The long optical base transmissometer (LAMS—Long Arm Marine Spectrophotometer) constructed in 2008 by NESTOR group is described. The data of the recent water transparency measurements in the NESTOR site and in the Capo Passero site in the wavelength range 378–522nm are presented

A long optical base line spectrophotometer designed to measure light transmission in deep sea waters is described. The variable optical path length allows measurements without the need for absolute or external calibration. The spectrophotometer uses eight groups of uncollimated light sources emitting in the range 370–530nm and was deployed at various depths at two locations in the Ionian Sea that are candidate sites for a future underwater neutrino telescope. Light transmission spectra at the two locations are presented and compared.