Development of an Optical Backside Thermography System Using a Pumped Two-Dye Fluorescence Technique

This work demonstrates the development of a novel backside thermography technique based on the temperature sensitivity of laser-induced fluorescence in flowing two-dye solutions. The approach utilizes visible light and optically transparent packaging materials to obtain spatially resolved transient thermal measurements. This makes it a relatively simple, inexpensive, and flexible approach for lab-scale experimental characterizations. This technique has the potential to achieve higher spatial and temporal resolutions compared to infrared thermography while using more economical cameras and optics. Additionally, both heating and cooling of the thermography stage can be achieved by passing temperature-controlled water through plastic packaging components. This flexibility allows for the characterization of backside surface temperatures during freezing and condensation as well as boiling and evaporation. A custom-built experimental setup was designed, constructed, and used to study the performance of seven two-dye Rhodamine B (RhB)-Rhodamine 110 (Rh110) fluorescent solutions. The effect of dye concentration ratio on sensitivity, maximum frame rate, and excitation area was characterized. Using an optimal dye concentration of unity, it was shown that frames rates of 265 to 3200 Hz are achievable for excitation areas with diameters of 15.5 to 4.3 mm, respectively, using the current setup. The calibrated system was used to demonstrate backside thermography using a simple droplet contact method as well as in situ thermography using serpentine flow channels of varying contour. Experiments were conducted in the range of 25 to 55 ºC with a spatial resolution of 30 µm. The experimental uncertainty of the temperature measurements was calculated to be ±2.1 ºC. Good agreement was found between the experimental and predicted results, showing the technique’s ability to account for error due to light fluctuations and non-uniform illumination. Finally, the effect of dye photobleaching during prolonged testing was studied and it was shown that photobleaching can be reduced, and even eliminated, by maintaining a nominal low flow rate of the pumped two-dye solution