A convenient practical notion (number) of the linear absorption coefficient is the introduction of the half-value thickness (HVT). It quantifies the penetrating power of radiation for a particular type of material and is defined as the thickness of a particular material necessary to reduce the intensity of a monochromatic beam of radiation by half, as shown in the figure. This HVT-value depends on the hardness of radiation.
Table 2-2 shows the average HVT-values for steel, table 3-2 shows the values for lead.
For a heterogeneous beam the HVT is not constant; the second HVT is slightly larger than the first. In general, in industry where relatively hard radiation is used, a fixed “average” HVT is applied.