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What is the effect of a radiation filter?
When a metal plate, usually lead or copper, is placed between the tube window and the object, radiation “hardening” occurs leading to a lower image contrast. This may be counter-balanced by a metal filter placed immediately behind the object (i.e. between object and film). This filter will cause the (softer) scattered radiation passing through the object to be absorbed by the filter to a greater extent than the primary (harder) radiation. This also improves the image quality.
If the edges of an object being radiographed are not close to the film (as in the case of a cylindrical body in figure 3-6) considerable scatter of the primary radiation can occur, leading to fogging. This scatter can be prevented by positioning sheets of lead foil between the object and the film as illustrated in this figure.
Reducing the contrast by filtration is also desirable when a radiographic image of an object of widely varying thicknesses has to be obtained on a single film.
Typical filter thicknesses are :
- 0.1 – 0.25 mm lead for 300 kV X-rays
- 0.25 – 1.0 mm lead for 400 kV X-rays
