The measurements of downward light attenuation coefficient (Kd) have been carried out using a spectrometer designed and built in IO PAS. Also the data collected by MER 2040 have been used for calculation of Kd since 1993 (MER data can be used for Ku calculation as well). Succeeding models of our apparatus have had similar mechanical part. They have been equipped with a rotating disc containing 8 interference filters. The spectral half bands of these filters range from 5.5 to 11 nm, maximum transmissions are 26-48%, maximum transmission wavelengths are: 403, 429, 466, 529, 532, 579, 625, 685 nm. The first versions of this apparatus measured depth profiles of the irradiation separately for each filter. The profile was taken with the first filter set up. Then the disc rotated until the next filter was set up in the optical path. Then the next depth profile was measured, etc. Since 1987, in the modern versions, the disc has rotated with speed 1 rev/sec. The values of irradiance are recorded in turn for individual filter , the depth and a reference value is measured once per each rotation.
The reference value is the irradiance above the sea surface. The reference unit equipped with Lambert cosine collector and a photovoltaic cell as a detector is positioned on the deck of the ship. This reference value is necessary to include the changes of solar irradiation during the measurement.
The most significant changes in our spectrometer were done in the electronic part. All but one version have had photomultiplier as a detector (there was one in 1989 with photovoltaic cell). The amplification and transmission of the signal were resolved following the progress in the electronics technology. The last version is equipped with microprocessor in underwater unit. The signal is transmitted in digital form using two wire cable to deck unit and then to the PC computer.
The measurements are performed in two sensitivity modes. Each complete measurements consist of two profiles (one with low sensitivity, in the surface layer, and the other with high sensitivity in the deeper zone). The obtained values of irradiation are not calibrated in physical units, for measurements of Kd the linearity of electronic part is sufficient.