Starting from a diamond material which shows a difference in its absorption characteristics after exposure to radiation with an energy of at least 5.5 eV (typically UV radiation) and thermal treatment at 798K, controlled irradiation is applied so as to introduce defects in the diamond material. After the controlled irradiation the difference in the absorption characteristics after exposure to radiation with an energy of at least 5.5 eV and thermal treatment at 798K is reduced.