The misconception about radiography is the idea that it can see and locate all discontinuities existing in a weldment. People think that it gives them an x-ray vision for looking into the inside of a weld just as "Superman" has the ability to look behind closed doors. Because of this, radiography has become the inspection technique of choice in inspecting critical welds. One reason for this is because radiographic discontinuities are reasonably simple to locate and most personnel can see the defect image with very little training. Darker areas in the film indicate some item that must be evaluated. The darkened areas are an indication of how much radiation the film has seen in that location.
The more radiation the film picks up, the less cross-section of the weld has at that location. Cross section differences can exist because of weld discontinuities. The more radiation that the film records, the darker it becomes. Rounded indications are normally characterized as porosity and sharp straight lines as incomplete penetration, even by the novice observer. The learning curve for evaluating radiographs made on welds at first glance seems very simple. It is more difficult than most realize. Without knowing the facts, many defects are not located or are misevaluated.
Radiographic Film Interpretation (RTFI)