LED’s represent an entirely different technology from the gaseous discharge-type lamps of old used in horticulture. Capabilities like spectral composition control and high light output with little radiant heat make this technology potentially one of the most significant advances in horticultural lighting since the development of high-intensity discharge (HID) lamps.
Lighting based on the use of light-emitting diodes (LEDs) is the biggest advancement in horticultural lighting in decades. LEDs can play many roles in horticultural lighting, including use in controlled environment research, tissue culture, and and photoperiod lighting for greenhouses. LED lighting systems have several unique advantages over traditional horticultural lighting, including the ability to control spectral composition, the ability to produce very high light levels with low radiant heat output when cooled properly, and the ability to maintain useful light output for years without replacement.
LEDs are the first light source to have the capability of true spectral composition control, allowing wavelengths to be matched to plant photoreceptors to provide more optimal production and to influence plant morphology and composition. LEDs are easily integrated into digital control systems, facilitating “daily light integral” lighting and sunrise and sunset simulations. LEDs are safer to operate than current lamps because they do not have glass envelopes or high touch temperatures, nor do they contain mercury.
The first sustained work with LEDs progressed from simple red-only LED arrays using the limited components available at the time to high-density, multicolor LED chip-on-board devices. As light output increases while device costs decrease, LEDs continue to move toward becoming economically viable for even large-scale horticultural lighting applications.
The first work with LEDs for plant lighting used red LED arrays made of individually lensed devices. At that time, only the red devices (≈660 nm) had light output adequate for plant growth. Arrays of these devices were not feasible for large-scale use as a result of cost, uneven performance of individual devices, and fabrication difficulties. With the advent in the 1999 to 2000 timeframe of high-output LEDs that lent themselves to automated manufacturing, the fabrication of solid-state lighting arrays greater than several square meters in area has become more economically feasible.
ADVANTAGES OF LED’S IN HORTICULTURE
LED-based lighting systems have several advantages over lamps currently used in horticulture. One is the ability to control the spectral output of the lighting system, something not easily done with broad-spectrum sources. The spectral output of an LED lighting system can be matched and optimized to provide maximum production without wasting energy. Spectra can be customized for specific crops or production protocols and the output even modified over the course of a growth cycle. Special lighting modes might possibly even be used to enhance disease or injury visualization (Schuerger and Richards, 2006).
- LED lighting systems can produce very high light levels (even in excess of full sunlight if desired), but even at high light outputs they can be operated in close to plants because they have very little radiant heat output when cooled properly.
- LEDs also have a very long operating life; current LED technologies are rated as maintaining 70% of their original luminous output after 50,000 h, and this is probably a conservative number as long as the devices are cooled adequately.
- LEDs turn on instantly and do not require warmup time. They also turn off instantly. LEDs are easily integrated into digital control systems. This allows complex control options not generally available with other light sources. LEDs can be continuously dimmed between zero and maximum, and custom spectra (Fujiwara and Sawada, 2006) and custom programs such as sunrise and sunset simulations can be programmed.
- LEDs have the potential for significant cost savings over current horticultural lamp types. Because of their long operational life, procurement and disposal costs for replacement bulbs is mostly eliminated along with associated labor costs. There are no ballasts to be replaced. The unique capabilities of LEDs can also significantly reduce power use over existing lamp types through the ability to operate in close proximity to plant tissue, the ability to optimize the light spectrum for productivity, and advanced control capabilities that allow optimization of lighting patterns.
- LEDs do not contain mercury that needs to be disposed of and do not have glass envelopes or high surface temperatures that can cause injury. They also do not produce damaging ultraviolet wavelengths (unless added for specific purposes) as HID lamps do if the envelope breaks. Because LEDs can be operated as a focused lighting source adjacent to the plant material, they should produce much less wasted light that can contribute to light pollution, obtrusive light that obscures the night sky and interferes with astronomical observatories (Narisada and Schreuder, 2004).
Advantages of solid-state lighting over gaseous discharge lamp technology include the ability to provide high light intensities with low radiant heat, adjustable spectral quality that allows optimization to improve photosynthetic efficiency and plant form and function, good safety characteristics, and operating capabilities that can significantly reduce power use. Improvements in LED “chemistry,” mounting and packaging, electrically efficient device drivers, heat sinking, and optics (lenses and reflectors) will all contribute to advances in LED lighting systems. The optimistic outlook for solid-state lighting technology along with the advantages it provides over existing lamp types makes LEDs a prime candidate for use in protected agriculture. (Source: http://hortsci.ashspublications.org/)
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