A simple method for increasing visibility through clean-burning flames, using ordinary blue LED light, could lead to 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. 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. Courtesy of National Fire Research Laboratory/NIST. Researchers placed a target object behind a 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. Developed by the National Institute of Standards and Technology (NIST), the imaging method 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.” The NIST narrow-spectrum illumination method for imaging through fire. Blue LED light is directed through a gas fire, reflects off the target object behind the flames, and is captured by a camera after passing through an optical filter. This reduces the observed intensity of the flame by 10,000-fold and yields highly detailed images. Courtesy of N. Hanacek/NIST based on a concept by M. Hoehler/NIST. 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 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. “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.” This approach could be a useful tool for obtaining visual data from large test fires, like those used in laboratory fire studies and fire-resistance standards testing, where high temperatures can disable conventional sensors. For their approach, researchers used commercially available blue LEDs with a narrow-spectrum wavelength of around 450 nm. The research was published in Fire Technology (doi:10.1007/s1069). Comparison of two different views of a laboratory fire spread test, one in normal light and the other with NIST's narrow-spectrum illumination system that uses ordinary blue light to see through the flames. In this example, blue-light imaging allows researchers to observe, track, and measure the charring of the wood sample. Courtesy of NIST, J. Gales/York University, and D. Sawyer/NIST.