Radiography in Modern Industry - Kodak

Figure 98: Characteristic curve of a typical film used for duplicating radiographs.Further, the gradients of the duplicating films are -1.0 over their useful density range. This meansthat, within this range, density differences in the original radiograph are faithfully reproduced inthe copy.ExposureThe exposure technique for duplicating film is simple and may be varied to suit the equipmentavailable. The radiograph to be copied is put in a glass-fronted printing frame (available fromdealers in photographic supplies). A sheet of duplicating film is placed on top of the radiograph,emulsion side in contact with the radiograph to be copied. The back of the printing frame is closedand an exposure to light is made through the glass. Any convenient light source can be used, andthus exposure conditions are difficult to specify. However, a conventional fluorescent x-rayilluminator is a usable light source. At a distance of about 2 feet between illuminator and printingframe, exposures range from several seconds to a minute. These exposures are long enough totime conveniently with simple means yet short enough to be efficient.When a satisfactory exposure time has been found by trial for a particular radiograph, exposuresfor others can be estimated much more accurately. The densities in the areas of interest of thetwo radiographs are read, and subtracted one from the other. The antilogarithm of this densitydifference is the factor by which the original exposure must be multiplied (if the secondradiograph is darker than the first) or divided (if the second radiograph is the lighter).Care should be taken to keep the glass front of the printing frame free from dirt, because anythingopaque adhering to the glass appears as a dark mark on the processed duplicate.Reproduction of DensitiesAs shown by the characteristic curve of Figure 98, the maximum density obtainable (that is, withno exposure on the duplicating film) may be below the maximum density in the radiograph to becopied. Thus, if the densities in the areas of interest in an industrial radiograph were 3.0 and 3.5,the duplicating film, the curve of which is shown in Figure 98, could not reproduce thesedensities. It would, however, reproduce the density differences exactly, and thus reproduceexactly the radiographic contrasts of the original. If the densities in the original were 3.0 and 3.5,these could be reproduced as 0.7 and 1.2, respectively or 1.0 and 1.5, or 1.3 and 1.8, dependingon the exposure given the duplicating film.In many cases, a reproduction of the radiographic contrasts alone is quite sufficient. (Note that ifa radiograph with densities of 1.0 and 1.5 were displayed on an illuminator, and a similarradiograph but with densities of 3.0 and 3.5 were displayed on an illuminator 100 times as bright,an observer would be unable to distinguish between them.)Radiography in Modern Industry 160