High-density failure of radiographic films

The recording of weak signals of high spatial frequency emanating from intensifying screens has been analysed in different films at varying densities. Even if the gamma value is kept constant over a wide range of densities, the recording of these signals is impaired at high densities above a certain optimum density, coinciding with the lower part of the straight part of the characteristic curve. The recording of these signals appears to be related to the number of silver halide grains per unit area available for signal recording rather than to the contrast as represented by the gamma value. When the density of the image is increased more grains are precipitated in creating the background density, leaving a smaller number of grains unaffected for the signals added on top of the background. If the signal frequency is high and the amplitude is low the remaining grains appear to become too few and too scattered for a proper recording, so that the signal cannot appear in the image. Since this occurs at densities where the film is not usually considered to be overexposed the phenomenon is called high-density failure. Different films may display great differences in high-density failure even if the sensitivity is the same.