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Light-emitting diodes (LEDs) are semiconductor devices that produce noncoherent, narrow-spectrum light when forward voltage is applied. LEDs range in wavelength from the UVC band to infrared (IR) and are available in packages ranging from milliwatts to more than 10 W. The first LED was an IR-emitting device and was patented in 1961. In 1962, the first practical visible spectrum LED was developed. The first high-power (1-W) LEDs were developed in the late 1990s. LEDs create light through a semiconductor process rather than with a superheated element, ionized gas, or an arc discharge as in traditional light sources. The wavelength of the light emitted is determined by the materials used to form the semiconductor junction. LEDs produce more light per electrical watt than incandescent lamps with the latest devices rivaling fluorescent tubes in energy efficiency. They are solid-state devices, which are much more robust than any glass-envelope lamp and contain no hazardous materials like fluorescent lamps. LEDs also have a much longer lifetime than incandescent, fluorescent, and high-density discharge lamps (U.S. Dept. of Energy). Although LEDs possess many advantages over traditional light sources, a total system approach must be considered when designing an LED-based lighting system. LEDs do not radiate heat directly, but do produce heat that must be removed to ensure maximum performance and lifetime. LEDs require a constant-current DC power source rather than a standard AC line voltage. Finally, because LEDs are directional light sources, external optics may be necessary to produce the desired light distribution. A properly designed LED light system is capable of providing performance and a lifetime well beyond any traditional lighting source.

Electrical cost, primarily for lighting, is one of the largest factors inhibiting the development of “warehouse-based” controlled environment agriculture (CEA). In a jointly sponsored collaboration, we have developed a reconfigurable LED lighting array aimed at reducing the electrical energy needed to grow crops in controlled environments. The lighting system uses LED “engines” that can operate at variable power and that emit radiation only at wavelengths with high photosynthetic activity. These light engines are mounted on supports that can be arranged either as individual intracanopy “lightsicles” or in an overhead plane of lights. Heat is removed from the light engines using air flow through the hollow LED strip mounts, allowing the strips to be placed in close proximity to leaves. Different lighting configurations depend on the growth habit of the crops of interest, with intracanopy lighting designed for planophile crops that close their canopy, and close overhead lighting intended for erectophile and rosette crops. Tests have been performed with cowpea, a planophile dry bean crop, growing with intracanopy LED lighting compared to overhead LED lighting. When crops are grown using intracanopy lighting, more biomass is produced, and a higher index of biomass per kW-h is obtained than when overhead LEDs are used. In addition, the oldest leaves on intracanopy-grown plants are retained throughout stand development, while plants lit from overhead drop inner-canopy leaves due to mutual shading after the leaf canopy closes. Research is underway to increase the energy efficiency and automation of this lighting system. This work was supported in part by NASA: NAG5-12686.

Electric supplemental lighting can account for a significant proportion of total greenhouse energy costs. Thus, the objectives of this study were to compare high-wire tomato (Solanum lycopersicum) production with and without supplemental lighting and to evaluate two different lighting positions + light sources [traditional high-pressure sodium (HPS) overhead lighting (OHL) lamps vs. light-emitting diode (LED) intracanopy lighting (ICL) towers] on several production and energy-consumption parameters for two commercial tomato cultivars. Results indicated that regardless of the lighting position + source, supplemental lighting induced early fruit production and increased node number, fruit number (FN), and total fruit fresh weight (FW) for both cultivars compared with unsupplemented controls for a winter-to-summer production period. Furthermore, no productivity differences were measured between the two supplemental lighting treatments. The energy-consumption metrics indicated that the electrical conversion efficiency for light-emitting intracanopy lighting (LED-ICL) into fruit biomass was 75% higher than that for HPS-OHL. Thus, the lighting cost per average fruit grown under the HPS-OHL lamps was 403% more than that of using LED-ICL towers. Although no increase in yield was measured using LED-ICL, significant energy savings for lighting occurred without compromising fruit yield.