Impact of water quality on Single Photon Avalanche Diode direct time-of-flight imaging

A detailed study on the effect of chlorophyll and sediment, two of the main constituents of ocean water on the image quality of a Single Photon Avalanche Diode (SPAD) direct time-of-flight (dToF) imaging system is conducted. This system consists of a 532nm laser and a 32×32 SPAD time-of-flight sensor. The degradation of laser power and the volume scattering function (VSF) are measured and the image quality of underwater objects imaged by the SPAD 32×32 time-of-flight sensor is examined. Classification accuracy of simple geometric shapes is used as an indicator of the quality of the SPAD images. Under lab conditions, controlled amounts of sediment and chlorophyll are added into a water tank for these experiments. The laser's output to input power ratio is found to be exponentially decreasing with increasing path length travelled by the beam. The laser's beam attenuation coefficient is calculated and found to increase linearly with increasing concentration of these constituents. Likewise, the volume scattering function of the beam is found to be larger when these two constituents are present in the water. All of these experimental results are in accordance with the predictions of the Beer-Lambert Law. Furthermore, the shape classification accuracy is shown to decrease with increasing concentrations of sediment. Overall, these results confirm that the quality of underwater images taken by the SPAD flash imaging system will rapidly degrade with increasing chlorophyll and sediment concentration.