International audienceThis paper presents a field platform for continuous measurement of fluorescence at the canopy level. It consists of a 21-m-high crane equipped for fluorescence measurements. The crane is installed in the middle of the fields dedicated to agricultural research. Thanks to a jib of 24 m and a railway of 100 m distance, fluorescence measurements can be performed at nadir viewing over various field crops. The platform is dedicated to the development and test of future passive or active airborne and space-borne vegetation sensors. A new fully automatic instrument, called TriFLEX, has been installed at the end of the jib. TriFLEX is designed for passive measurement of fluorescence in the oxygen A and B absorption bands. It is based on three spectrometers and allows for continuous measurements with a repetition rate of about 1 Hz. The data products are the radiances of the target, the fluorescence flux at 687 and 760 nm, and several vegetation indexes, including the photochemical reflectance index and the normalized difference vegetation index. A new algorithm for fluorescence retrieval from spectral bands measurement is described. It improves upon the well-known Fraunhofer line discriminator method applied to passive fluorescence measurement by taking into account the spectral shape of fluorescence and the reflectance of vegetation. A measurement campaign of 38 days has been carried out in summer 2008 over a sorghum field. The evolution of the signals showed that the crop was suffering from stress due to lack of water. After several rainy days, a reversion of the water stress was observed

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The soil line, a linear relationship between bare soil reflectance observed in two different wavebands. is widely used for interpretation of remotely sensed data. The basis on soil line was analyzed using a radiative transfer model in which reflectance was splitted into its single and multiple scattering components. The slope of the soil line corresponded to the ratio of the single scattering albedos corresponding to the two wavebands where the soil line was defined. The intercept originated from the difference in multiple scattering observed in each of the two wavelength bands used. The soil line concept was very robust over the whole optical domain as soon as soil types are separated, and when the effect of the view and source configurations as well as the surface roughness were considered. However, in the middle infrared spectral domain, the soil line concept failed when soil moisture was a factor of variation.