LEX will be the first mission designed to observe fluorescence by using a novel technique measuring the main part of the chlorophyll fluorescence spectrum that originates from the core of the photosynthetic machinery.  This offers the opportunity to derive the photosynthetic activity of the terrestrial vegetation layer.  Therefore, FLEX observations will provide a new possibility to assess the dynamics of actual photosynthesis through sun-induced fluorescence.  The enhanced information content from the spectrum originating from the two different photosystemsoffers a great advancement over current capabilities that can only detect potential photosynthesis as derived in passive reflectance measurements by conventional land surface monitoring satellites.