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Imaging Through Fire Using Narrow-Spectrum Illumination


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.

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  1. Although radiation in the infrared spectrum is not a problem for the image quality, the imaging equipment (camera, lenses, cables) must be protected from the energy in this range; e.g., by shielding, appropriate filtering barriers, or placement at a sufficiently large distance from the fire to prevent damage.

  2. Users should be aware that high-intensity light in this frequency range can be harmful to your eyes. Appropriate eye protection and/or measures to prevent direct viewing of the light at close range are required.

  3. Certain commercial products are identified in this paper to specify the materials used and the procedures employed. In no case does such identification imply endorsement or recommendation by the National Institute of Standards and Technology, nor does it indicate that the products are necessarily the best available for the purpose.


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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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Correspondence to Matthew S. Hoehler.

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Smith, C.M., Hoehler, M.S. Imaging Through Fire Using Narrow-Spectrum Illumination. Fire Technol 54, 1705–1723 (2018).

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  • Imaging
  • Fire
  • Narrow-spectrum illumination
  • Metrology
  • Blue light