Seeing People Moving Behind Smoke and Flames by Lensless Digital Holography at Long Infrared Wavelength

The working group of National Institute of Optics (INO-CNR), Italy. From left to right: Vittorio Bianco (behind), Pasquale Poggi (ahead), Lisa Miccio (behind), Melania Paturzo, Pietro Ferraro, Riccardo Meucci, Massimiliano Locatelli, Anna Pelagotti, Eugenio Pugliese.

Authors:
V. Bianco¹, M. Paturzo¹, M. Locatelli², E. Pugliese², A. Finizio¹, A. Pelagotti², P. Poggi², L. Miccio¹, R. Meucci², P. Ferraro¹.

Affiliations:
¹National Institute of Optics (INO-CNR), Pozzuoli (Naples), Italy.
²National Institute of Optics (INO-CNR), Florence, Italy.

Fire accidents are responsible every year for thousands of deaths, damages and permanent burn injuries, the most of them regarding home fires (3000 deaths occurring each year due to home fires according to the U.S. Fire Administration [1]). In such situations, firefighters and first responders are called to operate in hostile environments, where their skills are often strained to the limit by the reduced visibility due to the presence on the field of curtains of smoke and walls of flames. In these cases, the rescue operations can slow down and be more dangerous, depending on the fire and smoke spread which impair the naked-eye view as well as the vision obtainable with white-light detectors, hiding any target behind the “obstacle.”

Many efforts have been spent to try to extend the human eye capabilities in hostile environments. In order to cope with this problem many fire departments make use of the last generation of Infrared (IR) bolometers, now commercially available at contained costs, which operate in the range 7-14μm of the electromagnetic spectrum and, hence, they are much less affected by the scattering of smoke particles, or incandescent soot. However, if a flame is present on the line of sight between the detector and the target, the flame emission tends to saturate the detector elements, unavoidably resulting in blind areas in the image. This problem is more severe in presence of extended flames, as the whole fire scene becomes not observable. For example, in case of a room invaded by smoke and flames, the thermographic cameras return images with blind areas in correspondence with incandescent or burning objects, or the flame source itself, and also the mere detection of the presence of people trapped inside is not achievable with enough reliability.

On the contrary, we have proved that Digital Holography [2] at far Infrared radiation (IRDH) is a promising technique to substitute, in a forthcoming future, the existing thermographic devices, allowing to get a clear and sharp imaging of small objects as well as moving humans hidden behind smoke and flames. In principle this is possible exploiting some unique features of IRDH [3-5]. First of all, thanks to the IR radiation, the wavefront propagation is just slightly scattered by incandescent soot and smoke particles, making IRDH competitive with respect to the existing IR cameras. Moreover, employing a long wavelength (10.6µm in our experiments), the acquisition system is much less affected by seismic noise and vibrations, so that it can be brought outside the lab, to be exploited on the field. Thanks to the long wavelength, one of the most binding requirements of Digital Holography (DH) have been overcome for the first time. In fact, DH at visible radiation has been deeply employed so far in quantitative phase microscopy and, in general, to image small size objects. On the contrary, the IR wavelength allows to increase of about 20 times the maximum size of the target detectable, as this size is proportional to the wavelength employed [5]. However, the key feature of IRDH, which enables to overcome the limits of thermographic imaging, is the possibility to discard the flame emitted radiation and to recover the only useful information content behind it.

In fact, since in conventional IR imaging a zoom lens is employed, the radiating contributions emitted in several different directions and matching the numerical aperture of the lens are all conveyed on a few detector elements, and the focused energy causes their saturation. In turn, this results in blind areas in the retrieved image and the useful information of the targets beyond the flames is lost. On the contrary, IRDH in lensless configuration allows to perform the data capture by placing the detector out of the focus plane and the numerical focusing can be performed by means of numerical techniques simulating the diffraction integral, i.e. reproducing the propagation of the complex wavefield from the recording plane to the focus one. In this way, the disturbing radiation is distributed on the whole camera array and saturation is avoided. Taking advantage of the combination of all these features, we obtained the first holographic recording of a moving man standing behind a flame generated by a portable mini-stove [5].

To demonstrate the IRDH capabilities, in our first experiment we placed a bronze statuette 10cm high into a plexiglass box where smoke has been injected in controlled way, as shown in Fig.1a, and both thermographic and holographic acquisitions have been performed. From Fig.1b and Fig.1c it is apparent that both the investigated techniques are able to offer a clear view of the object inside the chamber, despite the presence of the dense curtain of smoke.

In our second experiment digital holograms of objects hidden by the flame of a candle have been acquired. The candle was placed on the path of the object wave so that it constitute an obstacle for thermographic cameras. The illustration of Fig. 2 shows the working principle of lensless IRDH which directly gets rid of the flame emission as previously described. As a result, in Fig. 2a the thermographic image of the bronze statuette placed behind the candles shows a blind area in the middle, impairing the vision of a part of the target. On the contrary, no information is lost by employing the lensless IRDH set-up, as shown in the holographic reconstruction of Fig. 2b.

Finally, in the last experiment we recorded holograms of a moving person behind a curtain of flames generated by a portable mini-stove. Figure 3a shows the result obtainable simply employing a thermographic sensor. The man shaking his hands is just barely visible through the extended blind area in the middle of the image and only his arms are appreciable. With such a sensor, a more extended flame would completely impair the detection of the target. In Fig. 3b the holographic reconstruction of the eye-glassed man imaged behind the same kind of flame is shown. No sensor saturation occurs, so that the reconstruction does not show blind areas and also details of the arm are appreciable in the region where the flame emission is stronger, thus demonstrating the capability of IRDH of rejecting it.

Thanks to these unique features of IRDH technique we believe that this work can open the route for the development, in the next future, of a new generation of portable holographic sensors to be exploited by firefighters and first responders in order to coordinate the rescue operations and work safely in hostile environments.

References
[1] U.S. Fire Administration, http://www.usfa.fema.gov/statistics.
[2] T. Kreis, “Handbook of holographic interferometry: optical and digital methods,” (Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2005).
[3] Melania Paturzo, Anna Pelagotti, Andrea Finizio, Lisa Miccio, Massimiliano Locatelli, Andrea Gertrude, Pasquale Poggi, Riccardo Meucci, and Pietro Ferraro “Optical reconstruction of digital holograms recorded at 10.6 μm: route for 3D imaging at long infrared wavelengths,” Opt. Lett. 35, 2112-2114 (2010). Abstract.
[4] Anna Pelagotti, Massimiliano Locatelli, Andrea Giovanni Geltrude, Pasquale Poggi, Riccardo Meucci, Melania Paturzo, Lisa Miccio, and Pietro Ferraro, “Reliability of 3D imaging by digital holography at long IR wavelength,” Journal of Display Technology, 6, 465-471 (2010). Abstract.
[5] M. Locatelli, E. Pugliese, M. Paturzo, V. Bianco, A. Finizio, A. Pelagotti, P. Poggi, L. Miccio, R. Meucci, and P. Ferraro, “Imaging live humans through smoke and flames using far-infrared digital holography,” Optics Express, 21, 5379-5390 (2013). Abstract.