Radiography in Modern Industry - Kodak

Density = 1.5 Density = 2.5Film Relative Speed Relative Exposure for D = 1.5 Relative Speed Relative Exposure for D = 2.5X 100 1.0 100 1.0Y 24 4.2 26 3.9Z 250 0.4 150 0.7Note that the relative speeds computed at the two densities are not the same because of thedifferences in curve shape from one film to another. As would be expected from an inspection ofthese curves, this is most noticeable for Film Z.Although the shape of the characteristic curve of a film is practically independent of changes inradiation quality, the location of the curve along the log relative exposure axis, with respect to thecurve of another film, does depend on radiation quality. Thus, if curves of the type mentionedabove were prepared at a different kilovoltage, the curves would be differently spaced, that is, thefilms would have different speeds relative to the film that was chosen as a standard of reference.Density-Exposure RelationThe most common way of expressing the relation between film response and radiation intensity isthe characteristic curve--the relation between the density and the logarithm of the exposure--which has been discussed above. If, however, density is plotted against relative exposure to x-rays or gamma rays--rather than against the logarithm of the exposure--in many cares there is alinear relation over a more or less limited density range (See Figure 119). If net density (that is,density above base density and fog), rather than gross density, is plotted against exposure, thestraight line passes through the origin.Figure 119: Density versus exposure curve for a typical industrial x-ray film exposed todirect x-rays or with lead screens. The density range over which a linear relationshipexists between density and exposure depends on both the film type and the development.The linear relation cannot be assumed, however, but must be checked for the conditions involvedin the particular application because the density range over which it is valid depends on the filmRadiography in Modern Industry 189