The fluorescence of chlorophyll in vegetation is a weak signal emitted between 650 and 850 nm that is mixed with the much more intense light reflected by the leaf, which is why active methods are commonly used (through the additional contribution of controlled artificial light) or using indirect measurements instead. So, the measurement is provided just in relative units in the first case, or the accuracy of the estimate in the second case is uncertain without proper direct validation. The Thesis presents a new device, called FluoWat, for passive measurement that allows direct measurement of the fluorescence emission of leaves in vivo under natural conditions in the field with sunlight. And it is part of the activities supporting the preparation of ESA’s FLEX mission for the global monitoring of vegetation fluorescence. The device consists of a small dark chamber implemented as a clip, so that the leaf can be housed inside without damaging it, with an opening to illuminate the sample by pointing at the sun, and a sliding filter holder with a low-pass filter that blocks sunlight in the same spectral range as fluorescence is emitted while allowing the excitation light to pass through, then a spectroradiometer connected to the clip measures the fluorescence spectrum without interference from sunlight. In addition, it is possible to measure the reflectance and transmittance factors of the leaf, which allows determining the absorptance, necessary to determine the photosynthetically active radiation (PAR) that has been absorbed (APAR). An essential parameter to properly interpret the fluorescence signal in relation to photosynthesis. Similarly, the reflectance and transmittance spectra in the visible range make it possible to determine the degree of photoprotection of the leaf and/or its chlorophyll content. A sensitivity analysis of different factors likely to affect the measurement has been carried out, such as the residual light that passes through the filter, or the effect of transients on fluorescence emission, among others. Processing methods have been developed to mitigate their effects on the fluorescence measurement, increasing the accuracy of the results. Finally, a series of experiments are presented in which the system is put to the test and that illustrate how, with the measurements provided by this new device, a better understanding of the dynamics of fluorescence emission while the vegetation adapts to different illumination changes, levels of stress and changing environmental conditions.