In recent years, LED lighting for horticulture applications, in particular plant-growth research and production in the controlled environment, has dramatically increased. The benefits of using LEDs for energy savings and performance improvement have been quickly recognized by researchers, plant growers, and greenhouse or controlled-environmental-chamber manufacturers, as well as government and energy savings experts. LED horticultural lighting is a fast-growing market, yet there are uncertainties as to how LED lighting products should be measured, compared, and qualified in regard to energy savings, performance, and safety.
Both energy efficiency and improved plant production are behind the uptake in solid-state lighting (SSL) usage in horticultural applications. A recent LEDs Magazine feature article covered some of the latest developments in applications and horticulture-centric products.
Lighting and plant research
Research into lighting and horticulture is not a new endeavor, although LEDs are changing the needs of the application community. For over fifty years in the field of photobiology, plant-growth experts and practitioners have studied and recognized the radiation, or light, effects for plant ecophysiology.
Plant reactions to light in relationship to the wavelength may be categorized within a range as the photosynthetically active radiation region, often abbreviated as PAR. PAR is defined as the radiation wavelength range from 400 to 700 nm but does not distinguish between different wavelengths within 400 and 700 nm. PAR assumes that wavelengths outside this range have no photosynthetic reaction.
Low-profile crops can be cultivated in multiple tiers with LED horticultural lighting enabling maximum production in commercial facilities (Courtesy of Urban Harvest; urbanharvest.com).
Studies show that photons around 660 nm (orange-red) have the highest amount of photosynthesis per photon. However, because short-wavelength photons carry more energy per photon, the weighting factor for photosynthesis may be used either with the photon-weighted curve or with the energy-weighted curve. Beyond the PAR region, experts also believe plants have other reactions to light as well as photosynthetic reactions. For longer than 700-nm wavelength, photomorphogenesis takes place as the light dependent change in morphology, which can include biochemistry. Plants may also be affected by radiation with shorter than 400-nm wavelengths. In all cases, the plant-growth community recognizes the differences between how human eyes see the light, or photometric effect. This community also observes how the plants react to the radiations but has not yet established agreed-upon standards, in particular the metrics for measurements.
During the 2014 American Society of Horticultural Science (ASHS) annual conference, in conjunction with the Symposium of LED Lighting for Horticultural Applications, an LEDs horticultural-lighting standardization roundtable discussion was conducted. Participation was strong with over 70 participants from university professors and researchers, many of them well-known plant growers, to LED and lighting manufacturers, test labs representatives, as well as lighting experts. The goal was to get a consensus from the plant-growth community regarding the need to standardize LED horticultural lighting.
The roundtable participants discussed three key topics. First, they addressed how to establish universal metrics for plant responses for radiations. Since such metrics don't exist, in many cases LED lighting fixtures used for horticulture applications are "qualified" for energy savings per human eye response, or luminous efficacy. However, in reality, luminous effective illumination for general lighting doesn't have much to do with how light effectively or efficiently affects plants. Therefore, correct and acceptable metrics must be established. Second, they discussed establishing standard methods for measuring and testing LEDs and LED light fixtures used for plant growth. The third topic was how to establish proper energy saving and performance measures for LED lighting in horticulture applications so that more correct specifications can be developed.
Lettuce and bok choy thrive after two weeks of growth (Courtesy of Urban Harvest; urbanharvest.com).
The roundtable ended with the organizers proposing a scope of work, evaluated by the American Society of Agricultural and Biological Engineers. ASABE is an ANSI (American National Standards Institute)-accredited standardization body and its Agricultural Lighting Committee published a standard titled "Lighting systems for agricultural," where the section "Greenhouse lighting system design" provides recommendations on three main design objectives for greenhouse lighting systems:
1) Instantaneous light level (sometimes converted to the daily light integral)
3) Uniformity (important for both instantaneous and photoperiod lighting)
Committee formation and plan
ASABE recognized the plant growth community's need for a standardization committee for developing LED horticultural lighting standard documents and formed the new Plant Growth LED Lighting Committee in early 2015. According to ANSI accreditation guidelines, this new committee consists of experts representing producers including LED and lighting fixture manufacturers as well as greenhouse or controlled-environment-chamber manufacturers; users such as plant growers, researchers from universities and other research institutions; and general interest parties including independent test libraries, government agencies, and specifiers.
The ASABE Plant Growth LED Lighting Committee established the overall work plan for developing new standard documents that meet the needs of the plant-growth community. Its overall objective is to reach consensus on identifying the proper metrics or measurables. The committee consists of three working groups to conduct the work in parallel.
The first working group is tasked with defining the metrics of radiation for plant-growth applications in the controlled environment. Currently, the metrics can be divided into two regions - within PAR and beyond PAR. The focus is to determine if a weighting factor is needed for plant responses when measuring the radiant flux or photon flux and, if so, what should the weighting factor be. The group will also explore if and how the photon-weighted curve or the energy-weighted curve should be used when representing the measured flux.
The second working group is tasked with establishing the methods of measurement. Many measurement and testing standard documents have been published by IESNA (Illuminating Engineering Society North America), or other standardization bodies, but the uniqueness of plant growth applications must be integrated. For instance, when measuring LEDs per IES LM-85 or measuring LED fixtures per IES LM-79, what parameters should be measured and reported? The working group also needs to provide guidelines for how to test longevity of LED products.
Unlike many general lighting applications, plant growth products are often operating continuously and the operational environment can have very high humidity and may be subject to chemical, biological, or other types of corrosions. If an LED lighting product is claimed to have a long-life and stable radiation output, the radiant flux maintenance test for LEDs per IES LM-80 or for light fixtures per IES LM-84 may need to be revised by adding the horticulture-application-related environmental conditions. This working group is also considering the suggestion from the plant-growth community that irradiance, or radiant flux density, on plant surfaces may be a performance measure. Such measurement should be well defined in terms of procedures, testing conditions, and recommended equipment.
The third working group will help link the society's standards and the specifications needed by government agencies, consortiums, and other program experts. With metrics defined by the first working group and methods of measurements defined by the second working group, this third working group will provide the recommended energy-saving measures. Instead of luminous efficacy used for general illumination, a new measure or definition of efficacy will be developed.
In addition to the energy-saving measures, the LED fixture is also subjected to performance requirements depending on applications. These performance requirements are a reflection of the needs from different plant-growth applications. In horticulture applications, plant species react differently to radiation. An energy-saving product used for growing one plant may not be applicable for growing another plant. Or, if an energy-saving fixture is used improperly, the plants may not get the necessary benefits, such as radiant flux density or irradiance. This working group will propose how energy-saving and performance measures can be integrated into a standardized practice.
The plant-growth community and the experts engaged in the ASABE Plant Growth LED Lighting Committee face many challenges. Not only do experts from each side have strong beliefs on what is the correct way to do things, they also face a rapidly growing industry that needs universal standards in a timely fashion.
DR. JIANZHONG JIAO, director of regulations and emerging technologies at OSRAM Opto Semiconductors, Inc., is an internationally recognized lighting expert. He has been actively involved in LED and SSL standard development activities. He serves as the past chairman of the SAE Lighting Committee, past chairman of NGLIA, past chairman of the NEMA SSL Technical Committee, active member of IESNA Testing Procedure Committee, Roadway Lighting Committee, and Computer Committee, ANSI SSL Working Groups, Standard Technical Panel of UL8750, standard committees in IEEE, CIE USA, SEMI, JEDEC and other organizations. He can be reached at email@example.com.