Photosynthetically active solar radiation can be either direct or diffuse. Due to atmospheric scattering, solar radiation is never fully direct. Under heavy overcast conditions however, it can be fully diffuse. Screens and glass that transform direct light into diffuse light are used under the assumption that diffuse light is more evenly distributed over the canopy, increasing crop photosynthesis rate. The Intkam crop growth model computes leaf photosynthesis rate in 5 leaf layers, for the sunlit and shaded leaf area and for the leaf areas receiving direct and diffuse light. It integrates instantaneous leaf photosynthesis rates to the crop photosynthesis rate. Instantaneous canopy photosynthesis is used to compute the seasonal growth of organs. This process approach enables a detailed analysis of the effects of variations in natural light. An analytical comparison was made between 100% direct and 100% diffuse light for a representative day in winter (day 24) and in summer (day 202). Sunlit leaf area is illuminated by both direct and diffuse light, whereas a shaded leaf area is illuminated by diffuse light only. These components vary within and among leaf layers, and were all quantified. On both days, a higher instantaneous crop photosynthesis was computed under fully diffuse light than under fully direct light. This difference is caused by the more homogeneous distribution of diffuse light than direct light at a certain canopy depth, in combination with a declining response to increasing light intensities of the photosynthesis rate. Experiments with three types of diffuse glass and a whitewash were conducted in 2011. Light scattering of the glass (haze) varied from 45-71%, with at least the same transmission as the reference. Tomato production under diffuse glass was increased by 8-11% in early June, and was maintained to November. The Intkam model simulated approximately the same relative seasonal production increases under diffuse glass.
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