Radiography in Modern Industry - Kodak

Film contrast refers to the slope (steepness) of the characteristic curve of the film. It depends onthe type of film, the processing it receives, and the density. It also depends on whether the film isexposed with lead screens (or direct) or with fluorescent screens. Film contrast is independent,for most practical purposes, of the wavelengths and distribution of the radiation reaching the film,and hence is independent of subject contrast.Definition refers to the sharpness of outline in the image. It depends on the types of screens andfilm used, the radiation energy (wavelengths, etc), and the geometry of the radiographic setup.Subject ContrastSubject contrast decreases as kilovoltage is increased. The decreasing slope (steepness) of thelines of the exposure chart (See Figure 44) as kilovoltage increases illustrates the reduction ofsubject contrast as the radiation becomes more penetrating. For example, consider a steel partcontaining two thicknesses, 3/4 inch and 1 inch, which is radiographed first at 160 kV and then at200 kV.In the table above, column 3 shows the exposure in milliampere-minutes required to reach adensity of 1.5 through each thickness at each kilovoltage. These data are from the exposure chartmentioned above. It is apparent that the milliampere-minutes required to produce a given densityat any kilovoltage are inversely proportional to the corresponding x-ray intensities passingthrough the different sections of the specimen. Column 4 gives these relative intensities for eachkilovoltage. Column 5 gives the ratio of these intensities for each kilovoltage.Column 5 shows that, at 160 kV, the intensity of the x-rays passing through the 3/4-inch section is3.8 times greater than that passing through the 1-inch section. At 200 kV, the radiation throughthe thinner portion is only 2.5 times that through the thicker. Thus, as the kilovoltage increases,the ratio of x-ray transmission of the two thicknesses decreases, indicating a lower subjectcontrast.Film ContrastThe dependence of film contrast on density must be kept in mind when considering problems ofradiographic sensitivity. In general the contrast of radiographic films, except those designed foruse with fluorescent screens, increases continuously with density in the usable density range.Therefore, for films that exhibit this continuous increase in contrast, the best density, or upperlimit of density range, to use is the highest that can conveniently be viewed with the illuminatorsavailable. Adjustable high-intensity illuminators that greatly increase the maximum density thatcan be viewed are commercially available.The use of high densities has the further advantage of increasing the range of radiation intensitiesthat can be usefully recorded on a single film. This in turn permits, in x-ray radiography, the use oflower kilovoltage, with resulting increase in subject contrast and radiographic sensitivity.Radiography in Modern Industry 89