IMAGING PYROMETRY OF SMOLDERING WOOD EMBERS AT VARIOUS DISTANCES AND ILLUMINATIONS

Abstract

Wildland fires in the WUI present a constant threat to life and property in the United States and across the globe. Many wildland fires are caused by ember spotting, a process in which firebrands are lofted significant distances away from the fire front by combinations of winds and gas flows. These firebrands have the potential to collect and cause new spot fires independent of the original wildland fire. While firebrand mechanisms such as ember generation and transport have been thoroughly studied and quantified, the capacity in which firebrands cause these fires is not as well known. Recent studies have made progress towards determining the surface temperature of these firebrands; however, none have provided repeatable temperature data from a variety of test conditions. This paper presents firebrand surface temperature using color imaging ember pyrometry techniques for various imaging distances and illuminations. A digital color camera was calibrated to a blackbody furnace with a temperature range of 600 – 1200 °C. Calibration to the blackbody allows the normalized pixel values of each image to be converted to temperature using G/R ratio, grayscale, and hybrid pyrometry. Signal to noise ratios of around 850 and 46 for grayscale and ratio pyrometry were obtained. Two simultaneous images of a single ember from distances of 0.5 and 1 m, as well as additional images from 4 m were observed and quantified. The firebrand surface temperature was determined to be independent of imaging distance. The mean surface temperature across all imaging distances was calculated to be 931 ± 6.2 °C. Ratio pyrometry was observed to be the preferred method of imaging pyrometry due to its independence from surface emissivity and transmissivity as well as it’s applicability to real fire scenarios for future research. Firebrands were also imaged in sequences containing various illumination and background color. Illumination was observed to disrupt G/R ratio pyrometry due to an overwhelming increase in green pixel values.