Unusual Thermally Induced Blueshift and Emission Amplification of Mn ²⁺ Ions Enable Filter‐Free Luminescent Thermal Imaging

ABSTRACT The shift from point‐based thermal sensing to filter‐free thermal imaging requires luminescent thermometers that exhibit pronounced and thermally driven spectral changes within spectral regions matching the sensitivity profiles of the red, green and blue (RGB) channels of a digital camera. In this work, we introduce such a system, enabled by the synergistic interplay between (i) thermal redistribution among the vibronic components of the ⁴ T ₁ excited state of Mn ²⁺ ions and (ii) thermally assisted population of this state via optical trap sites. These combined processes result in a simultaneous thermal enhancement and blueshift of the Mn ²⁺ emission band associated with the ⁴ T ₁ → ⁶ A ₁ electronic transition. Consequently, the emission intensity recorded in the G channel increases with temperature, while the luminescence signals detected in the B channel exhibit a corresponding decrease. As demonstrated, Ca ₁₉ Zn ₂ (PO ₄ ) ₁₄ :Mn ²⁺ , Ce ³⁺ supports not only sensitive filter‐free thermal imaging, but also two additional ratiometric readout schemes: one based on the intensity ratio of Ce ³⁺ and Mn ²⁺ emissions, and another based on two distinct spectral regions of the Mn ²⁺ emission band, yielding maximum relative sensitivities of 0.42 and 2.7%K ⁻¹ , respectively. This approach introduces a unique thermometric strategy that enables simple, robust, and cost‐effective two‐dimensional thermal imaging without the need for optical filters or specialized instrumentation.