Expanding the term plant lighting efficacy

18-07-2019    01:30   |    Heliospectra

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.

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.

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