Optical thermometry based on the red upconversion fluorescence of Er³⁺ in CaWO₄:Yb³⁺/Er³⁺ polycrystalline powder.

Based on the fluorescence intensity ratio method, the temperature-sensing behavior through thermally coupled levels (TCL) of the H_11/22 and S_3/24 states as well as the sub-levels of the F_9/24 state of Er³⁺ has been studied. The thermometry is observed to be dependent on the pump power for the H_11/22 and S_3/24 states, leading to an error of more than 20 K at 478 K. By utilizing the sub-levels of the F_9/24 state, such a problem could be solved. The maximum sensitivity is about 0.15%  K^-1 at 298 K. This will provide guidance on selecting appropriate and practical TCL for precisely sensing the temperature.