Quantification of photosynthetically active radiation inside sunlit growth chambers

Outdoor growth chambers allow plants to be grown under sunlight while other environmental variables such as air temperature and CO 2 concentration are controlled. Photosynthetically active radiation (PAR) inside these sunlit chambers could differ from ambient levels due to attenuations and reflections by the glazing materials. This study identified diurnal patterns and distribution of PAR inside various types of sunlit growth chambers, including soil-plant-atmosphere-research (SPAR) units using single-point and line quantum sensors. In comparison with the ambient levels, higher PAR was measured when reflections from the adjacent walls overlapped at specific locations in the SPAR unit. Within the crop growing area located in the northern part of a SPAR unit, daily integrals of PAR were between 93% and 105% of the ambient PAR on a clear day. A gradient in daily PAR existed, increasing from south to north inside the chamber. On a cloudy day, PAR within the crop growing area was between 92% and 95% of ambient PAR. A mathematical model was developed to predict incident PAR inside sunlit chambers. This model accounted for solar geometry, direct and diffuse radiations, chamber geometry, and the optical properties of the glazing material. The model was capable of simulating the diurnal patterns of PAR inside various types of sunlit chambers. The model predicted that the mean daily PAR inside sunlit chambers of acrylic sheets would usually be within 5% of the ambient PAR. Spatial distribution of the predicted daily PAR ranged from 75% to 120% of ambient levels inside various types of sunlit chambers including SPAR and open-top chambers.