Radiography in Modern Industry - Kodak

Figure 58: Overlay positioned so as to make the exposure chart and the nomogram applyto Film Y rather than to Film X. See No. 3, "Film type", above.4. Changes in processing conditions. It is difficult to make simple corrections for gross changes inprocessing conditions, because such changes affect both the speed and the contrast of the film,often to a marked degree (See "Effect Of Development Time On Speed And Contrast").For the sake of both economy and radiographic quality, avoid large departures fromrecommended times and temperatures.However, in manual processing of some industrial x-ray films, the development time may beincreased, for example, from 5 to 8 minutes at 68° F. The longer development may result inspeed increases that are frequently useful in practice, with little or no change in the shape of thecharacteristic curve--that is, with little or no change in film contrast. Exposure charts andnomograms of the types shown earlier can be made to apply to both development times.Assume, for instance, that increasing the development time of Film X from 5 to 8 minutes at 68° Fresults in a 20 percent gain in speed. The ratio of exposures required to achieve the same densitywould be 1:1.2. The logarithm of 1.2 is 0.08. A mark at log E interval of 0.08 above the arrowprinted on the exposure chart would allow the overlay to be positioned so that exposures for adevelopment time of 8 minutes could be read directly.5. Film density. Exposure charts apply only to a single density of the processed radiograph.However, by the use of data from the characteristic curve of the film, it is possible to supply asliding scale that can make the exposure chart applicable to any densities desired.Figure 44 was drawn for a density of 1.5. Let us assume that it is desired to make this chartapplicable to densities of 1.0, 2.0, and 3.0 also. The second column of the tabulation below isobtained from the characteristic curve of Film X in Figure 47.Radiography in Modern Industry 81