If the SFD is limited, the radius of beam coverage can be calculated by rearranging the formula:
Using the same cone half-angle of 15 degrees, Tan q = 0.268
Assume SFD is limited to 60 inches
If an area larger than a 34-inch diameter circle needs to be radiographed, more than one setup must be used.
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, Table 6-1 gives some multiplication factors for calculating
new exposure times to be used with the original kV and mA values.