A blue-light imaging method was developed that can be used to obtain 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.
Using DIC to study how fire affects structural materials presents the challenge of obtaining 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.
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. Commercially available and inexpensive blue light-emitting diode (LED) lights with a narrow-spectrum wavelength around 450 nanometers were used for the 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.
Just seeing the target wasn't enough to make the blue-light method work for DIC analysis. The researchers also had to reduce the image distortion caused by the refraction of light by the flame — a problem akin to the “broken pencil” illusion seen when a pencil is placed in a glass of water. The behaviors DIC needed to reveal, such as strain and deformation in a heated steel beam, are slow processes relative to the flame-induced distortion. Multiple images and large amounts of data were collected, and the measurements mathematically averaged to improve their accuracy.
To validate the effectiveness of the imaging method, it was applied 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 the case of material charring, blue-light imaging may one day help improve standard test methods. Using blue light and optical filtering, charring that is normally hidden behind the flames in a standard test can now be seen. The clearer view, combined with digital imaging, improves the accuracy of measurements of the char location in time and space.