Radiography in Modern Industry - Kodak

Sometimes, as when sets of reference radiographs are being prepared, it is required to reproduceboth the densities and the density differences of the original. This can be done by mounting auniform density filter behind the copy. In the example cited above, the densities of the originalwere 3.0 and 3.5, and the densities of the copy were 1.0 and 1.5. If a uniform density of 2.0 isadded, the total densities will be raised to 3.0 and 3.5, just as in the original.The least expensive and most convenient neutral density filter is a processed sheet of finegrainedphotographic film which has been uniformly exposed to light. This film should be coatedon clear base, so that the color of the copy is not changed. Further, the film chosen should bevery slow so that exposure is easy to control. Ideally, it should also be sensitive only to the blueportion of the visible spectrum and may be handled under the safelighting conditions used for x-ray films.In assembling copies and neutral density filters, the duplicating film and the neutral density filtershould be positioned so that their emulsion sides are toward the viewer.FluoroscopyFluoroscopy differs from radiography in that the x-ray image is observed virtually on a fluorescentscreen rather than recorded on a film. A diagrammatic sketch of an industrial fluoroscopic unit isshown in Figure 99.Figure 99: Schematic diagram of an industrial fluoroscope. Commercial models may differfrom the illustration. For more rapid examinations, industrial fluoroscopes may beprovided with material conveyors.Fluoroscopy has the advantages of high speed and low cost.However, fluoroscopy has three limitations: (1) Examination of thick, dense, or high-atomicnumberspecimens is impractical, because the x-ray intensities passing through them are too lowto give a sufficiently bright image on the fluorescent screen. (2) The sensitivity of the fluoroscopicprocess is not as great as that of radiography. This is caused in part by the lower contrast andcoarser grain of the fluoroscopic screen as compared to the film record, and in part by theRadiography in Modern Industry 161