- Parasitic radiation absorption by impurities located in transparent polycrystalline ceramic hosts may significantly impair their functioning. At least two classes of impurity which can act as parasitic absorbers were encountered: transition or rare‐earth metal cations and neutral species (molecules or atoms) trapped in lattice void spaces. The most ubiquitous parasitic absorbers, able to affect transmission in the visible domain, have been identified for the case of MgAl2O4‐spinel and Y3Al5O12‐garnet hosts; their spectral effect was described and explained. In the case of spinel, S8 type molecules and carbon atom clusters are the most damaging impurities, with Ce3+ and Y2+ also of significance for YAG host. Oxygen vacancy formation assists some of the redox processes which generate uncontrolled absorption. For instance, the Ti4+ reduction to Ti3+ is facilitated by such a process.