Cavity exploration by laser pulse stretching measurements with single photon counting

The recent development of single-photon avalanche diode (SPAD) based sensors has completely revolutionized the field of advanced imaging and contributed to the rapid increase of "computational imaging". In this paper, we show that it is possible to visualize the transient propagation wave when a laser pulse is entering into a cavity (e.g. an integrating sphere) though an opening, a skylight or any hole. The photon time-signature at the opening of the cavity depends on the volume or diameter of the cavity and on its spectral reflectance. By using ultra-short laser pulses, we demonstrated that the temporal observation of the reflected waves at such cavities can give a very fast information on the cavity volume and would allow to make a first evaluation before a more detailed analysis, for example with the NLOS technique. Our method of rapid cavity perimeter determination could be used in cavity assessment for disaster and damage scenarios and assistance for first responders, as well as for cavity exploration during geological/archaeological excavations or extraterrestrial missions to the Moon or Mars.