Filter used in X-ray tube is made up of:

'Filters' : They are metal sheets placed in the x-ray beam between the window and the patient that are used to attenuate the low-energy x-ray photons from the spectrum. Filtering is the removal of these low energy x-rays from the beam spectrum which would otherwise not contribute to image quality but would add to patient dose and scatter. Minimum total filtration of 2.5 mm of aluminium is required for x-ray tubes operating above 70 kVp.