LEDs for horticulture: not all the same

Traditional light technologies like high-pressure sodium, metal halide or fluorescent lights produce distinct light spectrum and wavelengths that are effective, but not necessarily optimized for plant growth. LEDs, on the other hand, have the flexibility to deliver specific wavelength combinations and lighting strategies that may yield faster and more favorable results for plant growers and researchers. However, this will depend on the type of LED used and the particular plant response they are seeking. The options for LEDs are numerous, and growers need to understand their objectives in order to choose the right manufacturer and fixture.

The use of artificial light to improve plant growth by providing longer photoperiods and higher daily light sums (DLI) has been in use for decades. Lights are designed to stimulate plant growth by emitting an electromagnetic spectrum that drives photosynthesis, which is the process plants use to convert light radiation into biomass.

What’s interesting to see is the advancement in lighting technologies used for horticulture in the past 15 years. The transition from T12 fluorescents to T8 and T5 lamps and the introduction of metal halide and high-pressure sodium light sources have provided indoor growers with new opportunities to improve plant growth significantly in controlled environments. The advent of LED technology now allows growers to isolate and mix wavelengths that are more effective in promoting consistent and healthy plant growth. LEDs can alter a plant’s strategy for energy use during photosynthesis by transmitting different information from the spectrum. Additional benefits of LEDs include longer lifespan, lower power consumption, significantly less radiant heat directed at the plants and less heat overall. In addition, LEDs produce consistent light across a wide range of temperatures, unlike fluorescent lamps that are very sensitive to the surrounding temperature and airflow. And lastly, compared to fluorescent lighting that contains mercury, the disposal of LEDs is friendlier to the environment.

Measuring Light Performance

There are multiple factors to consider when assessing LED manufacturers and their products. By evaluating light performance in terms of electrical efficiency, photosynthetic activity, and desired plant response, a grower can determine their optimum LED solution.

Electrical Efficiency

Traditionally, artificial light performance has been measured by how much radiation (µmol) the light source provides in the photosynthetically active radiation (PAR) area. In this way, efficiency is determined by how many µmol can be produced by each watt of energy input. Unfortunately, µmol/W does not reveal anything about the plant’s response to the light.

Many LEDs provide a pure red (660nm) spectrum (where all light is within the PAR region) and produce high electrical efficiency measured by µmol/W. In terms of plant growth, however, there are very few applications where a pure red light spectrum yields good plant growth results.

Photosynthetic Activity

Measuring the Relative Quantum Efficiency (RQE), which quantifies the relative photosynthetic reaction at each wavelength to differentiate a LED’s photosynthetic efficiency, is an alternative to measuring the radiation in the PAR area. Measuring photosynthesis, however, may give unreliable indications of spectrum performance due to tests being performed over a relatively short time period (usually only lasting minutes). Regardless, increased photosynthesis does not proportionately increase relative growth rate since enhanced carbohydrate availability may exceed the plant’s ability to utilize it fully.

Plant Response

Some LED lights are customizable to suit the objectives of the grower. The type of plant can strongly determine the choice of LED due to its reaction to the photoperiod and spectrum. For a lettuce grower, a plant needs to have biomass, whereas a rose grower requires a plant to grow quickly with a big flower and thick stem. To the lettuce grower, good taste and shelf-life are valuable traits and flowering is delayed or even inhibited. For this reason, the blue spectrum is essential during the vegetative phase of growth to promote leaf development with few flowers. For the rose grower, red spectrum light will trigger a greater flowering response initiating tall and narrow plant growth. Evaluating light performance in terms of electrical efficiency, photosynthetic activity, and desired plant response can determine the optimum LED solution. For a lettuce grower, a plant needs to have biomass, whereas a rose grower requires a plant to grow quickly with a big flower and thick stem.

Plants or Parking Lots?

Just as the high-pressure sodium lamp was initially designed for street lighting and not greenhouses; LEDs were not originally intended for specific use in horticulture. Unlike LEDs developed for general applications, some LEDs for horticulture are specifically configured for very high output with integration into controllable lighting systems. Though generic industrial and household LEDs have a clear price advantage, it is ultimately the value of LEDs in relation to the plant growth results that is most important to consider.

Narrow-band, red and blue LED grow lights are widely available at varying costs, depending upon the volume, quality, and performance. Blue (450-470 nm), red (660 nm), cool white and sometimes far-red (730nm) are more commonly available.

Trial First

With the plethora of LED options available, it is vital to assess the light spectrum from a manufacturer’s LED and understand its effect on the plant relative to the grower’s objectives. Setting up a small-scale trial of several fixtures and plants is one way to assess the effectiveness of a particular LED prior to outfitting a complete room or several rooms.

Form Factor

Size, shape, and uniformity of light are all factors to consider when devising a lighting strategy. Designing lit areas with standard lengths in mind will avoid custom configurations that could be costly to implement. As LED technology advances, so will their adaptability and availability in more standard sizes. Detailed lighting simulations are useful in determining how many light fixtures to use and where to place them in a grow room.

Managing Heat

LEDs produce less radiant heat than metal halide and high-pressure sodium lamps, which allows LEDs to be positioned much closer to the plants. This proximity results in a higher intensity of light and a higher concentration of photons that ultimately lead to better photosynthetic productivity.

Even though, LEDs run cooler than traditional lighting strategies, their fixtures still produce heat that needs to be managed. A high-quality LED fixture converts 30-40% of energy to light and 60-70% of heat. A LED fixture with insufficient cooling will convert more than 70% of its energy to heat that tends to overheat the circuitry and cause them to “burnout” quicker if not properly cooled. LEDs can be cooled actively using fans or circulating water, or passively using heat sinks. Both strategies reduce energy consumption without compromising the lifespan of the LED diodes.

Certification & Safety

Safety certification is an important factor to consider when selecting a LED. CE markings, UL or cETLus certifications, warranty and decay test results are all necessary when determining quality and security. Color rendering index (CRI) can be used to estimate how comfortable the light is to human eyes. Values under 50 are considered difficult to work under for a long period. CRI values for HPS are 20-40 (depending on lamp type), while traditional red-blue LEDs are zero.

Test standards like LM79 (Test environment), LM80 (decay measurement step), and TM21 (projection of decay) are useful when comparing performance of LEDs. These standards, however, only pertain to the LED components, not the fixtures that provide the cooling and working conditions of the LED. Currently, there are no standards that address LED fixture testing and how they behave over time.