Digital radiography needs a measured resolution figure, because unlike film there is no grain to set a floor – the detector, the scan pitch and the geometry all contribute and the result cannot be assumed. The measurement tool is the duplex wire IQI to ISO 19232-5: thirteen pairs of parallel wires, each pair with wires of diameter d separated by a gap also equal to d, so each pair has a period of 2d.

Imaged through a system with unsharpness U, a pair whose period is much larger than U shows two clear peaks with a deep valley between them. As the pairs get finer the valley fills in. The convention is that a pair is resolved while the dip between the peaks is at least 20 percent of the peak height. The first pair that fails that test defines the system unsharpness as U = 2d, and the basic spatial resolution is half that value, SRb = d – equal to the wire diameter of that pair.

SRb is the digital analogue of film graininess and is the quantity ISO 17636-2 uses to qualify a technique. It also feeds the normalised signal-to-noise ratio, which is why the two measurements are made from the same image. Reported in micrometres, a good direct digital detector array gives 50 to 100 micrometres and a standard imaging plate 150 to 250.

Total unsharpness combines the detector term with the geometric term. Adding them in quadrature, U_total = sqrt(Ug^2 + Ui^2), shows immediately which one is worth attacking: if the geometric term is already half the detector term, backing the source off further buys almost nothing.

Worked example

First unresolved wire pair	d7
Geometric unsharpness	0.3 mm
Maximum SRb permitted	250 um
Wire diameter of that pair	0.2 mm
Basic spatial resolution SRb	200 um
Image unsharpness Ui	0.4 mm
Total unsharpness	0.5 mm
Equivalent line pairs per mm	2.5

Duplex pair D7 has 0.20 mm wires, so image unsharpness Ui = 2 x 0.20 = 0.400 mm and SRb = 200 um. Combined in quadrature with a 0.300 mm geometric term, U_total = sqrt(0.30^2 + 0.40^2) = sqrt(0.25) = 0.500 mm. The detector term dominates, so more standoff would buy little.