The depth position (d) of a defect can be determined by the parallax-method, as shown in figure 4-18. The radiograph is exposed from two opposite angles. The required quantity of radiation is equally divided over positions A and B. Only one film is used. The shift in defect image on the film (G in mm) is a measure for the depth position; the shift of the source (A to B in mm) and the source-to-film distance (H in mm) are important data. The depth position is calculated with the formula: d = (GxH) / (AB+G).
Another, much more complex method of depth determination is stereo-radiography, by which two separate films are exposed which are viewed simultaneously via mirrors. However, this method is rarely used.