Laser ranging can see 3D objects melting in fires

It may help overcome practical challenges posed by structural fires, which are too hot to measure with conventional electromechanical sensors mounted on buildings.

(a) Experimental ranging setup. A target obstructed by flames (depicted is an acetylene flame) is scanned with a FMCW LADAR. The target is placed ∼0.5  m behind this wall of flames. The total stand-off is 2 m. Range measurements are taken continuously at a 1-kHz update rate. A fast-steering mirror (FSM) sweeps the beam across the target, and the resulting 3D point cloud is then transformed to obtain Cartesian 𝑥𝑦𝑧 points. (b) 𝑥𝑦𝑧 3D point clouds. Left panel, machined aluminum step-block. Right panel, video (see Visualization 1) of a piece of chocolate, showing the deformation due to the flame heat (the frame rate is accelerated 60× to 1 Hz).
(a) Experimental ranging setup. A target obstructed by flames (depicted is an acetylene flame) is scanned with a FMCW LADAR. The target is placed ∼0.5  m behind this wall of flames. The total stand-off is 2 m. Range measurements are taken continuously at a 1-kHz update rate. A fast-steering mirror (FSM) sweeps the beam across the target, and the resulting 3D point cloud is then transformed to obtain Cartesian 𝑥𝑦𝑧 points. (b) 𝑥𝑦𝑧 3D point clouds. Left panel, machined aluminum step-block. Right panel, video (see Visualization 1) of a piece of chocolate, showing the deformation due to the flame heat (the frame rate is accelerated 60× to 1 Hz).

Using Laser detection and ranging (LADAR) system, scientists at the NIST have imaged three-dimensional (3D) objects melting in flames. Doing this, they got a precise, safe and compact way to measure structures as they collapse in fires.

The technique maps distances to objects melting behind flames that produced varying amounts of soot. The test estimated 3D surfaces with an accuracy of 30 micrometers (millionths of a meter) or better from 2 meters away. This level of exactness meets necessities for most auxiliary fire research applications.

Project leader Esther Baumann explained said, “We needed something that doesn’t melt too fast or too slow but you still see an effect. And I like chocolate.”

The technique is exceptionally sensitive and can image objects notwithstanding when little measures of soot are available in the flames. The technique additionally works at a distance, from sufficiently far away that the gear is safe from the intense heat of a fire. Moreover, the instrument can be minimized and compact, depending on fiber optics and straightforward photodetectors.

NIST structural engineer Matthew Hoehler said, “The project came together somewhat serendipitously when we got ‘fire people’ talking with ‘optics people’. The collaboration has not only been fruitful, it’s been fun.”

In the 3D mapping system, a laser clears ceaselessly over a band of optical frequencies. The underlying laser yield is joined with the reflected light from the objective. The subsequent “beat” signals are distinguished, and this voltage is then dissected by digital signal processing to produce time-delayed data, comparable to distance.

Scientists successfully applied LADAR to measure and map 3D “point clouds”—points are the “voxels” constituting an image—even in a turbulent fire environment with strong signal scattering and distortion. For comparison, the team also made videos of the chocolate as it melted and images of a more complex plastic skeleton.

For the melting chocolate, each LADAR frame comprised of 7,500 focuses adequately to catch the chocolate deformation procedure. The plastic skeleton was scarcely noticeable in the traditional video, however, the 3D point cloud uncovered complex shapes generally holed up behind blazes—subtle elements of the ribcage and hips.

The scientists discovered that the LADAR framework was quick enough to overcome signal distortions and that avoidances of the laser beam because of the flares could be suited by averaging the signs after some time, to hold high exactness.

The preliminary results suggest that the LADAR technique could be applied to larger objects and fires. The NIST team now plans to scale up the experiment, first to make 3D images of objects through flames about 1 meter wide, and if that works, to make quantitative observations of larger structural fires.

The results of this study are published in the journal Optica.