The film characteristic that is the reverse of contrast or gamma is film latitude. The higher the contrast, the smaller the
latitude and the lower the contrast, the greater the latitude. Latitude is, therefore, the range of radiation intensities that
a film is capable of recording.
The radiation reaching the film may be in part caused by the use of intensifying screens to reduce the exposure time.
The intensification factor for lead or calcium tungstate screens depends on the energy converted to either electrons or
light to which the screen is sensitive. This factor varies with kilovoltage and type of film. The film must be selected to
achieve highest efficiency of energy conversion from the screens used. The use of screens is covered more thoroughly
in Section III.
The characteristics of X-ray equipment must be known to properly operate the unit and obtain maximum results. The
utilization of X-ray equipment with the least amount of lost time requires a set of technique charts which show the
exposure times required for various thicknesses of material under stated conditions. These charts are generally
available from the manufactures of X-ray machines. (See Figure 6-34). However, due to the differences between
individual machines, it may be necessary or desirable to prepare additional technique charts for the specific purposes
and conditions to which the machine will be applied. If published technique charts are available; these charts can be
used as a guide in preparing the detailed charts.
Figure 6-34. A Typical X-ray Exposure Technique Chart.