<p>A spectral upconversion glass made from oxyfluorophosphate containing Er³⁺/Yb³⁺ ions (OFP-ErYb glasses) was fabricated using the melt/quenching technique. A host glass network composed of 50P₂O₅-20PbO-15CaF₂-14MgF₂-1Er₂O₃ (OFP:Er³⁺) was prepared and incorporated with 2.5, 5, and 7.5 mol% of Yb₂O₃, substituting for CaF₂, resulting in OFP:Er³⁺/Yb³⁺-1, OFP:Er³⁺/Yb³⁺-2, and OFP:Er³⁺/Yb³⁺-3 glasses. Structural variations from including Yb³⁺ ions into the host OFP:Er³⁺ glass were analyzed via X-ray diffraction (XRD), density measurements, density-based parameters, and Fourier-transform infrared (FTIR) spectra. The high density of Yb₂O₃, local environmental changes, and the high polarizability of Yb³⁺ ions, significantly impacted the structure of the host OFP:Er³⁺ glass. As a crucial factor in spectral conversion materials, differential scanning calorimetry (DSC) and ultrasonic velocity measurements assessed thermal stability and elasticity. The fabricated glasses exhibited high thermal stability and adequate elasticity, indicating their potential as conversion materials for various applications. Distinctive absorption bands of the Er³⁺ ions detected in the 200–1100 nm region. The energy was successfully transferred from Yb³⁺ to Er³⁺ upon excitation at 980 nm, generating two intense red emissions at 648 nm and 734 nm, along with two weaker green emissions at 550 nm and 578 nm. The chromaticity coordinates for OFP:Er³⁺/Yb³⁺-1, OFP:Er³⁺/Yb³⁺-2, and OFP:Er³⁺/Yb³⁺-3 correspond to yellowish-white, yellowish-white, and pinkish-red, with color purities of 20.91%, 35.91%, and 24.25%, respectively. A significant increase in emission intensity was observed at 5 mol% of Yb³⁺ (OFP:Er³⁺/Yb³⁺-2 glass), whereas a quenching effect of 7.5 mol% (OFP:Er³⁺/Yb³⁺-3 glass), caused a reduction in emission intensity. Therefore, the 5:1 Er³⁺/Yb³⁺ ratio (OFP:Er³⁺/Yb³⁺-2 glass) in the oxyfluorophosphate glass demonstrated highly efficient upconversion of NIR light at 980 nm to visible light at 550, 578, 648, and 734 nm, along with excellent thermal stability and good elasticity, making it an ideal option for photonics and optoelectronics materials.</p>