Greenhouse Lighting

Environmental factors such as light and radiation, temperature, relative humidity, carbon dioxide (CO₂), water, nutrition, insect, and disease stresses are the key environmental components affecting optimum plant growth and yield. And greenhouse growers have been successfully using a number of mechanisms for modulating each and every one of these components for better plant growth and yield including artificial lighting. Light energy is required directly to sustain the growth of all green plants in the first place while light intensity (quantity), light quality (wavelength), and light duration (photoperiod) all influence plant development, and plants respond primarily to the intensity and duration of light within photosynthetically active radiation (PAR) range, the visible portion of the spectrum. Light has very strong interactions on photosynthesis as well particularly with ambient temperature and CO₂ concentrations. And the greenhouse location, orientation, and glazing determine the amount of free natural light that is available for plant growth and development in a greenhouse. The best greenhouse location therefore is the location where the free solar energy enters the greenhouse all day long with no shadows cast on the greenhouse. When the intensity and/or duration of natural light is not sufficient under normal operating conditions, then artificial lighting might be considered for increasing the intensity of light and extending the duration of light.

This chapter covers essential light definitions, units, and conversions in lighting systems first, and then moves to the lighting strategies and different light sources used in greenhouses and other controlled environments. Outdoor annual average daily light integral (DLI) maps for different parts of the world are provided. Examples are provided for radiometric to quantum and quantum to photometric units conversions and light comparisons; photometric light from high-pressure sodium (HPS) and metal halide (MH) lamps into quantum conversion; quantum to radiometric conversion; calculating the DLI deficiency for supplemental lighting; calculating how long to operate supplemental lighting lamps for DLI deficiency; calculating how much of total DLI is coming from supplemental lighting lamps as well as calculating the DLI from footcandle or lux measurements. Photoperiodic lighting, photosynthetic (supplemental) lighting, and sole-source lighting are covered in depth. Different lighting sources, including high intensity discharge (HID) lamps, light-emitting diodes (LEDs), and incandescent (INC) lamps and compact fluorescent lamps (CFLs) are introduced as well; and finally, the chapter covers the future perspectives of greenhouse lighting.