Focal Penumbra
Image edge blur in industrial radiography results from the finite spatial dimensions of the X-ray tube focal spot. This penumbral shadowing, termed geometric unsharpness, limits the smallest defect size visible during non-destructive inspection of printed circuit board solder joints.
Ray Geometry
X-rays emit across the full surface area of an electron beam target rather than a single mathematical point. Rays passing the edge of an internal feature project a transition zone between full shadow and full illumination onto the digital detector plane. The width of this blurring zone equals the focal spot diameter multiplied by the distance from object to detector, divided by the distance from source to object.
Moving the circuit board closer to the detector minimizes edge blurring, whereas moving the board closer to the X-ray tube increases optical magnification at the cost of broader penumbra boundaries. High-density component packaging demands minimal unsharpness to separate adjacent microvia structures and void boundaries.
Resolution Bound
Radiographic acceptance standards like IPC-A-610 define minimum feature resolution based on focal spot geometry and tube positioning. Excessive unsharpness hides micro-cracking and wetting defects along component leads. Microfocus X-ray tubes control geometric unsharpness by focusing electron beams onto microscopic target areas.
