<s>[INST] context: Inherent filtration consists of the materials that x-ray photons encounter as they travel from the focal spot on the target to form the usable beam outside the tube enclosure. These materials include the glass wall of the x-ray tube, the insulating oil that surrounds many dental tubes, and the barrier material that prevents the oil from escaping through the x-ray port. The inherent filtration of most x-ray machines ranges from the equivalent of 0.5 to 2 mm of aluminum.
Added filtration may be supplied in the form of aluminum disks placed over the port in the head of the x-ray machine. 
Total filtration  is the sum of the inherent and added filtration. Governmental regulations require the total filtration in the path of a dental x-ray beam to be equal to the equivalent of 1.5 mm of aluminum for a machine operating at up to 70 kVp and 2.5 mm of aluminum for machines operating  at  higher voltages.
Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 10,11 [/INST] question: The inherent filtration of X-ray machines range from
___________ thickness of aluminium | options: A: 0.5 - 2 mm B: 1.5 - 2.5 mm C: 2.5 - 5 mm D: 1.0 - 2.5 mm | answer: A </s>