Classification of cavities by time correlated multi-bounce photon counting

Remote cavity exploration is a matter of probing hollow structures such as caves and cavities and is applied in security and military scenarios, used for assessing cavities in disaster and damage scenarios or geological and archaeological excavations, and attracted interest for space applications. In contrast to detailed exploration and mapping approaches, we have developed a rapid and simple assessment of cavities that allows initial remote evaluation and classification of cavity sizes prior to further exploration by other means. Our method uses time-correlated single photon counting (TCSPC) to measure the temporal response of cavities to excitation by short laser pulse illumination. We observe a temporal signature consisting of multiple peaks due to multiple-bounce photon path before the onset of intermixing of signatures lead to an integration and homogenization of intensity. The observed signatures represent photon paths with reflections from different surface areas within the cavity. Based on the temporal sequence and shape of the signatures, we can determine the size of the cavity investigated. In this publication we focus on the analysis of the recorded signatures using a multi-peak fitting algorithm and a propagation model assuming a spherical cavity shape.