Smartphone‐Based Luminescent Thermometry via Temperature‐Sensitive Delayed Fluorescence from Gd$_{0}$O$_{2}$S:Eu$^{3+}$

Thermal images generated from infrared radiation are useful for monitoring many processes; however, infrared cameras are orders of magnitude more expensive than their visible counterparts. Methods that allow visible cameras to capture thermal images are therefore of interest. In this contribution, thermal images of a surface coated with an inexpensive inorganic micropowder phosphor are generated from the analysis of a video taken with a smartphone camera. The phosphor is designed to have a temperature‐dependent emission lifetime that is long enough to be determined from the analysis of a 30 frames‐per‐second video recording. This proof‐of‐principle work allows temperatures in the 270–320 K range to be accurately determined with a precision better than 2 K, even in the presence of bright background illuminance up to 1500 lm m$^{-2}$. In the broader context, this inspires further development of phosphors to bring time‐resolved sensing techniques into lifetime long enough ranges to allow smartphone‐based detection.