Filters absorb the low energy photons from the x-ray spectrum. These low energy photons will not contribute to the image formation. In dental x-ray units, the glass of the tube acts like a filter (inherent filtration) and, in addition, a piece of aluminum is added in front of the x-ray tube for additional filtration. The total filtration is expressed as an equivalent to millimeter of aluminum. If the kVp of the tube increases, the total filtration should increase.3,6,11