Radiography in Modern Industry - Kodak

Figure 61: Abridged form of the exposure chart derived from a previous figure, butshowing exposures at 180 kV to produce a density of 1.0 on Film X. Dotted lines indicatethe metal thickness corresponding to the log E interval of the figure above. If theseparation of lines ABC and DEF is maintained, they can be moved up and down the chart.They will then mark off a large number of thickness ranges on the various kilovoltagelines, all of which will completely fill the density range which has been assumed to beuseful.A simple means for applying this method to routine work is as follows:Parallel lines are drawn on a transparent plastic sheet, such as a fixed-out x-ray film, in themanner shown in the figure below. The spacing between the base line and the line immediatelyabove is the log relative exposure interval for Film X between D = 1.0 and D = 3.5. It is laid off tothe same scale as the ordinate (vertical) scale of the exposure chart. Similarly, the distance fromthe baseline to any other line parallel to it can be made to correspond to the log relative exposureinterval for other density limits and films. This transparent guide is moved up and down on theexposure chart with its lines parallel to the thickness axis. The two guidelines being used form arectangle with the two vertical lines of the exposure chart that mark the thickness limits of thespecimen. The correct kilovoltage is the one whose graph intersects diagonally opposite comersof the rectangle. If the film type used is the one for which the chart was prepared, the correctexposure is indicated at the intersection of the upper guide line with the exposure scale (Point CFigure 1). If a different film is used, a suitable correction factor obtained either from tables ofrelative speeds or by the method described in Example 2 in "Overlay Methods" and "NomogramMethods", must be applied to the exposure as determined from the chart.Radiography in Modern Industry 84