Read about Radiographic Film Interpretation(RTFI) as offered by Base Line Data
 

There is a misconception with the thought that radiography can see and locate all discontinuities that exist in a weldment. People think that it gives them x-ray vision for looking into the inside of a weld just like "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 "lay" personnel can see the defect image with very little training. Darker areas on 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 sees, the less cross section the weld has at that location. Cross section differences can exist because of weld discontinuities. The more radiation that the film sees, 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 you think. Without knowing the facts, many defects are not located or are misevaluated.