Inherent filtration of X-ray machines:
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 aluminium. Added filtration may be supplied in the form of aluminium 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.
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 10,11
0.5-2 mm
1.5-2 mm
1.5-2.5 mm
0.5-1 mm