Geometric unsharpness

A radiographic source is not a point. It has a finite focal spot of size F, so every edge in the object casts a penumbra rather than a sharp shadow. The width of that penumbra measured on the film is the geometric unsharpness, Ug = F x t / d, where t is the distance from the object feature to the film and d is the distance from the source to that feature. The relationship is pure similar triangles: the shadow spreads in the same proportion as the source size spreads over the gap behind the object.

Two consequences follow directly. First, unsharpness grows linearly with the air gap behind the object, which is why the film is pressed hard against the part and why a raised weld cap on the film side is a real penalty. Second, unsharpness falls as the source is backed off, so the cure for a blurred radiograph is almost always more standoff, paid for with a longer exposure that goes as the square of the distance.

ASME BPVC Section V, Article 2, T-274.2 caps Ug by nominal material thickness: 0.020 in (0.51 mm) under 2 in, 0.030 in (0.76 mm) from 2 to 3 in, 0.040 in (1.02 mm) from 3 to 4 in, and 0.070 in (1.78 mm) over 4 in. For double-wall shots the relevant t is the full outside diameter, not the wall, because the source-side wall sits an entire pipe diameter away from the film.

Geometric unsharpness is only one of the contributors. Total image unsharpness combines Ug with the inherent unsharpness of the film or detector, and on digital systems the detector term often dominates once the standoff is adequate.

Ug = F x t / d
F = focal spot (source) size
t = object-to-film distance (OFD)
d = source-to-object distance (SOD)
SFD = d + t
SFD entry mode: d = SFD - t