T.O. 33B-1-16-14If the SFD is limited, the radius of beam coverage can be calculated by rearranging the formula:R TanSFD= qUsing the same cone half-angle of 15 degrees, Tan q = 0.268Assume SFD is limited to 60 inchesR inches= 02866017.If an area larger than a 34-inch diameter circle needs to be radiographed, more than one setup must be used.6.2.4.5.2 Beamcoverage.The greater the field size available from an X-ray unit, the greater its radiographic inspection capacity. Except atextremely high voltages, the X-ray beam has an angle of coverage that is a function of the X-ray target angle, thegeometry of the focal spot and the X-ray port size. As indicated in the discussion of heel effect in the previousparagraph, the physical size of the field of uniform intensity increases directly with the distance from the target to thefilm. However, the beam intensity decreases proportionally with the square of the distance, so the exposure (theproduct of amperage and time) must be increased to produce equivalent density on the radiograph. If a technique hasbeen established but the situation requires a different SFD, Table 6-1 gives some multiplication factors for calculatingnew exposure times to be used with the original kV and mA values.
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