This article describes a theoretical model for computing light penetration into a double-row crop canopy The inputs of the model are diffuse and beam irradiance above the canopy, position of the sun, and a description of the crop canopy in termsof plant stand dimensions and foliage characteristics. The outputs of the model are beam, diffuse and scattered irradiance at any location within a plant stand. The first step in the model is to compute attenuation of beam irradiance traveling through the canopy. Next, the model computes transmission of diffuse irradiance from the space above the canopy into the canopy. Lastly, downward scattered irradiance is computed from direct and diffuse irradiance in the canopy and average leaf transmittance. Themodel was validated using irradiance data collected in a cucumber crop grown during the fall of 1996 in a glass greenhouse located at Harrow, Ontario. Solar irradiance levels were monitored outside the greenhouse, inside the greenhouse above the canopy,and at three heights within the canopy Comparison between measured and predicted values showed that this model can be used to predict the average hourly light levels at any position within a double-row greenhouse crop with mean bias differences less than10. By substituting functions that describe different plant stand dimensions and foliage characteristics, this model can easily be applied to different crops. This model can be combined with plant physiology models for estimating canopy photosynthesisand transpiration of greenhouse crops, and with greenhouse climate models for estimating in-canopy and leaf surface microclimate in greenhouses.
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