where
Ug = geometric unsharpness
F = source size: the maximum projected dimension
of the radiating source (or effective focal spot)
in the plane perpendicular to the distance D
from the weld or object being radiographed, in.
D = distance from source of radiation to weld or
object being radiographed, in.
d = distance from source side of weld or object being
radiographed to the film, in.