LED grow lights are gaining popularity and the technology behind them has improved over the years. The efficacy of a grow light is one of the factors you need to understand in order to make an informative purchase decision and reach a high-quality plant production. The efficacy number of a grow light fixture is often listed by the lighting manufacturer along with other horticulture lighting metrics.
Rucola in Ljusgårda's premises under Heliospectra's ELIXIA
Efficiency and efficacy are very much related terms. In horticulture lighting, “efficiency” is a measure of how much power there is in the light output per input power to the grow light and is measured in Watts per Watt (W/W). Plants are not primarily sensitive to how much power there is in the light, but to how many photons there is.
Efficacy, on the other hand, measures how many photons you get out of the fixture per input watt. Because of this, efficacy is used for plant lighting fixtures and is a more relevant measure in the horticultural lighting industry.
The metrics used to evaluate the efficacy of a lighting solution today is photosynthetic photon efficacy (PPE).
PPE (photosynthetic photon efficacy) is the PAR photon output of a light source (measured in micromoles per second or μmol/s) divided by the input power (W) to produce that light. PPE (measured in μmol/J) will provide growers with insights about how competent the light is at converting watts into photons.
Although PPE is a great metric helping growers today to evaluate grow lights offered in the market, it has some limitations.
PPE does not include the photons outside the PAR range. Wavelengths outside of the PAR range like far-red (FR) have proven to have a positive effect on growth and steer morphological responses of plants. The efficacy of light sources emitting FR is typically lower than the efficacy of light sources emitting only PAR radiation even if they may give better results in plant biomass production and healthy growth. For this reason, spectral impact on crop performance is not reflected by the efficacy number.
We have a tendency to go very technical when measuring and evaluating the efficacy of grow lights. While this is needed and a great start when evaluating lighting fixtures for our grow facility, we must not forget to place our focus back on the plants and ask ourselves what the growth efficacy of the fixture is.
Growth efficacy (kg/kWh)
You should first prioritize concerns like “what light intensity” and “what light spectrum” is needed to get your desired growth goal then proceed to calculate the number of fixtures and power needed.
Light distribution is also another imperative factor to have in mind and focus on as all the light levels should be the same over the whole canopy area in order to have a uniform high-quality yield.
PPFD is a ‘spot’ measurement of a specific location on a grower’s plant canopy. To keep your crop production consistent, the entire plant canopy must receive even light levels. It is important to note that PPFD is dependent on the number of lights over the surface and how efficient those lights are.
Heliospectra’s ELIXIA LED grow lights have secondary optics. These secondary optics allow the fixture to focus on providing a higher average PPFD. Secondary optics steer photons to the plant canopy, minimizing light waste to aisles and other non-cultivation zones while also ensuring a uniform light distribution over the surface.
When comparing the amount of photons that actually hit the target area for light fixtures with and without optics, the amount is higher for those lights with optics than without. The optics enable more photons to hit the target plant area and vastly improve light uniformity. In-house testing at Heliospectra showed that the target hit rate is 16% higher with secondary optics than without for a typical setup.
Heliospectra researchers, along with a global community of scientists, have found that light quality is as important as light quantity. Good light quality is achieved through a balance of wavelengths selected to create an ideal environment, specifically for plants. LEDs with certain wavelengths have a lower PPE than others thus lowering the PPFD provided by the fixture. Be that as it may, some lower PPE LEDs are critical in creating spectral designs that promote plant growth and quality.
Research conducted by Heliospectra and its partners revealed that a high biomass is not always correlated to the intensity and efficacy, but a combination of intensity and spectral quality. A test was conducted where two different spectra compositions were evaluated and compared to each other. The power consumption was the same and all other conditions were kept exactly the same for both test subject groups. One had a spectrum that had a high fixture PPE (HE) and the other was designed to have a high growth efficacy (GE) but a lower fixture PPE (LE). The fixture with the spectrum designed to promote plant growth had a better result across the board, with an average between 12 - 28% higher biomass production.
Growth efficacy test results by Heliospectra and its partners