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Which supplemental greenhouse lighting technology is best to use?

While spectrum from light emitting diodes (LEDs) can be customized to a specific program, most lights come with a red-blue combination.
Photo courtesy of Ricardo Hernández

Often greenhouse growers must decide which supplemental lighting system is better: high-pressure sodium (HPS) or light-emitting diodes (LEDs). Meeting the necessary light requirement for certain crops during low-light periods can often be a challenge to greenhouse growers. Balancing operational energy costs, capital investment, and optimizing yield are the main concerns of selecting your lighting needs to overcome this common, yet unpredictable, limiting factor. Currently, the conventional standard in greenhouse supplemental lighting is HPS. Meanwhile, new horticulture-based light emitting diodes (LEDs) may be an option to replace or supplement HPS.

Before deciding which supplementary lighting is best for you, it is important to understand the differences in electrical efficiency, spectral distribution and other unique characteristics of the two lighting systems (HPS vs. LEDs).

Electrical efficiency

High pressure sodium (HPS) heat energy, emitted as near infra-red (NIR) may result with a leaf temperature increase of up to 2°C.
Photo courtesy of Ricardo Hernández

Horticultural Electrical Efficiency defines the ratio of electrical power that is converted to photons of light (micromoles per second: µmol s-1) in the photosynthetic active radiation spectrum (PAR: 400 to 700 nm). In other words, it is measured by how many photons (µmol) a particular fixture emits every second (s) per every Watt (W) of electrical power provided.

This number is either provided or can be directly calculated from the manufacturer-fixture specs. For example, if the spec sheet gives the emission rate (µmol s-1) and the power draw (W), you simply divide the emission rate by the total W. Alternatively, the specs will directly show the efficiency with the units of µmol s-1 W-1.

The efficiency of multiple fixtures ranges between 0.9 and 2.7 µmol s-1 W-1 for LEDs and 1.6 and 1.9 µmol s-1 W-1 for HPS. From the two highest advertised fixture efficacies, you can see that LED has around 40 percent greater efficiency than the new double-ended HPS. This, in theory, will generate a 40-percent energy savings. In the article, “Do the math, see the light,” published in the January 2016 issue, you learned that this number is key for electrical savings. As a warning, we have often seen horticultural LED fixtures with lower efficacies than HPS fixtures. For this reason, always make sure to obtain the efficiency information before you buy any horticultural light. Simply put, greater µmol s-1 W-1 is better.

All the power that is not converted into light photons is then emitted as heat. It is fair to note that because LEDs have relative comparable efficiency to HPS, they also have comparable heat production. This means LEDs produce almost as much heat than HPS, but in different forms. LED fixtures release most of their heat through convection cooling (most heat generated on the circuits), while HPS fixtures undergo radiative cooling (heat generated by the light spectrum). Although not visible, HPS heat energy is emitted as near infra-red (NIR). This can result in a 0.5 to 2°C increase in leaf temperature. Typically, plants under higher temperature have a greater growth rate, which may be beneficial during winter months; however, it may also be detrimental for cool-season crops or during high-temperature summer periods. Under identical light intensities, plants under HPS can have higher leaf/canopy temperature.

Spectral “tuning” of LEDs

The light emitted from LEDs can be customized, meaning that certain spectrum of color can be delivered in varying dosages. LED arrays can be comprised of many one-color diodes (making it all blue or all red, for example) or of several different diode types that would comprise a multiple color array (this can even include radiation that falls outside of the PAR spectrum such as UV and far-red radiation). Most commonly marketed LED panels contain some combination of blue and red light.

The advantage in customizing spectrum is the application of eliciting morphological responses by using different light qualities (colors) of light. This could potentially replace or reduce the use of chemical plant growth regulators in your operation.

For example, in one greenhouse study, cucumber transplants were grown under supplemental HPS, red-LEDs, and blue-LEDs. And cucumber seedlings under supplemental HPS and blue-light were 46 to 61 percent taller (less compact) than plants grown under supplemental red-LED (see Figure 1) (Hernández and Kubota, 2015). This could be a useful trait to utilize if a grower needs sturdier and more compact seedlings.

Fig. 1 Cucumber transplants were grown under blue light, red light and high pressure sodium (HPS) light to show how the quality (color) of light can affect the growth of the plant.
Photo courtesy of Ricardo Hernández

In another study, researchers found that petunia cuttings supplemented with LED lighting (consisting of 70 percent blue and 30 percent red light) had a larger accumulation of both leaf and root dry mass than when compared to those cuttings under supplemental HPS lighting (Currey and Lopez, 2013).

In contrast to LED spectral versatility, HPS has a set spectrum and the composition of the spectrum varies slightly between fixtures. However, the spectrum roughly contains 5 percent blue (400 to 500 nm), 53 percent green/yellow (500 to 600 nm) and 42 percent red (600 to 700 nm) (spectroradiometer scan on a 600W HPS lamp). The set spectrum limits the grower’s ability to manipulate a crop at the photomorphogenic level in novel ways.

It should also be noted that each crop species will react differently to specific lighting regimes, and thus more research is needed to know how horticultural plants will respond.

Future of LEDs

Haitz’s law states that the amount of light coming out of a LED diode will increase 20 times in the next 10 years (not to be confused with efficiency) and that the cost per diode will decrease 10 times in the next 10 years (Haitz and Tsao, 2011). This theoretical estimation is supported by actual data from LED technology development. This translates to more affordable LEDs in the very near future. Growers must calculate the return on investment (ROI) on the two fixture types (HPS and LEDs) by calculating the electrical cost (based on the fixture efficiency) and initial capital investment. If HPS is the desired technology, growers must take NIR radiation into consideration, which can increase plant canopy temperature. Unique and customized light spectrums obtained with LED lighting can elucidate desirable plant characteristics in horticultural crops and LEDs are expected to decline in price in the near future.

Ricardo is an assistant professor in Controlled Environment and Sustainable Energy, Department of Horticultural Sciences at North Carolina State University.

Hans is a Ph.D. student in Controlled Environment, Department of Horticultural Sciences at North Carolina State University.

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Currey, C.J., and Lopez, R.G. (2013). Cuttings of Impatiens, Pelargonium, and Petunia Propagated under Light-emitting Diodes and High-pressure Sodium Lamps Have Comparable Growth, Morphology, Gas Exchange, and Post-transplant Performance. Hortscience 48, 428-434.

Haitz, R., and Tsao, J.Y. (2011). Solid-state lighting: ‘The case’ 10 years after and future prospects. physica status solidi (a) 208, 17-29.

Hernández, R., and Kubota, C. (2015). Physiological, morphological, and energy-use efficiency comparisons of LED and HPS supplemental lighting for cucumber transplant production. HortScience 50, 351-357.