LEDs offer spectral output control for horticulture applications

Article By : Yoelit Hiebert

Research shows that crops respond in different ways to exposure to different parts of the visible spectrum. LEDs conveniently offer the ability to select spectral output.

It’s no secret that LED lighting has revolutionized indoor grow facility operations, mainly due to the flexibility LEDs afford in spectral output and their drastically lower heat output. The American Society of Agricultural and Biological Engineers (ASABE) has recently released ANSI/ASABE S642 SEP2018, “Recommended Methods For Measurement And Testing Of LED Products For Plant Growth And Development,” the second in a series of three standards focusing on specifying the performance of LED lighting products for horticultural applications.

The new standard defines test methods for LED products emitting optical radiation in the range of 280–800 nm (vs. lumens in the visible spectrum of about 390 to 700 nm).

“LED lighting for use in horticultural applications has generated one of the highest levels of interest of any of our projects in the past two decades,” according to Scott Cedarquist, ASABE’s Standards and Technical Director.

Unsurprisingly, horticultural lighting uses plant-focused terminology; here are a few of the more widely used terms:

Photosynthetically active radiation (PAR) – the spectral range of solar radiation plants use in the process of photosynthesis. Traditionally considered to be the range of 400 nm to 700 nm, but recent studies demonstrate that wavelengths up to 740 nm can impact plant growth and development.

Photosynthetic photon flux (PPF) – the number of photosynthetically active photons created by a lighting system per second, analogous to luminous flux (lumens) and measured in micromoles/second (µmol/s).

Photosynthetic photon flux density (PPFD) – the number of photosynthetically active photons striking a surface per unit area per unit time, analogous to illuminance (lux) and measured in µmol/ m²s.

The focus of horticultural lighting is delivered photons, as these are what initiate photosynthesis and other processes in plant development through the excitation of electrons. LED products used in horticultural applications differ from those used for general illumination in a couple of important ways, the first of which is the wider spectral output typical for these products. This is useful because different plants respond to different parts of the spectrum.

Industry and academic research has demonstrated that for each type of plant, there is a specific “light recipe” to yield more of whatever is being grown in a shorter amount of time. This recipe defines the variation in light spectra to optimize each phase of the plant’s life cycle and to improve desirable plant characteristics, for example, to increase the flavor of vegetables or the potency of cannabis.

LED lighting products also have the unique capability of not only being able to provide a precise output spectrum, but also to be “tuned” to the optimal spectrum for different plant life cycle phases.

To address the elevated temperatures of some grow environments, active cooling, although by no means universal, is the other significant characteristic of LED horticultural lighting products. Active cooling is mainly accomplished by means of a fan, but water-cooling systems are also in use. Water resistant housings are also a consideration, especially in facilities where the LEDs are mounted in close proximity to the plants, e g., vertical farms.

Speaking of vertical farms, LED lighting products are being adopted due to their much lower heat output as compared to traditional lighting. LED lights can be interspersed in close proximity to the plants without causing damage, which allows grow facility managers to make maximum use of available space. In additional to large scale operations like the one shown below, vertical farming has also become increasingly popular in urban areas where abandoned buildings are being converted to grow food, making fresh produce available at a lower cost.

A vertical farm project in Millis, MA using LED lighting Source: Crop One Holdings

Note that thus far not one word has been said about the main reason LED lighting has been so widely adopted for general illumination, to wit, energy savings. And that’s because, for grow facility managers, the name of the game is yield. For high value crops like cannabis, which unsurprisingly, was the first industry to adopt LED lighting, the cost of energy is trivial in comparison with the revenue increase resulting from higher yield and shorter life cycle. And even for more traditional crops like leafy vegetables and flowers, energy cost reduction is not as compelling an argument as the ability to produce more crops in a shorter amount of time.

LED lighting adoption for horticultural applications is growing by leaps and bounds (no pun intended) and is sparking a major shift in how our food will be grown in the future. So the next time you’re at the grocery store buying blackberries in the depth of winter, consider that your treat may well have been grown indoors using LED lighting.

Yoelit Hiebert has worked in the field of LED lighting for the past 10 years and has experience in both the manufacturing and end-user sides of the industry.

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