By Suzanna Brooks
For those of us here in the U.S. and across the northern hemisphere, spring is on the way—growing season for farmers, gardeners, and enthusiasts. But with new technology they no longer have to rely on the sun to grow their favorite plants. The latest horticulture LEDs are providing new options to grow year round, even in places which couldn’t support regular farming—such as urban factories and multistory warehouses. 
Research has shown that particular wavelengths of light play different roles in how plants grow. Plants respond better to wavelengths specifically in the 400 to 500nm (blue) and 600 to 700nm (red) spectra. Growth is strongly influenced by the number of photons absorbed in the photosynthetically active radiation (PAR) region, which covers only 45 percent of the total light spectrum. Improvements in LED technology, including the ability to deliver specific wavelengths in individual LEDs, are paving the way for horticulture LEDs to radically change the way crops and other plants are grown. In fact, LED-based “plant factories” are already operating commercially in Japan!
The environmental and financial benefits of still-developing horticulture LED technologies are vast. With LED lighting, more plants can be grown faster, over a longer period of time, allowing for reduced use of chemical fertilizers. Certain lighting, such as UV-C light, and color balances can reduce plant diseases and pests, and therefor require fewer pesticides. When switching from more traditional lighting to LEDs for growing indoor crops, less heat is generated, meaning lower cooling and humidity-control costs. LEDs are highly energy efficient, requiring lower power and offering longer bulb life than other lights.
Scientists are currently studying how to cook up lighting “recipes” for specific horticultural uses. Some have found that plant responses to light filters also vary the growth results. For example, two special filters—UV-O and Luminance films—produce greater plant mass in herbs, depending on the species. It also appears that UV-O lessens the infestation of a type of aphids on iceberg lettuce. Different filters are clearly suited to different growth patterns, increasing the complexity of possible lighting recipes that electrical engineers can design.
In horticultural applications, using Lux or lumens as a measure of light output is deemed inappropriate, as they imply a heavily biased human perception of light. Photosynthetic photon flux density (PPFD) is a more appropriate measure for the light energy that reaches the surface of plants in a certain area over a given time. PPFD is measured in mol/m2/day or µmol/m2/s. The PPFD of sunlight reaching sea level on a clear day, is around 2000µmol/m2/s, which is more than most plants can absorb and can even cause damage to some. Philips Lumileds says bedding plants require 10 to 12mol/m2/day, while 4 to 6mol/m2/day is suggested for plants grown from cuttings.
The way that LED wavelengths, PPFD, and lighting filters are distributed and mixed is critical and each plant needs a different recipe. A U.S. technology firm called Nano Labs has released an LED-based “intelligent illumination system” for greenhouse horticulture. It adapts light output to suit the plant’s needs at different stages of development and uses pulsed lighting, instead of continuous lighting, to reduce energy use even further. Some different suppliers of LEDs for horticultural lighting include Lumex, OSRAM Opto Semiconductors, LedEngin, and Philips Lumileds.
Suzanna Brooks joined Mouser Electronics in 2011 as a Technical Content Specialist and writes web content about the newest embedded and optoelectronic products available. Suzanna holds a Bachelor of Science degree from Embry-Riddle Aeronautical University and is a private pilot.
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