Two-dimensional surface temperature measurements of burning materials

A new technique for two-dimensional temperature measurements of burning surfaces is presented. Laser-induced phosphorescence from a thermographic phosphor material applied to a surface of investigation was measured with a fast framing camera. The phosphor was excited by the 4th harmomic from a pulsed Nd:YAG operating at 10 Hz. The phosphorescence images measured by eight consecutively gated CCD detectors enable pixel-by-pixel lifetime evaluation of the phosphorescence by interpolating an exponential decay curve to the counts of the corresponding pixel positions of the sequential CCD images. The temperature at each pixel position was evaluated using a calibration procedure of temperature against lifetime. These measurement procedures were used for surface temperature measurements of the evolution of flame spread on low-density fiber boards. The results from experiments showed the possibility of measuring surface temperature during all phases of the flame spread. The total time window used for each two-dimensional temperature measurement was 800 mus to obtain high accuracy and precision at high temperatures, 680-780 K, temperatures characteristic of burning surfaces. The best precision, better than +/-5 K, was obtained at these temperatures. In this region, evaluation by the lifetime method shows a higher sensitivity to temperature than what can be expected from methods based on spectral line intensities. The results of the experiments were in accordance with those reported from previous one-point measurements. In the low-temperature region close to room temperature, the accuracy deteriorated considerably. The results obtained from the two-dimensional imaging experiments are presented and discussed.