Effect and Beam Coverage. For simplicity’s sake, most literature states that the intensity of radiation of the primary beam is constant. This is not quite correct. There is a variation in intensity due to the angle at which the X-rays are emitted from the focal spot. This is called the heel effect.
The intensity of the beam diminishes fairly rapidly from the central ray toward the anode side and increases slightly toward the cathode side. In general practice the heel effect is not evident, provided the maximum lateral dimension of the object to be radiographed is less than half the source-to-film distance (SFD). In other words, coverage of a 14 by 17inch film requires an SFD of approximately 36 inches to provide a field intensity of plus or minus 12 percent over the whole film. This is based upon using part of the radiation field within a cone having a 30-degree included angle. (The source for an X-ray tube is the focal spot.) For a single exposure of larger areas requiring multiple films, the SFD must be increased. For example, to determine the SFD to cover an area that fits within a circle with a diameter of 56 inches, do the following calculation:
If the SFD is limited, the radius of beam coverage can be calculated by rearranging the formula:
If an area larger than a 34-inch diameter circle needs to be radiographed, more than one setup must be used.
Beam coverage. The greater the field size available from an X-ray unit, the greater its radiographic inspection capacity. Except at extremely high voltages, the X-ray beam has an angle of coverage that is a function of the X-ray target angle, the geometry of the focal spot and the X-ray port size. As indicated in the discussion of heel effect in the previous paragraph, the physical size of the field of uniform intensity increases directly with the distance from the target to the film. However, the beam intensity decreases proportionally with the square of the distance, so the exposure (the product of amperage and time) must be increased to produce equivalent density on the radiograph. If a technique has been established but the situation requires a different SFD.
