A digital detector cannot resolve a feature that falls inside a single sampling element. Sampling theory sets the floor: at least two samples are needed across a feature before it produces a measurable modulation, so the smallest resolvable detail is roughly twice the basic spatial resolution of the system.

Basic spatial resolution is not the same as pixel pitch. A direct digital detector array is close – light or charge is collected within the pixel, so SRb is approximately equal to the pitch. A computed radiography imaging plate is much worse, because the stimulating laser spot spreads inside the phosphor and the emitted light scatters before reaching the photomultiplier. A standard imaging plate scanned at 100 micrometres typically delivers SRb around 200 micrometres; a high-resolution plate does better but never reaches its own scan pitch.

Magnification works in the technique’s favour here, and it is the one lever that changes the answer. Detector unsharpness is fixed in detector coordinates, so projecting the object larger divides that unsharpness by the magnification when referred back to the object: SRb_object = SRb_detector / M. The penalty is geometric unsharpness rising in proportion, so the gain only lasts while the focal spot is small enough to keep up.

Convert the object-plane resolution into a wire diameter and the result can be checked directly against the IQI requirement before a single shot is fired. If the finest resolvable wire is coarser than the essential wire the code demands, the technique will not qualify and no amount of exposure will fix it – the answer is a finer detector, more magnification or a smaller focal spot.

Worked example

Pixel pitch / scan pitch	100 um
Detector type	cr_std
Magnification	1.15
Penetrated thickness	20 mm
Essential wire diameter required	0.4 mm
Detector factor	2
SRb at the detector	200 um
SRb referred to the object	173.91 um
Smallest resolvable wire	0.35 mm
Sensitivity at that wire	1.74 %
Finest ISO 19232-1 wire likely resolved	10
Diameter of that wire	0.4 mm
Sensitivity at that standard wire	2 %

A standard imaging plate at 100 um pitch gives SRb of about 2 x 100 = 200 um at the detector. At 1.15x magnification that is 200/1.15 = 173.91 um referred to the object, so the smallest resolvable wire is 2 x 173.91 = 347.8 um = 0.35 mm. The nearest standard ISO wire not finer than that is W10 at 0.40 mm, giving 0.40/20 x 100 = 2.00 percent sensitivity - exactly meeting a 0.40 mm essential wire requirement.

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