837 
The laser transmitter of the lidar system is a Nd:YAG laser with frequency doubling or tripling. In order 
to induce chlorophyll fluorescence while still staying eyesafe, the output from the frequency-tripled laser 
was Raman-shifted in a high-pressure deuterium cell to achieve 397 nm. The radiation was transmitted in a 
divergent beam towards the target area. The backscattered fluorescence light was collected by the lidar 
telescope and directed into a second Cassegrainian telescope. The image plane of the lidar telescope 
coincided with the object plane of the Cassegrainian telescope. In the image plane of the Cassegrainian 
telescope an intensified CCD camera was placed. The telescope has its first mirror cut into four segments 
that can be individually adjusted. By tilting the four mirror segments, each segment produces an image at 
the image intensifier, arranged as four quadrants of the detector. Before each mirror section an interference 
filter or coloured-glass filter was placed. The peak transmission of the filters were chosen as to match the 
interesting features of the fluorescence spectra. 
When the system was used in the spectrally resolving point monitoring mode, a flip-in mirror was 
placed in the focal plane of the lidar telescope. The detected light was directed towards a white screen in 
which a hole had been pinched. The target area could be seen on the screen. An optical fibre was mounted 
in the centre of the hole, which transmitted light from the point that was to be analyzed. The fluorescence 
light was guided through the optical fibre to a small spectrometer. The detector was an image-intensified 
1024 channel diode array. The signal was displayed on the screen of the OMA mainframe. 
In both types of measurements the detection was gated in order to suppress ambient daylight. In 
order to increase the signal-to-noise ratio, averaging was used. In the measurements performed in 
Avignon, a new detector was employed for the spectrally resolved mode, a Spectroscopy Instruments 
Model ICCD-576. The sensitivity of this device was several times larger than that for the one used before. 
This meant that less shots were necessary to get an adequate signal-to-noise ratio. 
3. RESULT AND DISCUSSION 
The choice of wavelength to use for exciting vegetation fluorescence must be done with great care. Fig. 3 
shows spectra taken in the lab for different excitation wavelength on the same leaf of maize. 
Maize 
Fig. 3. Fluorescence spectra from the upper side of a maize leaf recorded inside the van using different 
excitation wavelengths. Magnification of parts of the spectra are included in the figure. 
The wavelengths chosen in this example were the first Anti-Stokes component, the fundamental, and the 
first and second Stokes component generated from a deuterium-filled Raman cell, pumped by 355 nm 
radiation. The wavelengths were selected one at a time and transmitted through an optical fibre to the