2017 ASHS Annual Conference

Inconsistent crop production can result from the high degree of variability in the natural lighting environment that greenhouse-grown crops are exposed to. This is due to spatial and temporal variability; and the interaction of these factors. Long-term temporal variability refers to seasonal changes in natural sunlight, which in turn depends on geographic location (affecting the angle of the sun and daylength) and seasonal weather patterns. For example, inside a greenhouse, in Leamington, Ontario, CA, (40.1ºN; 83.6 ºW), the mean DLI can vary from 2.5 to 25 mol m^-2 d^-1, with an average daily PPFD of approximately 70 to 500 mmol m^-2 s^-1, between winter and summer. There is also short-term variability in light intensity throughout the day which depends on cloud cover and the angle of the sun. These factors interact with the crops’ position within the greenhouse relative to structural components and light transmission properties of the glazing. The goals of supplemental lighting are effectively to reduce the spatial and temporal variation in the lighting environment of the crop production areas, and increase crop productivity, and uniformity. Supplemental lighting in greenhouses has traditionally been achieved by lighting directly over the crop, with minimal assurance of lighting uniformity.

Using light modeling software to consider different lighting scenarios we determined that when fixtures are hung in a square grid, an area of uniform light distribution is created at a distance below the light consistent with the grid size. For example, if a one-acre greenhouse is illuminated with 400 fixtures on a 10x10 foot grid, uniformity of light is achieved at a distance 10 feet below the fixtures for 90% of the area, (uniformity defined as within 20% of the target PPFD.) The edge where the uniformity requirement was not met, can be successfully mitigated by varying the hang height of the fixtures associated with the edge. This analysis gives similar results for both LED and HPS luminaires.

We will present empirical data to demonstrate the variability in natural lighting found in greenhouse environments, modeling data on how (fixed-intensity) supplemental light plans can be modified to improve the lighting environment, and introduce a strategy for optimizing light uniformity using dynamic light sensors that measure ambient light levels and adjusts supplemental (LED) lighting, when and where necessary, to create uniform lighting throughout the growing environment.