A new instrument for passive remote sensing: 1. Measurements of sunlight-induced chlorophyll fluorescence

Under natural sunlight illumination, the chlorophyll fluorescence emitted by the vegetation represents less than 3% of the reflected light in the near infrared part of the spectrum. This small amount is difficult to quantify except at certain wavelengths, where the solar spectrum is attenuated (Fraunhofer lines). An instrument measuring the in-filling of the atmospheric oxygen absorption band at 760 nm by chlorophyll fluorescence has been designed and constructed at the "Laboratoire pour l'Utilisation du Rayonnement Electromagnetique" in Orsay, France. The system was calibrated against a pulsed fluorimeter (FIPAM), especially developed for monitoring chlorophyll fluorescence at distance. The penetration of diuron, a herbicide acting on photosynthesis, was monitored by the passive instrument for several days on a corn canopy. A good agreement was found between gas exchange and variable chlorophyll fluorescence at the canopy level and variable fluorescence at the leaf level. The potential application of the passive chlorophyll fluorescence measurements for long range vegetation remote sensing is discussed.