Radiography in Modern Industry - Kodak

greater length later. Although an image may be formed by light and other forms of radiation, aswell as by gamma rays or x-rays, the properties of the latter two are of a distinct character, and,for this reason, the sensitive emulsion must be different from those used in other types ofphotography.Figure 69: Cross section showing the distribution of the developed grains in an x-ray filmemulsion exposed to give a moderate density.Selection Of Films For Industrial RadiographyIndustrial radiography now has many widely diverse applications. There are many considerationsin obtaining the best radiographic results, for example:• the composition, shape, and size of the part being examined--and, in some cases, itsweight and location as well• the type of radiation used--whether x-rays from an x-ray machine or gamma raysfrom a radioactive material• the kilovoltages available with the x-ray equipment• the intensity of the gamma radiation• the kind of information sought--whether it is simply an overall inspection or the criticalexamination of some especially important portion, characteristic, or feature• the resulting relative emphasis on definition, contrast, density, and the time requiredfor proper exposureAll of these considerations are important in the determination of the most effective combination ofradiographic technique and x-ray film.The selection of a film for the radiography of any particular part depends on the thickness andmaterial of the specimen and on the voltage range of the available x-ray machine. In addition, thechoice is affected by the relative importance of high radiographic quality or short exposure time.Thus, an attempt must be made to balance these two opposing factors. As a consequence, it isnot possible to present definite rules on the selection of a film. If high quality is the decidingfactor, a slower and hence finer grained film should be substituted for a faster one--for instance,for the radiography of steel up to 1/4-inch thick at 120-150 kV. Film Y (See Figure 47) might besubstituted for Film X. If short exposure times are essential, a faster film (or film-screencombination) can be used. For example, 11/2-inch steel might be radiographed at 200 kV usingfluorescent screens and a film particularly sensitive to blue light, rather than a direct exposure filmwith lead screens.Radiography in Modern Industry 96