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 safer to operate than current lamps because they do not have glass envelopes or high touch temperatures, and they do not contain mercury.



The spectral output of an LED lighting system can be matched to plant photoreceptors and optimized to provide maximum production without wasting energy on nonproductive wavelengths (Dougher and Bugbee, 2001; Sager et al., 1982). The ability to dynamically control the spectral output can also be used to influence plant morphology (Heo et al., 2002). Spectra can be customized for specific crops or production protocols and the output even modified over the course of a photoperiod or growth cycle. Special lighting modes might possibly even be used to enhance disease or injury visualization (Schuerger and Richards, 2006).


LEDs turn on instantly and do not require warmup time. They also turn off instantly. Because they are solid-state devices, 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. When used in arrays, LEDs can be configured for control by zone, ensuring only areas containing plants are illuminated. More advanced capabilities include the ability to integrate sensors that can dynamically control light level based on inputs such as available solar lighting or the proximity of plants.



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 either.


Taking into account lamp ballasts and drivers, preliminary calculations indicate that the LED array provides three times more light output for the same watt of input power on an equivalent area basis. 



each decade, LED prices have fallen by a factor of 10 while performance has grown by a factor of 20. This phenomenon is known as Haitz' Law. 

conomy of scale will drive significant cost decreases.



Devices in the ultraviolet wavebands, however, still have low output. Ultraviolet A wavelengths (320 to 400 nm) are of particular interest because they can play a role in preventing a physiological disorder termed intumescence injury (Morrow and Tibbitts, 1988) that impacts a variety of plants grown in protected environments. Also, ultraviolet B radiation (290 to 320 nm) may be useful in manipulating phenolic phytonutrients to improve dietary value in greenhouse or controlled environment-grown crops like lettuce



Over time, as new LED chip technologies became available, LED modules using very high density chip-on-board technology were developed. One such module (Fig. 1), often referred to as a “light engine,” was 1 inch2 (6.5 cm2) in area and contained 132 LEDs in five colors (Emmerich et al., 2004). This technology is too expensive for large-scale use, but is ideal for specialty or research applications that require high light output at several independently controllable spectral bands. pic: