Luminescence from rare earth ions provides spectral signatures that identify the type of site. Substitution of the ions in a lattice introduces distortions scaled by the relative size of the host and dopant. For LaF3 or Bi4Ge3O12 several different co-ordination numbers are used; charge compensation is also needed in BaF2, SrF2, CaSO4 etc. When the trapping and recombination sites closely interact, the glow peak temperatures are strongly dependent on the size of the rare earth ion. Three types of pattern are noted with (i) minimal change; (ii) peak shifts scaling with RE ion size over similar to30 degrees or (iii) over 200 degrees from precipitate formation.