From the up-converting multimodal luminescent thermometer to ratiometric visual power density meter based on Er3+,Yb3+ emission.

This study demonstrates that thermally induced variations in the spectroscopic properties of Na₃Sc₂(PO₄)₃:Er³⁺,Yb³⁺ can be effectively harnessed for multimodal remote temperature sensing. As shown, Na₃Sc₂(PO₄)₃:Er³⁺,Yb³⁺ supports multiple ratiometric sensing modes based on the intensity ratios of (i) ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2; (ii) ²H_9/2 → ⁴I_13/2 and ⁴S_3/2 → ⁴I_15/2; and (iii) green-to-red emission intensity ratio, achieving maximum relative sensitivities of 2.8% K^-1, 3% K^-1, and 1.8% K^-1, respectively. The synergy between thermal changes observed in the green-to-red emission intensity ratio of Er³⁺ ions, combined with the efficient optical heating of Na₃Sc₂(PO₄)₃:Er³⁺,Yb³⁺ at elevated Yb³⁺ concentrations enables the development of a visual optical power density sensor, exhibiting relative sensitivities of S_Rx = 1.0% W^-1 cm² and S_Ry = 0.9% W^-1 cm² at 15 W cm^-2 when quantified using CIE 1931 chromaticity coordinates. To the best of our knowledge, this is the first report of a visual luminescent optical power density sensor. Furthermore, it was demonstrated that Na₃Sc₂(PO₄)₃:Er³⁺,Yb³⁺ can be successfully applied for two-dimensional imaging of optical power density, thereby enabling spatial visualization of power distribution within an illuminated field.