Spatial distribution of photon flux density created by LED grow lights

LED lighting created new opportunities and also brought significant complexity in horticultural lighting. Requirements of horticultural lighting differ significantly from human-based lighting applications in terms of spectral and spatial distribution of light. LED luminaires can be placed much closer to plants compared to HID light sources therefore in a typical LED grow light application the luminaire cannot be regarded as a point like source. As a consequence, far field goniometry measurements do not provide proper information to calculate spatial distribution of Photosynthetic Photon Flux Density (PPFD) for vertical farms because the inverse square law is no longer applicable in case of LED luminaires having size comparable to the distance between the luminaire and the illuminated surface. We discuss the dependence of PPFD uniformity as a function of geometrical parameters and estimate the efficiency of LED lighting from the crop growth point of view.