Wireless Nano Irradiance Meter Based on Upconversion Luminescence Lifetime

Abstract Measuring light irradiance, or power density, within a material is essential for ensuring the precision, efficiency, and safety of light‐based technologies, such as photodynamic therapies, optical sensing, and material processing. However, this task becomes particularly challenging in scenarios involving highly scattering or absorbing media, dynamic systems with varying properties, or confined and hardly accessible compartments. These challenges can be addressed by using colloidal luminescent nanomaterials with luminescence lifetimes strongly dependent on excitation irradiance. Building on this, an upconversion lifetime‐based Nano Irradiance Meter (nIM) is proposed, employing, on SrYb^{3 +}F₅:@CaF₂ upconverting nanoparticles. This nIM operates within the first biological window and allows the direct estimation of laser irradiance, without prior knowledge of the area illuminated by the excitation beam. The feasibility of the sensor is validated through a calibration process, correlating the lifetime of the 770 nm upconversion luminescence of Tm^{3 +} ions with the 967 nm excitation irradiance. The sensor achieved a sensitivity of 0.9% W⁻¹ cm² at low irradiances (≈17 W cm⁻²) and 0.008% W⁻¹ cm² at high irradiances (≈5 kW cm⁻²), surpassing previously reported results based on ratiometric luminescence approaches. Finally, its robust performance under real‐life conditions across various media is demonstrated.