accommodate a wide variety of complex components with a multiple film exposure. Experience provides the required
In some cases, it is desirable to know the location of a given discontinuity relative to one of the plane surfaces of the
object. If repairs are to be made, it is desirable to know from which surface the repair should be started. A single
radiograph will not reveal this information. This information can be obtained by making a double exposure with
suitable markers placed on the object. Markers are placed on both the source side and on the film side. Two exposures
can be made on one film where discontinuity is very prominent or on two separate films that can later be superimposed.
These radiographs can be used for measurement purposes to obtain the desired information. (See Figure 6-18.)
Figure 6-18. Triangulation Technique Used to Determine Flaw Depth in an Object
In this method, small lead markers usually in the form of triangles are attached to the two surfaces of the object, one set
of three or four markers on the source side and one set on the film side. If two separate exposures are to be made, each
film must be carefully aligned with the object so that both films occupy the same position. After the markers are
positioned, one exposure is made with the normal source, object, and film position. A second exposure is made with all
conditions the same between the object and film with the exception that the source is shifted 10 to 45 degrees from the
initial position. The greater the shift, the greater the accuracy of determining the position of a given discontinuity from
one of the object's surfaces.