Using Blue Light to See Through Fire in Optical Imaging

Interview conducted by Alina Shrourou, B.Sc. (Editor)Aug 28 2018

An interview with Matthew S. Hoehler, PE, PhD, conducted by Alina Shrourou, BSc

Please give an overview of how optical methods are used in fire research.

A wide range of optical measurement techniques are used in fire research. As a structural engineer, I am interested in measuring the position of objects in a burning room, as well as how they deform as a fire progresses. A typical object of interest would be a steel beam or column holding up a building during a fire. Image-based techniques, such as Digital Image Correlation (DIC), that provide full-field displacements on the surface of an object by comparing changes in two or more evolving digital images, offer several potential advantages over conventional measurement methods.

© kzww/Shutterstock.com

What are the challenges associated with using image-based techniques to investigate how fire impacts structural materials?

All optical techniques that rely on light in the visible spectrum must contend with three challenges when fire is present: (1) extinction of light due to smoke and soot, (2) obscuration of the target by thermal radiation (particularly red light) emitted by the flames and glowing hot targets and (3) distortion of the images as a result of light refraction due to density changes in the hot air between the camera and the target. In structural fire research, we can frequently use clean-burning surrogate fuels to replicate the burning contents of a building, which means extinction is less of a problem and we primarily need to deal with obscuration and distortion.

Please outline the Narrow-Spectrum Illumination Technique.

The narrow-spectrum illumination technique uses high-intensity light emitting diode (LED) lights with a mean wavelength of 450 nm (blue light) to illuminate the target object that is engulfed in flames. The target is then imaged using one or more cameras equipped with bandpass optical filters which are frequency matched to the wavelength of the LED light. Various methods can then be used to analyse the images.

© Graphic created by N. Hanacek/NIST based on a concept by M. Hoehler/NIST

How does a blue light source improve the signal received by a camera through flames?

Most of the radiant energy given off by a fire is in the infrared and red portions of the electromagnetic spectrum. By using blue light, we boost the intensity of light reflected off the target in the lower portion of the visible spectrum and then filter out the red and infrared signals using the optical filter. This allows us to increase the signal-to-noise ratio by several orders of magnitude, while still working with common digital or scientific cameras; i.e. it is an inexpensive way to improve your image signal-to-noise ratio. Blue light has been used by others in the past to image glowing hot metal, but we found that the approach also works for seeing through large clean-burning fires.

Please describe your recent research involving the use of blue LED lights to reduce flame-induced image distortion.

Narrow-spectrum illumination is an enabling technology for subsequent optical analysis. It allows one to image targets engulfed in flames that were previously not visible. These images, however, are still distorted due to light refraction by the temperature gradients in the flames. When you watch a video of a stationary object behind flames it appears to waver (distort) over time; much like when you look at road pavement on the horizon on a hot day. Luckily, the target objects we are imaging move slowly relative to the flame-induced distortions in the images, so we can use temporal and spatial averaging to reduce the effects of the distortion on our displacement measurements.

What are the applications of using blue light in this way?

A big benefit of using the blue light in our research has been to see things more clearly through fires and to make previously impossible qualitative observations about material damage during a fire. We recently performed exploratory large-scale tests of structural steel members subjected to fire and analysed them using Digital Image Correlation. We have also used the technique to measure char location in engineered wood elements that are burning and believe there may be other potential applications for the investigation of burning materials.

©  M. Hoehler/NIST

Where do you hope to take these findings in the next steps of your research?

We are now deploying this technique in daily operations at the NIST National Fire Research Laboratory and intend to continue to explore quantitative application of narrow-spectrum illumination to make dimensional measurement of objects in fire.

Where can readers find more information?

More information about using narrow-spectrum illumination to image through clean-burning fires can be found in a recent article published in Fire Technology.

C.M. Smith and M. Hoehler. Imaging Through Fire Using Narrow-Spectrum Illumination. Fire Technology. Posted online July 23, 2018. DOI: https://doi.org/10.1007/s10694-018-0756-5

Examples of the deployment of the technique to large-scale fire test can be seen in:

M. Hoehler and C.M. Smith. Application of Digital Image Correlation to Structures in Fire (PRESENTATION SLIDES); https://www.nist.gov/publications/application-digital-image-correlation-structures-fire-presentation-slides

About Dr. Matthew Hoehler

Matthew Hoehler is a Research Structural Engineer at the National Institute of Standards and Technology (NIST) where he provides program support to the National Earthquake Hazards Reduction Program and the National Fire Research Laboratory to further NIST´s mission to ensure a resilient national infrastructure.

Dr. Hoehler received his B.S.E. from Princeton University in 1998, his M.Sc. from the University of California, Berkeley in 2000 and his Dr.-Ing. from the University of Stuttgart, Germany in 2006. He has nearly twenty years of experience in experimental testing and analysis of the performance of materials, components, and structures associated with structural collapse, natural disasters, or human-initiated events. Dr. Hoehler is a licensed Professional Engineer in the State of California.