When Ho3+:Yb3+:CaF2 crystalline powders prepared by combustion synthesis were exposed to near infrared (lambda similar to 975 nm) radiation, intense photon up-conversion (UC) was observed at the visible with emission bands peaked at similar to 545, similar to 650 and similar to 750 nm identified as 4f-4f transitions from higher levels (F-5(4), S-5(2)) and F-5(5) to lower levels I-5(8) and I-5(7) of Ho3+. The emission bands at the green and red, in particular, have been demonstrated to be useful for temperature sensing based on luminescence intensity ratio technique. However, no model is available in literature to explain the change of the electronic populations of states (F-5(4), S-5(2)) and F-5(5) with temperature. The UC phenomenon was studied from both theoretical and experimental points of view. A rate equation model with temperature dependent parameters for Ho3+ and Yb3+ electronic populations considering a high sensitization of Ho3+ ions by Yb3+ ions was used. High Yb3+ -> Ho3+ energy transfer efficiency was found (similar to 88% at room temperature). The change with temperature predicted by the model for the luminescence intensity ratio of the UC green and red emission lines agrees well with the experimental data. (C) 2017 Elsevier B.V. All rights reserved.