A delayed gating approach for interference-free ratio-based phosphor thermometry

Accurate surface temperature measurements present vast difficulties in numerous technical applications, especially when imaging fast temperature changes. One example is spray-induced surface cooling, where temperature variations occur on the sub-millisecond timescale. Phosphor thermometry relies on changes in the photoluminescence properties (typically the lifetime or the emission spectrum) of phosphor materials for temperature determination. For temperature imaging in situations where short measurement durations are critical such as on fast moving objects or short and intense heat transfer events, the spectral method is preferred. However, in many situations, fluorescence signals over a broad spectral range originate from fuel, flame radicals, optical windows or even the chemical binder used to coat the phosphor material and may interfere with the measurements. In this work, we investigate a delay strategy for the spectral method that prevents interference from fluorescence sources by using a phosphor with a microsecond-range-lifetime. By applying a short delay (<1 mu s), between excitation and detection, short-lived interfering fluorescence signals can be efficiently avoided. The temporal and spectral content of the fluorescence from the substrate, binder and fuel are investigated with a photomultiplier tube and a spectrometer respectively. The delayed gating strategy is then implemented for temperature imaging of the wetted side of a spray impinged surface with a tin-doped phosphor, (Sr,Mg)(3)(PO4)(2):Sn2+; selected for its suitable lifetime (26 mu s @ 300 K) and high temperature sensitivity. Calibration measurements for coated surfaces show that by avoiding fluorescence from the binder, the temperature sensitivity is improved from 0.3% K-1 to 0.8% K-1 at 293 K. Furthermore the calibration curve repeatability between two similar coatings is enhanced. Finally, temperature measurements of the cooling induced by a fluorescing impinging gasoline spray was successfully performed with this strategy owing the same level of measurement precision (0.5 K) as with a non-fluorescing fuel (n-hexane UV grade).