The ability to see behind flames is a key challenge for the industrial field and particularly for the safety field. Development of new technologies to detect live people through smoke and flames in fire scenes is an extremely desirable goal since it can save human lives. The latest technologies, including equipment adopted by fire departments, use infrared bolometers for infrared digital cameras that allow users to see through smoke. However, such detectors are blinded by flame-emitted radiation. Here we show a completely different approach that makes use of lensless digital holography technology in the infrared range for successful imaging through smoke and flames. Notably, we demonstrate that digital holography with a cw laser allows the recording of dynamic human-size targets. In this work, easy detection of live, moving people is achieved through both smoke and flames, thus demonstrating the capability of digital holography at 10.6 μm.
In the United States, for example, fire departments respond to about 1.6 million fire calls per year, and domestic house fires make up the majority of them (3000 deaths occurring each year in house fires).
The recent generation of infrared (IR) bolometer detectors, commercially available for imaging in the IR spectrum in the range of 7–14 µm, are uncooled (i.e., they operate without liquid nitrogen), thus they are lighter in weight and have reached high density and resolution array (680×480 pixels; pixel size down to 25 µm). Also, the cost of such devices is no longer so high, considering their brilliant performance. Such devices allow passive or active clear vision (i.e., with laser IR illumination) through smoke or fog since IR electromagnetic radiation is scattered just slightly by fog drops or smoke particles. However, visible radiation is strongly affected by scattering, and vision can be completely impaired in such situations. Many fire departments use IR cameras based on a bolometer for exploring fire scenes in order to have clearer vision and to allow the rescue of human lives, or to operate safely in such a hostile environment. As explained above, while imaging through smoke is possible in the range of 7–14 µm, flames can completely blind the detector. In fact, electromagnetic radiation emitted by flames can severely saturate the detector, occluding the scene behind them.
Target imaging through smoke. (a) Metal object in Plexiglas™ box. Images recorded by a standard white-light photo camera before and after letting smoke into the box. (b) Thermographic imaging of the metal object through smoke. (c) Holographic amplitude reconstruction. This confirms that holography has the same capability of IR imaging to see through smoke