Blue-light imaging method sees through flames
The new imaging method published in the Fire Technology journal uses digital image correlation (DIC), an optical analysis 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, researchers can accumulate detailed information about how the material performs over time, including behaviors such as strain, displacement, deformation, and even the microscopic beginnings of failure.
To ensure that DIC would provide high-quality images even when bright, rapidly moving flames came between the sample and the camera, NIST scientists borrowed from the methods that glass and steel manufacturers use to monitor hot and glowing materials during production.
“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,” said research structural engineer Matt Hoehler. “We figured if it works with heated materials, it could work with flaming ones.”
“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 said.
Researchers conducted a series of experiments to determine the effectiveness of their method, using it to image how fire bends steel beams and also to image the progressive charring of a wooden panel when partial combustion occurs. In experiments, the method demonstrated the ability to take images of objects in natural gas fires up to 1000 kW using 200 W of illumination power.
Compared to white light, the required illumination to detect objects engulfed in flames with the blue light/optical filter method was reduced by a factor of 104.
“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”, explained Hoehler.
For more, see “Imaging Through Fire Using Narrow-Spectrum Illumination.”
NIST – www.nist.gov
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