Radiography in Modern Industry - Kodak

Figure 115: Characteristic curve of a typical industrial x-ray film. Density differencescorresponding to a 20 percent difference in x-ray exposure have been evaluated for thetwo values of gradient illustrated in Figure 114.In general, if the gradient of the characteristic curve is greater than 1.0, the intensity ratios, orsubject contrasts, of the radiation emerging from the specimen are exaggerated in theradiographic reproduction, and the higher the gradient, the greater is the degree of exaggeration.Thus, at densities for which the gradient is greater than 1.0, the film acts as a "constant amplifier".Similarly, if the gradient is less than 1.0, subject contrasts are diminished in the radiographicreproduction.A minimum density is often specified for radiographs. This is not because of any virtue in aparticular density, but rather because of the gradient associated with that density. The minimumuseful density is that density at which the minimum useful gradient is obtained. In general,gradients lower than 2.0 should be avoided whenever possible.The ability of the film to amplify subject contrast is especially significant in radiography with verypenetrating radiations, which produce low subject contrast. Good radiographs depend on theenhancement of subject contrast by the film.The direct x-ray characteristic curves of three typical x-ray films are shown in the first figurebelow. The gradients of these curves have been calculated, and are plotted in the second figurebelow against the density. It can be seen that the gradients of Films X and Y increasecontinuously up to the highest densities that can conveniently be used in radiography. This is thebasis for the recommendation that with these films one should use the highest densities that theavailable illuminators allow to be viewed with ease. The gradient versus density curve of Film Zhas a form different from the others in that the gradient increases, then becomes essentiallyconstant over the density range of about 1.5 to 2.5, beyond which it decreases. With this film, thegreatest density difference corresponding to a small difference in transmission of the specimen isobtained in the middle range of densities and the maximum, as well as the minimum, usefuldensity is governed by the minimum gradient that can be tolerated.Radiography in Modern Industry 185