Unambiguous calibration of power dependence in ratiometric luminescent nanothermometry through multiple intensity ratios and symbolic regression

-activated nanothermometers and, besides the well-known 525/545 nm ratio, define a second luminescence intensity ratio involving the emission at 660 nm to achieve a power-calibration-free nanothermometry. The intensity of this emission is strongly correlated with the power and is available anyways when using standard spectroscopic instrumentation. We apply symbolic regression to find an unambiguous mathematical expression that describes the experimental data. From this mathematical expression, we determine the mean temperature deviation resulting from the fitting error to be 0.16 K and a maximum temperature precision as small as 6 mK (0.22 K on average). In summary, our approach makes excitation power measurements in ratiometric luminescent nanothermometry superfluous.