Radiation hardness (beam quality) depends on wavelength. Radiation is called hard when its wavelength is small and soft when its wavelength is long. In industry the quality of the X-ray tube ranges from very soft to ultra hard. The beam quality is related to a tube voltage (kV) range, or keV for isotopes.
The first two columns of the table on the right side indicate the relationship hardness/tube voltage range applied in NDT. The third column gives the related qualification of the radiation effect, i.e. half-value thickness (HVT), described in detail in a later blog article.