Composite lighting for controlled-environment plant factories

One of the consequences of employing artificial lighting to supplement solar irradiance either in a greenhouse [1–3] or in a controlled-environment plant growth chamber, wherein solar irradiance is transmitted through optical cables from solar concentrating systems [4–6], is the subjection of the growing crops to lighting profiles that differ from the conventional lighting profile. The daily lighting profile of a conventional electric-based plant-lighting system can generally be represented by a rectangular wave (Figure 1A) whose height represents the magnitude of the instantaneous photosynthetic photon flux (PPF, in μmol m^{– 2} sec^{– 1}), whose length represents the daily photoperiod (P, in hr), and whose area represents the daily integrated PPF (Q, in mol m^{– 2} day^{– 1}). For a hybrid solar and artificial lighting system, the daily lighting profile that results is a composite lighting profile (Figure 1B), typically consisting of an approximately bell-shaped curve, representing the solar component, that is superimposed over a rectangular wave, representing the artificial lighting component. The total of the area under the solar curve and the area of the rectangular wave represents the daily integrated PPF.