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To develop novel luminescent materials for optical temperature measurement, a series of Yb³⁺- and Er³⁺-doped Ca₃Sc₂Si₃O₁₂ (CSS) upconversion (UC) phosphors were synthesized by the sol–gel combustion method. The crystal structure, phase purity, and element distribution of the samples were characterized by powder X-ray diffraction and a transmission electron microscope (TEM). The detailed study of the photoluminescence emission spectra of the samples shows that the addition of Yb³⁺ can greatly enhance the emission of Er³⁺ by effective energy transfer. The prepared Yb³⁺ and Er³⁺ co-doped CSS phosphors exhibit green emission bands near 522 and 555 nm and red emission bands near 658 nm, which correspond to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, respectively. The temperature-dependent behavior of the CSS:0.2Yb³⁺,0.02Er³⁺ sample was carefully studied by the fluorescence intensity ratio (FIR) technique. The results indicate the excellent sensitivity of the sample, with a maximum absolute sensitivity of 0.67% K⁻¹ at 500 K and a relative sensitivity of 1.34% K⁻¹ at 300 K. We demonstrate here that the temperature measurement performance of FIR technology using the CSS:Yb³⁺,Er³⁺ phosphor is not inferior to that of infrared thermal imaging thermometers. Therefore, CSS:Yb³⁺,Er³⁺ phosphors have great potential applications in the field of optical thermometry. © 2023 by the authors.