Adaptive underwater optical imaging: Development and evaluation of a range-enhanced, LED-based, SPAD imaging system

Kurzfassung

Underwater optical imaging systems carried by robotic platforms play a critical role in detailed close range observations as required in structural health monitoring, archeology, habitat monitoring or the inspection of critical underwater infrastructures. Together with an ongoing trend towards longer autonomous underwater robotic missions, either with extra large uncrewed underwater motherships or with resident vehicles recharging in underwater docking stations, this leads to operations facing a larger variety of environmental conditions with reduced opportunities for intervention by a human operator in the meantime. This is accompanied by an increased demand for optical imaging systems adapting to the ambient conditions. In particular in coastal waters underwater visibility can change quickly and drastically. To address this challenge, the development of an LED-based underwater imaging system, capable of 1) passive imaging in the presence of ambient light, 2) high power flood illumination in dark, but clear environments and 3) range-gating in the absence of light in turbid waters, is proposed. Range-gating enhances the underwater imaging range through tight synchronization of nanosecond light pulses with a delayed camera exposure, suppressing backscatter in front of the instrument, typically relying on laser sources. With regard to the proposed system, there are knowledge gaps about the value of the proposed operational mode switching, about the feasibility of LEDs in range-gating and the value of potentially complementary methods such as multi-spectral imaging. The visibility range of all three operational modes is among other things limited by forward scattering inducing image blur. The spectral characteristics of forward scattering were rarely studied, especially not experimentally and with regard to image restoration through deconvolution. These characteristics were addressed in an experimental study, showing especially the value of red illumination in scattering waters and the importance of correct wavelength selection for long wavelength channel deconvolution. The feasibility of LED illumination in nanosecond pulsed gated operation was estimated in a first study, proposing the combination of a spotlight consisting of approximately 100 single LEDs with a single-photon avalanche diode (SPAD) camera operating at MHz repetition rate. Nevertheless, achieving high powers with nanosecond pulsed LEDs is a major challenge, because of their inherent electro-optical principle. Developments, modifications and a comparison of different LED driver designs led to the successful development of three large-array LED light sources capable of 8 ns long optical pulses, providing approximately 100 mW optical power at 1 MHz repetition rate. With the developed underwater imaging system combining three (RGB) such light sources with a 512x512 pixel SPAD camera, LED-based range-gating was demonstrated, achieving gates as short as 0.9 m. In a turbid lake test, range-gated images were acquired at up to 4.7 attenuation lengths which is close to the imaging ranges of laser-based systems. The three proposed operational modes were systematically compared with each other in field experiments in three different natural waters. Range-gating almost always provides the best contrast, but the contrast-to-noise ratio indicates a higher image quality with flood illumination at up to roughly 4 to 4.5 attenuation lengths in highly scattering waters, when applying contrast enhancing methods such as histogram equalization. With minor light availability improving updates, the range-gating mode is expected to achieve imaging ranges comparable to laser-based systems. Due to its range-gating capability, the system brings the tools for underwater optical property estimation, enabling a physics-based operational mode switching. In particular the developed LED light source with its scalability and the potential for nanosecond white light pulses may spark research towards hyperspectral range-gating, which would especially address needs in underwater ecosystem monitoring.