The log relative exposure range for two films of different speed, when used together in this manner, is the difference in log exposure between the value at the low-density end of the faster film curve and the high-density end of the slower film curve. Figure 47 shows that when Films X and Y are used, the difference is 1.22, which is the difference between 1.57 and 2.79. It is necessary that the films be close enough together in speed so that their curves will have some "overlap" on the log E axis.
Limitations Of Exposure Charts
Although exposure charts are useful industrial radiographic tools, they must be used with some caution. They will, in most cases, be adequate for routine practice, but they will not always show the precise exposure required to radiograph a given thickness to a particular density.
Several factors have a direct influence on the accuracy with which exposures can be predicted. Exposure charts are ordinarily prepared by radiographing a stepped wedge. Since the proportion of scattered radiation depends on the thickness of material and, therefore, on the distribution of the material in a given specimen, there is no assurance that the scattered radiation under different parts will correspond to the amount under the same thickness of the wedge. In fact, it is unreasonable to expect exact correspondence between scattering conditions under two objects the thicknesses of which are the same but in which the distribution of material is quite different. The more closely the distribution of metal in the wedge resembles that in the specimen the more accurately the exposure chart will serve its purpose. For example, a narrow wedge would approximate the scattering conditions for specimens containing narrow bars.
Although the lines of an exposure chart are normally straight, they should in most cases be curved--concave downward. The straight lines are convenient approximations, suitable for most practical work, but it should be recognized that in most cases they are only approximations. The degree to which the conventionally drawn straight line approximates the true curve will vary, depending on the radiographic conditions, the quality of the exposing radiation, the material radiographed, and the amount of scattered radiation reaching the film.
In addition, time, temperature, degree of activity, and agitation of the developer are all variables that affect the shape of the characteristic curve and should therefore be standardized. When, in hand processing, the temperature or the activity of the developer does not correspond to the original conditions, proper compensation can be made by changing the time according to methods described in "Control of Temperature and Time". Automated processors should be carefully maintained and cleaned to achieve the most consistent results. In any event, the greatest of care should always be taken to follow the recommended processing procedures.