Calibration of the LISST-VSF to derive the volume scattering functions in clear waters

The recently commercialized LISST-VSF instrument measures the volume scattering function (VSF) from 0.1 degrees to 15 degrees with a traditional laser diffraction unit (LISST) and from 15 degrees to 155 degrees with an eyeball component. Between these two optical components, only the LISST unit is calibrated. The eyeball measurements are scaled using the VSFs at 15 degrees that are measured by both components. As this relative calibration relies on a valid measurement at 15 degrees by the LISST, it might fail in clear oceanic waters, where the forward scattering is relative weak either due to a lack of large particles or an overall low concentration of all particles. In this study, we calibrated the LISST-VSF eyeball component through a series of lab experiments using standard polystyrene beads. Validation with the beads of two different sizes showed a median difference of 11.1% between theoretical and calibrated values. Further evaluations with in situ data collected by the LISST-VSF and an ECO-BB3 meter indicated that the new calibration worked well in both turbid and clear waters, while the relative calibration method tended to overestimate VSFs in clear waters. (C)2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement