876 
Table I 
Laser 
Wavelength 
Energy 
Pulse width 
Pit 
nm 
mj 
ns 
Hz 
XeQ 
308 
100 
16 
200 Max 
Nd:YAG 
355 IH harm 
30 
1 
10 
rsch 4 
338 
5 
16 
1 
” 
374 
1.5 
16 
1 
" 
421 
0.3 
16 
1 
The LIF signal, originated by laser excitation on the investigated target, is collected by a Newto 
nian telescope and focussed onto a glass fiber optic (F.O.) bundle after passing suitable optical filters, which are 
needed either to cut most of the backscattered laser radiation or to select a specific wavelength range. The F.O. 
bundle feeds the collected radiation to the entrance slit of a small monochromator whose F# has been properly 
matched by the F.O. termination. The dispersed radiation is finally read by a gated OMA linear array detector. 
On the field, the telescope usually looks directly towards the fluorescing target, while in the labo 
ratory the last transmitter mirror and the first collecting mirror are mounted at approximately 45 deg on top of a 
metal pylon. The sample holder, placed at the base of the pylon, is either a small basin, to monitor the water 
surface, or a high cylindrical vessel, to detect signals from water columns of different heights (up to 200 cm), 
depending on the transparency of the solution. Laboratory experiments with two lasers pump-probe technique, 
have been performed on a small photobioreactor by placing the end of the F.O. near to the investigated sample. 
Gating of the OMA detector, and synchronization of photodiode scans with the transmitted laser 
pulses is accomplished by a suitable choice of trigger schemes which overcome the problems related to laser 
jitters. These problems, especially important in two laser experiments (pump-and-probe technique), have been 
solved by building home-made electronic module with adjustable delays [5]. 
2.Z Data analysis 
A personal computer is used to control the lasers and the OMA via GPEB interfaces and is further dedicated to 
both data acquisition and preliminary data analysis. Data are finally elaborated and stored in a VAX computer 
connected to a DECNET network. The data analysis procedures adopted for our measurements are described 
below: 
• All the measured LIF spectra are corrected for the OMA spectral sensitivity, as given by the manufacturer, 
through the whole visible region. 
• The small passive background emission from the ambient light within the gate time of 200 ns, is automati 
cally subtracted from the data by the OMA acquisition program. 
• Before data normalization (to 100 pulses), the accumulated signal intensity on both detectors is checked to 
be exactly proportional to the number of pulses. 
• Normalization to the water Raman signal itself has been performed for measurements on water column of 
depth greater than 20 cm. 
For all the investigated targets, complete LIF spectra are routinely acquired and stored by the OMA in the wave 
length region 300 nm to 800 nm. The spectra are usually piecewise integrated with a fixed bandwidth around 
several characteristic wavelengths. Spectral contents are extracted in the UV (for monitoring the residual back- 
scattered laser power and/or the water Raman signal), in the Blue and in three Red wavelengths, the latter being 
especially important in monitoring pigments characteristic of different phytoplankton species. Ratios among 
spectral contents at different wavelengths (spectral ratios) are finally used to infer various features of a given 
target and to measure the absolute concentration of phytoplankton in the water.