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Dispersion relation is established for Er(NbO3)3 phosphor, which is an excellent candidate for optical thermometry based on thermal effect of Er3+ fluorescence intensity ratio. To achieve it, Er(NbO3)3 phosphor was dispersed into α-bromonaphthalene liquid to form a suspension. Then, refractive index values of the suspension were measured at wavelengths 473, 589.3, 632.8, 980, 1311 and 1553 nm by dropping it onto a plastic sheet. Subsequently, refractive indices of the Er(NbO3)3 phosphor were evaluated from these measured index values using Maxwell-Garnett relation. Finally, based on these Er(NbO3)3 index values, which have an error 0.03, a three-oscillator Sellmeier equation that considers contributions from Nb–O group, plasmons in far UV regime and phonons in infrared region is established. In parallel, a Cauchy dispersion equation is also established. Both equations can well fit the experimental data and the refractive indices predicted by the two equations can be regarded as identical within the error 0.03.

980-nm-upconverted 530 and 550 nm Er3+ green fluorescence spectra of Er3+/Yb3+-codoped NaGd(WO4)2 submicron crystals were measured in the temperature range of 298–383 K. A sextuple ratiometric thermometry is proposed. It is established on the basis of six schemes of fluorescence intensity ratio (FIR) that considers three component peaks of the 530 nm emission band and two component peaks of the 550 nm emission band, which involve electronic transitions between two Stark sublevels of Er3+. The study shows that the phosphor shows strong green fluorescence, which is verified by measured quantum yield, and thermally stable spectral structure desired for the sextuple ratiometric thermometry. All of the six FIR schemes display highly efficient sensing performances with slightly different thermal sensitivities. Each scheme gives a temperature value and the six schemes give an averaged result. In parallel, we have also carried out an ex vivo experimental study on the temperature characteristics of the green fluorescence of the phosphor. Almost same results have been obtained, verifying biological applicability of the phosphor. The ex vivo experimental results also show that the sextuple thermometry increases considerably the accuracy and reliability of temperature measurement in comparison with the conventional intensity integration method.