Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd³⁺ is a promising candidate for that purpose due to its intense radiative transitions within biological windows (BWs) I and II and the simultaneous efficient excitability within BW I. This makes Nd³⁺ almost unique among all lanthanides. Typically, emission from the two ⁴F_3/2 crystal field levels is used for thermometry but the small ~100 cm⁻¹ energy separation limits the sensitivity. A higher sensitivity for physiological temperatures is possible using the luminescence intensity ratio (LIR) of the emissive transitions from the ⁴F_5/2 and ⁴F_3/2 excited spin-orbit levels. Herein, we demonstrate and discuss various pitfalls that can occur in Boltzmann thermometry if this particular LIR is used for physiological temperature sensing. Both microcrystalline, dilute (0.1%) Nd³⁺-doped LaPO₄ and LaPO₄: x% Nd³⁺ (x = 2, 5, 10, 25, 100) nanocrystals serve as an illustrative example. Besides structural and optical characterization of those luminescent thermometers, the impact and consequences of the Nd³⁺ concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd³⁺ concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd³⁺ concentrations, cross-relaxation processes enhance the decay rates of the ⁴F_3/2 and ⁴F_5/2 levels making the decay faster than the equilibration rates between the levels. It is shown that the onset of the useful temperature sensing range shifts to higher temperatures, even above ~ 450 K for Nd concentrations over 5%. A microscopic explanation for pitfalls in Boltzmann thermometry with Nd³⁺ is finally given and guidelines for the usability of this lanthanide ion in the field of physiological temperature sensing are elaborated. Insight in competition between thermal coupling through non-radiative transitions and population decay through cross-relaxation of the ⁴F_5/2 and ⁴F_3/2 spin-orbit levels of Nd³⁺ makes it possible to tailor the thermometric performance of Nd³⁺ to enable physiological temperature sensing.

Herein, temperature dependencies of emission and emission decay of Zn₂SiO₄:Mn²⁺ powder in the range from room temperature to 300 °C is reported. These dependencies allow temperature sensing from luminescence measurements, since Mn²⁺ emission intensity, emission band spectral position, and emission decay are rapidly changing over the entire temperature range. For fluorescence intensity ratio method, measurement that relays on emission intensity changes with temperature, sensitivity of 12.2% is found. In the case of lifetime thermometry, method that relays on emission decay changes with temperature, sensitivity of 0.48% is determined. Both sensitivity values are among the highest ever recorded for inorganic materials.