- Researched used commercial LIDAR system to image 3D structures melting in the fire.
- They successfully measured 3D objects with an accuracy of 30 micrometers from a 2-meter distance.
- The system can generate precise details in the presence of high signal deflection and distortion caused by flames.
To study the influences of fire on buildings and other structures, it’s necessary to analyze the deforming objects like pipes, columns or beams affected by hot flames. Usually, it requires measurements with sub-millimeter precision, which are extremely difficult to perform in intense fire conditions.
We could use optical range measurement technique to deal with practical problems caused by structural fires, which cannot be analyzed by traditional electromechanical sensors installed on buildings.
Now a team of scientists at the National Institute of Standards and Technology has imaged 3D objects melting in the fire, using a LIDAR (LIght Detection And Ranging) system. It provides a small, reliable and safe way to measure objects as they deform in flames.
The team used a commercial LIDAR system to demonstrate their research. They mapped distances to melting items behind fire producing a large quantity of soot. They were able to measure 3D objects with an accuracy of 30 micrometers from the 2-meter distance.
They wanted an object that does not melt too slow or too fast, and you could see its internal structure being affected by hot surrounding. Therefore, they focused on two things: a plastic toy and pieces of chocolate.
Experimental ranging setup | Courtesy of researchers
LIDAR measures distance to an object by illuminating it with pulsed laser light and monitoring the reflected pulses through a sensor. The technology offers numerous advantages for imaging through fire: it’s sensitive and capable of detecting small items even when flames carry some soot particles.
Moreover, the technique works at certain distances that are far enough to keep instruments safe from fire heat. These equipment are small, portable, and rely on conventional photodetectors and fiber optics.
How Does It Work?
A laser beam continually sweeps through an optical frequency band in the three-dimensional mapping system. The initial light beam is merged with the beam reflected by the target object.
Then the voltage of the final pulse is examined via digital signal processing to produce time-variant data, which represents the distance between object and instrument. The frequency difference between the initial light beam and the reflected one amplifies with distance.
The heterodyne (a signal processing technique) makes measurements at low return signals possible and masks out background radiation of flames, whereas the fast update rate enables the system to efficiently work in the presence of distorted signals.
LIDAR was used for measuring and mapping 3D points clouds (voxels forming a picture) in extreme fire conditions with high pulse scattering and distortion. For instance, in melting chocolate, every single LIDAR frame comprised of 7,500 points, which is enough to accurately image the process of chocolate deformation.
3D shape of the plastic skeleton | Courtesy of researchers
For plastic skeleton, the LIDAR frame unveiled complex shapes behind flames (including details of hips and ribcage), which were hardly visible in a typical video. Overall, the system is effective enough to generate precise details in the presence of high signal deflection and distortion caused by flames.
The early experiments were performed with only 50-millimeters wide flames produced via laboratory burners. However, the LIDAR method can be deployed to bigger structures and fires. The researchers plan to scale up their demonstration: they will generate 3D pictures of larger objects in one-meter wide flames for quantitative observations.