“…The commonly used LIR technique involves the intensity ratio of the two thermally coupled energy levels (TCLs) of Ln 3+ ion with an energy gap between 200 and 2000 cm –1 . Among various Ln 3+ ions, Pr 3+ , Dy 3+ , Er 3+ , Ho 3+ , Gd 3+ , Nd 3+ , and Tm 3+ have TCLs in their 4f electronic configuration. − The LIR technique involving Ln 3+ ions with TCLs is governed by the Boltzmann distribution law, and therefore, the relative temperature sensitivity is proportional to the energy gap of the two TCLs. However, the thermal coupling of the two energy levels has an upper limit on the energy separation of about 2000 cm –1 , and this imposes restriction on the maximum sensitivity that can be reached from a single Ln 3+ ion.…”