As a fundamental physical parameter reflecting the microscopic motion within matter, the measurement of temperature holds a critical significance in industrial production, biomedical fields, and aerospace engineering. Conventional contact-based thermometry, requiring direct interaction with measured objects, demonstrates limitations in extreme environments due to the susceptibility to external interference and inherent measurement perturbations, thus failing to meet modern technological demands for high sensitivity, non-invasiveness, and rapid response. These challenges propel the development of non-contact thermometric technologies featuring enhanced measurement sensitivity, robust anti-interference capability, non-invasive characteristics, and fast response. Fluorescence-based temperature sensing has emerged as an innovative solution via leveraging the distinctive energy level configurations of rare-earth ions and their temperature-dependent luminescent properties. This technique has attracted substantial research attention due to its rapid response kinetics, exceptional sensitivity, and remarkable adaptability to harsh operational environments.Fluorescence thermometry based on the correlation between fluorescence intensity and temperature represents one of the earliest developed methods in temperature measurement technology. This approach primarily falls into two categories, i.e., single-energy-level fluorescence intensity thermometry and fluorescence intensity ratio (FIR) thermometry. The single-energy-level method, as the most straightforward technique, determines temperature via monitoring the intensity variation of a specific emission peak with temperature changes, directly demonstrating the relationship between fluorescence intensity and temperature. However, its applications reduce in recent years due to the inherent limitations such as susceptibility to fluorescence loss during detection and strong dependence on excitation light intensity, hindering precise control in measurement processes.In contrast, FIR thermometry has attracted much attention due to its insensitivity to external disturbances, high measurement accuracy in complex environments, and excellent reproducibility. This technique is further divided into thermally coupled energy level FIR thermometry and non-thermally coupled energy level FIR thermometry. The former operates based on the principle that particles in thermally coupled energy levels (denoted as Level 1 and Level 2) reach a thermal equilibrium within a short timeframe, where the population redistribution between these levels induces measurable changes in fluorescence intensity ratio. While a larger energy level spacing generally enhances temperature sensing performance, the inherent limitation of thermal coupling energy gaps poses some challenges for improving relative sensitivity. To address this constraint, non-thermally coupled FIR thermometry through co-doping dual luminescent centers is porposed. This strategy enhances relative sensitivity and measurement reliability via leveraging the distinct temperature-dependent luminescence characteristics of two different centers. The intensity ratio between dual centers provides more accurate temperature determination through mutual calibration and effectively compensates for measurement errors caused by external factors. Common dual-center configurations include rare-earth ion pairs and rare-earth/transition metal ion combinations, showing significant improvements in relative sensitivity for advanced thermometric applications.The application of rare-earth-doped fluorescence intensity-based thermometry focuses on two primary domains, i.e., temperature measurement within the physiological range and in ultra-high-temperature environments. Distinct requirements for the upper temperature limit and sensitivity arise across these applications. For instance, in physiological temperature monitoring, slight temperature variations can lead to significant biological effects, necessitating exceptionally high relative sensitivity in low-temperature regimes to ensure sufficient measurement accuracy. Conversely, in high-temperature scenarios, the primary objective is to achieve an upper measurement limit compatible with extreme thermal conditions. Consequently, the design and development of novel materials should be closely aligned with specific application contexts, emphasizing performance optimization tailored to operational demands, such as tunable temperature thresholds, enhanced thermal stability, and environment-specific signal responsivity. This approach ensures that material systems balance sensitivity, durability, and temperature range adaptability for targeted technological implementations.Summary and ProspectsThe development of fluorescence intensity-based thermometry faces two major challenges, i.e., enhancement of temperature measurement performance and expansion of application domains. Regarding performance improvement, the existing fluorescence intensity thermometric methods primarily focus on fluorescence intensity ratio (FIR) techniques involving thermally coupled and non-thermally coupled energy levels. However, several critical issues have emerged.Firstly, rare-earth ions and certain transition metal ions exhibit significant thermal quenching effects at elevated temperatures, thus leading to substantial attenuation of fluorescence signals in high-temperature environments. Secondly, in thermally coupled energy level-based FIR thermometry, despite the abundant energy levels of rare-earth ions, some studies on potential thermally coupled energy levels for specific rare-earth ions remain insufficient. Moreover, systematic investigations into performance variations among different thermally coupled energy level pairs for FIR thermometry are notably lack. Thirdly, the fundamental mechanisms underlying non-thermally coupled FIR thermometry, particularly the operational principles of dual-luminescent-center systems, have yet to establish universally accepted theoretical explanations. The energy transfer processes and interaction mechanisms between different luminescent centers’ energy levels require a further elucidation. In terms of luminescent center selection, more combinations such as rare-earth–rare-earth ion pairs and rare-earth–transition metal ion systems warrant a comprehensive exploration. Lastly, although numerous temperature-sensitive materials demonstrate either high upper measurement limits or superior relative sensitivity, materials simultaneously with a high relative sensitivity in a broad temperature range remain scarce. Therefore, the design and development of novel temperature-sensitive materials will constitute a crucial research frontier.Concerning application expansion, fluorescence intensity thermometry has attracted considerable attention due to its unique characteristics. The existing thermometric materials exhibit diverse advantages, each possesses inherent limitations. A critical challenge lies in rationally designing application scenarios based on material properties to maximize their functional advantages for practical implementations. Current research predominantly focuses on material development itself, with insufficient exploration of potential application fields. Note that raw materials cannot be directly employed in industrial production but require integration into temperature sensor architectures coupled with complete testing systems. However, the research framework spanning from material development to practical applications, encompassing complete testing systems and material optimization, remains underdeveloped. Furthermore, although applications have extended to biomedical and aerospace fields, investigations under extreme conditions such as ultra-low temperatures and highly corrosive environments remain inadequate.It is anticipated that these challenges above will be progressively addressed with sustained research efforts. The development of advanced temperature-sensitive materials with enhanced performance and their subsequent integration into industrial applications are expected to drive significant advancements in fluorescence intensity-based thermometry technology.