838 
sample. As can be seen in Fig. 3, the light cannot excite the chlorophyll molecules very well for the lower 
wavelengths. This is partly due the UV-protecting outer layer of the leaf, consisting of wax and UV- 
screening pigments, and partly due to the smaller absorption cross section of the chlorophyll molecules at 
these wavelengths. The strong blue fluorescence has its origin in the the outer layer of the leaf. When 450 
nm is used, the excitation wavelength coincides with the maximum absorption peak of chlorophyll, which 
is the reason for the very high chlorophyll fluorescence exhibited. This wavelength is, however, a bit too 
long to efficiently excite the blue fluorescing compounds that might be used to decide on the plants status. 
Furthermore, eye safety regulations do not allow high-power optical radiation in the visible region. All this 
considered, our choice of exciting wavelength was 397 nm, which can provide chlorophyll fluorescence as 
well as blue fluorescence, while making it relatively easy to comply with eye safety regulations. 
Figure 4 shows examples of characteristic fluorescence spectra for four different plants. They 
were recorded remotely with the spectrally resolving point monitor during the field test in Avignon. The 
plants are maize, cypress, sorghum and poplar. 
Fig. 4. Characteristic fluorescence spectra for maize, cypress, sorghum and poplar. 
An example of the signal-to-noise ratio can be studied in Fig. 5. Measurements were performed 
with the spectrally resolved system equipped with the new sensitive detector. 
Fig. 5. Fluorescence spectra of a maize plant at a distance of 40 m. The spectrum to the right was 
integrated over 100 laser shots while the spectrum to the left was recorded with a single laser shot only. 
The laser pulse energy was about 30 mJ.