Experiments

Mid ir digital holography: vision through smoke and flames

Digital holography in the Mid IR range (Mid IR DH) is a well-established research activity at INO. Indeed, the extension of digital holography from the visible to the IR spectral region has been realized, for the first time, at INO in 2003, and, since then, the technique has been significantly improved.
Digital Holography is an interferometric imaging technique which gives access to both amplitude and phase of the wavefront coming from an object irradiated with coherent radiation. The wavefront information is encoded in the so-called hologram (the interferometric pattern created by the superposition of the radiation scattered by the object and a reference beam) so that it can be digitally recorded and, then, numerically reconstructed.
Thanks to the long wavelength, Mid IR DH benefits of a 20 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view, with respect to DH in the visible range. Mid IR DH is thus well-suited for remote monitoring of human size dynamic scenes out of laboratory condition and has various possible applications (virtual museum, non-destructive test and quality control, 3d television, etc.).
Recently, we have demonstrated one of the most promising capabilities of Mid IR DH: vision through smoke and flames. Various elements contribute to achieve this result: first of all, digital holograms are recorded in a lensless configurations so that flame-emitted radiation is scrambled over the whole detector area, avoiding saturation; secondly, flame-emitted radiation is not coherent with the laser radiation so that it does not disrupt the interferometric pattern containing the wavefront information; finally, IR radiation is just slightly scattered by smoke. This result has received the attention of the international media and scientific community for its obvious potential application in the homeland security field, as a aid to fire rescue teams.

Imaging of an Augustus statuette through candles flames. (Left) Thermo-graphic image. (Right) Holographic reconstructed image.

Imaging of a human being through flames. (From left) Thermographic image, visible range image, holographic image.