This topic focuses on the optimization of a contactless temperature measurement technique based on multispectral optical pyrometry for nuclear environments. The scientific objective is to improve the reliability of an instrumentation chain capable of measuring the temperature of a fuel cladding under extreme conditions, particularly during a loss-of-coolant accident. The method relies on collecting the infrared radiation emitted by the investigated surface and transporting it through optical fibers to a multispectral detection system. A key challenge is the simultaneous estimation of temperature and emissivity, two parameters that are strongly coupled in pyrometry. The work also aims to improve optical calibration, channel-by-channel transmission stability, and signal acquisition speed. Particular attention is given to the design of micro-sensors and optical collection heads compatible with pressurized, irradiating, and thermally constrained environments. The project includes the study of lower temperature measurement limits in order to extend the sensor’s operating range. Tests in a pressurized chamber will be carried out to validate sealing, optical transmission, and metrological robustness. From a scientific perspective, this postdoctoral project combines optics, radiometry, signal processing, metrology, and instrumentation for harsh environments. Ultimately, this technology could be transferred to other nuclear experiments requiring fast, accurate, and non-intrusive temperature measurements.