a typical duplicating film is shown in Figure 98. The negative slopeindicates that increasing the exposure to light decreases developed density. Thus, if a sheet ofthis film is exposed to light through a radiograph and then processed, the dark parts of theradiograph (which transmit little light to the duplicating film) are reproduced as high densities, andthe low density areas of the original are reproduced as low densities.Radiography in Modern Industry 159