Better Imaging Through Fire

To improve the ability of researchers to see through fire, NIST has developed an imaging system using ordinary blue light to dramatically clear the picture. (Source: NRFL / NIST)

Researchers at the National Insti­tute of Standards and Tech­nology NIST have demons­trated that ordinary blue light can be used to signi­ficantly improve the ability to see objects engulfed by large, non-smoky natural gas fires like those used in labora­tory fire studies and fire-resis­tance standards testing. The blue-light imaging method can be a useful tool for obtaining visual data from large test fires where high tempera­tures could disable or destroy conven­tional electrical and mechanical sensors.

The method provides detailed infor­mation to researchers using optical analysis such as digital image corre­lation (DIC), a technique that compares successive images of an object as it deforms under the influence of applied forces such as strain or heat. By precisely measuring the movement of indi­vidual pixels from one image to the next, scientists gain valuable insight about how the material responds over time, including behaviors such as strain, displace­ment, defor­mation and even the micro­scopic beginnings of failure. However, using DIC to study how fire affects structural materials presents a special challenge: How does one get images with the level of clarity needed for research when bright, rapidly moving flames are between the sample and the camera?

“Fire makes imaging in the visible spectrum difficult in three ways, with the signal being totally blocked by soot and smoke, obscured by the intensity of the light emitted by the flames, and distorted by the thermal gradients in the hot air that bend, or refract, light,” said Matt Hoehler, a research structural engineer at NIST’s National Fire Research Labora­tory (NFRL). “Because we often use low-soot, non-smoky gas fires in our tests, we only had to overcome the problems of bright­ness and distor­tion.”

To do that, Hoehler and colleague Chris Smith, a research engineer formerly with NIST and now at Berk­shire Hathaway Specialty Insu­rance, borrowed a trick from the glass and steel industry where manu­facturers monitor the physical charac­teristics of materials during produc­tion while they are still hot and glowing. “Glass and steel manufacturers often use blue-light lasers to contend with the red light given off by glowing hot materials that can, in essence, blind their sensors,” Hoehler said. “We figured if it works with heated materials, it could work with flaming ones as well.”

Hoehler and Smith used commer­cially available and inex­pensive blue light-emitting diode (LED) lights with a narrow-spectrum wave­length around 450 nano­meters for their experi­ment. Initially, the researchers placed a target object behind the gas-fueled test fire and illu­minated it in three ways: by white light alone, by blue light directed through the flames and by blue light with an optical filter placed in front of the camera. The third option proved best, reducing the observed inten­sity of the flame by 10,000-fold and yielding highly detailed images.

However, just seeing the target wasn’t enough to make the blue-light method work for DIC analysis, Hoehler said. The researchers also had to reduce the image distortion caused by the refraction of light by the flame. “Luckily, the behaviors we want DIC to reveal, such as strain and defor­mation in a heated steel beam, are slow processes relative to the flame-induced distortion, so we just need to acquire a lot of images, collect large amounts of data and mathe­matically average the measure­ments to improve their accuracy,” Hoehler explained.

To validate the effec­tiveness of their ima­gining method, Hoehler and Smith, along with Canadian colla­borators John Gales and Seth Gatien, applied it to two large-scale tests. The first examined how fire bends steel beams and the other looked at what happens when partial combustion occurs, progres­sively charring a wooden panel. For both, the imaging was greatly improved. “In fact, in the case of material charring, we feel that blue-light imaging may one day help improve standard test methods,” Hoehler said. “Using blue light and optical filtering, we can actually see charring that is normally hidden behind the flames in a standard test. The clearer view combined with digital imaging improves the accuracy of measure­ments of the char location in time and space.”

Hoehler also has been involved in the develop­ment of a second method for imaging objects through fire with colleagues at NIST’s Boulder, Colorado, labora­tories. In an upcoming paper, the researchers demons­trate a laser detection and ranging (LADAR) system for measuring volume change and move­ment of 3D objects melting in flames, even though moderate amounts of soot and smoke. (Source: NIST)

Reference: C. M. Smith et al.: Imaging Through Fire Using Narrow-Spectrum Illumination, Fire Technol., online 23 July 2018; DOI: 10.1007/s10694-018-0756-5

Link: National Fire Research Laboratory Group, National Institute of Standards and Technology NIST, Gaithersburg, USA

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