Radiography in Modern Industry - Kodak

Figure 38: A filter placed near the x-ray tube reduces subject contrast and eliminatesmuch of the secondary radiation, which tends to obscure detail in the periphery of thespecimen.Column 3 of the table below shows the percentage of the original x-ray intensity remaining afterthe addition of the filter, assuming both exposures were made at 180 kV. (These values werederived from actual exposure chart data.)RegionSpecimenThickness(inches)Percentage of Original X-ray IntensityRemaining After Addition of a FilterOutsidespecimenThin sectionMedium section0 less than 5%1 / 4 about 30%1 / 2 about 40%Thick section 1 about 50%Note that the greatest percentage change in x-ray intensity is under the thinner parts of thespecimen and in the film area immediately surrounding it. The filter reduces by a large ratio the x-ray intensity passing through the thin sections or sticking the cassette around the specimen, andhence reduces the undercut of scatter from these sources. Thus, in regions of strong undercut,the contrast is increased by the use of a filter since the only effect of the undercutting scatteredradiation is to obscure the desired image. In regions where the undercut is negligible, a filter hasthe effect of decreasing the contrast in the finished radiograph.Although frequently the highest possible contrast is desired, there are certain instances in whichtoo much contrast is a definite disadvantage For example, it may be desired to render detailvisible in all parts of a specimen having wide variations of thickness. If the exposure is made togive a usable density under the thin part, the thick region may be underexposed. If the exposureis adjusted to give a suitable density under the thick parts, the image of the thin sections may begrossly overexposed.Radiography in Modern Industry 54