During x-ray generation, off-focus radiation refers to the emission of x-ray photons which originate outside of the anode focal spot. Essentially a form of scatter, photons produced in this manner may result in blurring and are of no use for diagnostic purposes. They are shielded as much as possible by the construction of the x-ray tube itself.

During x-ray generation, only a portion of the photons generated at the anode continues along the desired focal path through the tube output port and collimator. Most off-focus photons are generated secondarily, due to electrons backscattered from the anode following a curved path as they are accelerated back towards the anode and impact somewhere other than the focal spot. ^{1, 2}. Since these are secondarily (or even tertiarily) generated, these photons tend to be of lower energy.

Several features of x-ray tubes serve to decrease off-focus radiation, both to maximize image quality and reduce dose to subject and operator. A metal housing surrounds the x-ray tube insert and attenuates the majority of block off-focus photons (save for a small amount of leakage radiation). Just beyond the tube output port, the metal collimator is used to shape the x-ray field and also blocks many off-focus photons.

Terminology

"Off focus radiation" to may be used more generally to refer to any photons originating from the x-ray tube device which do not follow the focal path. This may include those which were secondarily generated by striking other parts of the tube insert, the insulating oil layer, etc ².