Final answer:
X-ray machines operating at over 70 kV must have at least 2.5 mm of aluminum equivalent for tube filtration to absorb non-contributory photons and reduce patient dose.
Step-by-step explanation:
An x-ray machine operating at potentials above 70 kV requires at least 2.5 mm of aluminum equivalent for total tube filtration. This level of filtration is necessary to ensure that lower energy x-ray photons, which do not contribute to image formation but increase patient dose, are absorbed before reaching the patient. X-ray photon energy is directly related to the kV of the machine, with higher potentials resulting in more energetic photons capable of deeper penetration into materials. For instance, at 100 kV, the most energetic photon would be 100 keV.