This practical paper describes a novel fluorescence imaging experiment to study the three processes of photochemistry, fluorescence and thermal energy dissipation, which compete during the dissipation of excitation energy in photosynthesis. The technique represents a non-invasive tool for revealing and understanding the spatial heterogeneity in leaf performance caused by internal and external factors, such as development and abiotic stress. Fluorescence yield (F), effective PSII quantum yield (Y(II)) and non-photochemical quenching (NPQ) were analysed by fluorescence imaging in leaves of "Avena sativa" L. and two species of "Brassica" ("B. fruticulosa" and "B. oleracea") plants during the first stages of plant development and under high illumination. In young leaves, F was 0.1 or less, Y(II) between 0.7 and 0.8 and NPQ less than 0.3, indicating that photochemical efficiency was high (70-80%) and that fluorescence emission (less than 10%) and heat dissipation (less than 30%) were low. As leaf age increased, differences were observed between species, indicating different development and senescence patterns in each. "Avena sativa" and "B. fruticulosa" (a wild species adapted to high light intensity in its natural habitat) showed greater tolerance to high illumination than "B. oleracea" (an agricultural species), which experienced photoinhibition caused by excessive light. This practical paper is intended for advanced students of plant biochemistry and plant physiology. (Contains 2 figures and 1 table.)