The K-absorption edge for lead occurs at 88kev
so that the photons whose energies lie between 60 and 88 kev are much less attenuated than those between 88 and 110 kev
with a net result that more soft radiation than is desirable is transmitted and the high energy photons are excessively removed
PRINCIPLES IN CHOOSING A FILTER MATERIAL
The material chosen must attenuate principally by means of photoelectric effect in the photon energy range being dealt with in order to discriminate against low energy photons
The material must not have an absorption edge close to the energies of the photons that it is desired to use
The thickness of the material must not be too small – it should be uniform
Too thin might produce pin holes which might produce completely unfiltered beams
In order to prevent the shadows cast onto the film by the grid from interfering with visualization of diagnostic detail, certain principles must be followed:
the lead should be as thin as possible to be consistent with adequate absorption of scattered radiation. The thinner the lead, the narrower the shadow it will produce on the film and the less visible it will be to the eye.
Also, the thinner it is the less absorption of primary radiation will be in the grid. BUT it must be noted that adequate absorption of scattered radiation is the function of the grid and lead must be thick enough to provide this function.
When strips are not progressively angulated but are perpendicular to the surface of the grid, the grid is termed “parallel” .
USED in fluroscopy & mobile imaging.