Imaging Through Fire Using Narrow-Spectrum Illumination
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This paper demonstrates a simple method to enhance visibility through clean-burning flames that allows new opportunities to perform optical metrology in fire research. The method combines narrow-spectrum, blue illumination and matched optical filters to reduce the influence of optical emissions from a glowing hot target and a large natural gas diffusion flame. The paper describes how the required illumination strength and filtering can be estimated from basic combustion and optical principles. Compared to white light, the required illumination to detect objects engulfed in flames with this method is reduced by a factor of 104. A series of experiments are conducted to determine the effectiveness of this method, successfully demonstrating the ability to take images of objects in natural gas fires up to 1000 kW using 200 W of illumination power. These experiments corroborate the estimation of relative optical intensities, as it is observed that the target can be imaged successfully when the estimated signal-to-noise ratio exceeds 2:1. The temporal averaging of successive images is also demonstrated to further improve the image quality.
KeywordsImaging Fire Narrow-spectrum illumination Metrology Blue light
The authors would like to thank Behrang Hamadani at the National Institute of Standards and Technology (NIST) for his assistance with the spectroradiometer measurements and Jeffrey Fagan (NIST) for characterizing the filter transmittance spectra. We also thank Matthew Bundy, Laurean DeLauter and Anthony Chakalis at the National Fire Research Laboratory for their support in the execution of the fire experiments.
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