The most important advantages of applying the lidar in marine campaigns is that the results of lidar measurements are obtained in real time without any disturbance to the aquatic environment (Babichenko et al. 1993, Determann et al. 1994, Patsayeva 1995, Barbini et al. 2001). The lidar light penetrates the seawater, where part of it is absorbed, emitted as fluorescence quanta or transformed into some other kind of energy. The emitted light disperses equally in all directions, but only the fraction reaching the telescope’s field of view is recorded by the lidar. So the important parameter of the geometry set-up is the ratio of the solid angle from which the light is collected by the telescope to the full solid angle (4π). It is the ratio of the telescope area to the surface area of the sphere into which the light is dispersed. The radius of this sphere r is equal to the distance between the target that emits the return signal and the telescope. The number of photons reaching the telescope decreases with the square of r and is proportional to the surface area of the telescope.
Intended to create a database of in situ fluorescence spectra of seawater, the lidar experiments were performed on board r/v ‘Oceania’ with the FLS-12 (LDI, Estonia) lidar system (Babichenko et al. 1989). This consists of an excimer laser (308 nm) used as the pumping source to a tuneable dye-laser, the lidar light source, and the receiving block, which includes the telescope, polychromator and electronic block. The tuneable range of emission is 320–670 nm. Time-gated fluorescence spectra of seawater are recorded in the 400–850 nm range.
The time-gated detection of the return signal permits control of both the optimal distance to the sensing layer (time-gate delay of the receiver) and the thickness of the sensing layer (time-gate duration of the receiver). The return signal is a continuous spectrum and can be divided into separate spectral bands due to Rayleigh scattering (elastic scattering of the laser emission at the water surface and in the water column), Raman scattering (inelastic scattering of the laser emission at water molecules, shifted 3420 cm−1 from the excitation wavelength) and CDOM and Chl a fluorescence (Fig. 2). The total intensity of the recorded fluorescence signal comes from the upper layer. The Raman scattering signal is used to normalise the fluorescence signal in order to obtain the fluorescence parameter describing the relative concentration of the fluorescing molecules (Klyszko & Fadeev 1978, Babichenko 2001, Drozdowska et al. 2002).