Why a “ball”?
A globally recognized detection sensitivity standard is always a spherical object in nominal sizes (diameters) of a metal ball that the inspection system “catches” (positively detects) at its least sensitive location.
The sphere is a homogeneous object in the three-dimensional space (having the same size in all three axes) which, when passing through the metal detector, causes a constant disturbance of the electromagnetic field, irrespective of its orientation to the detection orifice. Any other object, such as a wire, will cause a field disturbance of a different size as it passes through the detection opening, depending on its orientation. In this case, the sensitivity of the detector would also depend on the position (rotation) of the object passing through the detector. For a ball like the sensitivity standard, this dependency is not.
In the case of X-ray inspection systems operating on a completely different principle from metal detectors, the fact that a sphere is a homogeneous object in space and that the detection capabilities of the system are not influenced by the position of the test standard relative to the X-ray is again exploited. beam. Although in practice, spherical contaminants are not commonly found, this shape is considered the most suitable for testing and comparing the performance of inspection systems. In a very simplified way and in general, there is also a direct proportion – the smaller the diameter of the ball the detection system can detect, the smaller the contaminant of another shape is also able to detect.