Looking beyond smoke and flames by lensless infrared digital holography

To reveal what’s behind the flames is a key and challenging aim both in the industrial and, above all, in security field. A wide variety of industries relies on furnaces and boilers for manufacturing processes. A failure of furnace and boiler equipment can cause quality problems and, in some case, they can also shut down an entire process line. Special thermal imaging cameras are often used to detect most of equipment problems during operation, so that failures can be prevented. This cameras uses a spectral waveband filter that only allows thermal radiation with those specific wavelengths through. Usually the filter is around 3.80 micron, where no hot gases are emitting. In fact, gases present in the flame have discrete absorption bands in the IR spectrum: the maximums for CO2 are at 2.7, 4.4 and 15 μm; for H2O they are at 1.4, 1.9, 2.7, 6 and 17 μm. Therefore, thermal imaging cameras equipped with such a filter ideal for furnace inspections, but cannot be used to see through any kind of flames. For example, in the flames generated during the combustion of forest fuels, the emitted radiation comes also from the solid particles of incandescent soot. Soot particles emit radiation in a continuous spectrum over a wider region from the visible to the IR. They are considered to behave as grey bodies (i.e. absorptivity is independent of wavelength and temperature). The relative contribution of soot radiation and band radiation (associated to hot gases) depend on the flame thicknesses: for thin flames (<0.20 m) band radiation dominated, whereas the contribution of soot radiation to flame radiation dominated with increasing flame thickness. Another contribution to the continuous spectrum, comes from the burning vegetation litter that emits in a stronger manner and in a continuous way in the infrared. In other cases the continuous emission in flames may be due to processes such as recombination of ions, or associations of atoms and radicals[1-3]. Therefore, also the last generation of Infrared (IR) cameras, based on a microbolometric sensor, commercially available at contained costs, operating in the range 7-14μm of the electromagnetic spectrum, that are generally used to see trough smoke, cannot be used to see through flames. In fact, they are blinded by flame-emitted radiation whenever a flame is present on the line of sight between the target and the detector. Here we propose a different approach to the through-flame imaging, based on an interferometric technique, that is not bound to the frequency of the emission spectrum and, therefore, can be applied to all kind of flames. In fact, we demonstrate a fully different approach to the through-flame imaging, based on an interferometric technique, i.e. Digital Holography [4] (DH). In particular we demonstrate that, thanks to lensless DH at far infrared, i.e.10.6 μm we are able to acquire in real-time holograms of moving alive people by CW laser and to obtain clear images through smoke and flames.