Radiography in Modern Industry - Kodak

where the slope of the characteristic curve is greatest. For Film Z, as has been pointed out, theregion of greatest slope is in the central part of the curve. For Films X and Y, however, the slope--and hence the film contrast continuously increases throughout the useful density range. Thecurves of most industrial x-ray films are similar to those of Films X and Y.Use Of The Characteristic CurveThe characteristic curve can be used to solve quantitative problems arising in radiography, in thepreparation of technique charts, and in radiographic research. Ideally, characteristic curves madeunder the radiographic conditions actually encountered should be used in solving practicalproblems. However, it is not always possible to produce characteristic curves in a radiographicdepartment, and curves prepared elsewhere must be used. Such curves prove adequate formany purposes although it must be remembered that the shape of the characteristic curve andthe speed of a film relative to that of another depend strongly on developing conditions. Theaccuracy attained when using "ready-made" characteristic curves is governed largely by thesimilarity between the developing conditions used in producing the characteristic curves andthose for the film, whose densities are to be evaluated.A few examples of the quantitative use of characteristic curves are worked out below. In theexamples below, D is used for density and log E for the logarithm of the relative exposure.Example 1: Suppose a radiograph made on Film Z (See Figure 48) with an exposure of 12 mAminhas a density of 0.8 in the region of maximum interest. It is desired to increase the density to2.0 for the sake of the increased contrast there available.1. Log E at D = 2.0 is 1.622. Log E at D = 0.8 is 1.003. Difference in log E is 0.62Antilogarithm of this difference is 4.2Therefore, the original exposure is multiplied by 4.2 giving 50 mA-min to produce a density of 2.0.Radiography in Modern Industry 71