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Published on:April 19, 2017
By Josh Gerovac
Fluence Bioengineering
The true efficacy of LED fixtures for horticultural lighting depends on many factors, explains JOSH GEROVAC, and lighting manufacturers and growers need to take a broad view to ensure optimal yields and energy efficiency.
There are multiple considerations that a business faces when evaluating light sources to use for horticultural lighting, including but not limited to: light intensity, spectrum, uniformity of light distribution, energy efficiency, and fixture lifespan. Horticultural lighting systems convert electrical energy into light that plants use to drive photosynthesis for growth and development, and LED-based sources can offer a spectrum tuned for the application. Still, ascertaining the efficiency or efficacy of such solid-state lighting (SSL) systems is a challenge. There are several factors that impact the overall efficiency of a lighting system, relative to the specific application at hand. This article will discuss how the design of a lighting fixture influences energy efficiency and how in turn this reality can influence the overall profitability of a controlled-environment plant growth facility.
Interested in articles & announcements on horticultural lighting?
Indeed, the efficacy with which a horticultural light fixture converts electrical energy into usable light for plant growth is critical to the success of any controlled environment plant growth facility - often called CEA (controlled environment agriculture). Fig. 1 shows a vertical farm that is one example of CEA. Realize that the efficacy consideration is necessarily far different for lighting tuned for plants compared to lighting tuned for humans.
FIG. 1. The ability to deploy lighting solutions within inches of a crop canopy is a breakthrough for vertical farming applications. Properly designed LED solutions enable higher yields per square foot compared to poorly designed LED solutions and other lighting technologies such as HPS and fluorescent.
Metrics for horticulture
We will start by defining the proper metrics used for horticultural lighting applications to add context for the rest of the article. Plants primarily use wavelengths of light within the visible range of 400-700 nm to drive photosynthesis (Fig. 2), which is why this range is also called photosynthetically active radiation (PAR). Photosynthetic photon flux (PPF) measures the total amount of PAR that is produced by a lighting system each second. This measurement is taken using a specialized instrument called an integrating sphere that captures and measures essentially all photons emitted by a lighting system. The unit used to express PPF is micromoles per second (μmol/s).
FIG. 2. The graph depicts the average plant response to photosynthetically active radiation (PAR).
Photosynthetic photon flux density (PPFD) measures the amount of PAR that arrives at the plant canopy. PPFD is a spot measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second (μmol/m2/s).
Last, we will discuss photon efficacy, which refers to how efficient a horticultural lighting system is at converting electrical energy into photons of PAR. If the PPF of the light is known along with the input wattage, you can easily calculate photon efficacy for a horticultural lighting system. Given the unit for PPF is μmol/s, and the unit to measure watts is joules per second (J/s), the seconds in the numerator and denominator cancel out, and the resulting unit becomes μmol/J, which is the unit of measure used to represent efficacy. The higher this number is, the more efficient a lighting system is at converting electrical energy into photons of PAR.
By Josh Gerovac
Fluence Bioengineering

The true efficacy of LED fixtures for horticultural lighting depends on many factors, explains JOSH GEROVAC, and lighting manufacturers and growers need to take a broad view to ensure optimal yields and energy efficiency.
There are multiple considerations that a business faces when evaluating light sources to use for horticultural lighting, including but not limited to: light intensity, spectrum, uniformity of light distribution, energy efficiency, and fixture lifespan. Horticultural lighting systems convert electrical energy into light that plants use to drive photosynthesis for growth and development, and LED-based sources can offer a spectrum tuned for the application. Still, ascertaining the efficiency or efficacy of such solid-state lighting (SSL) systems is a challenge. There are several factors that impact the overall efficiency of a lighting system, relative to the specific application at hand. This article will discuss how the design of a lighting fixture influences energy efficiency and how in turn this reality can influence the overall profitability of a controlled-environment plant growth facility.
Interested in articles & announcements on horticultural lighting?
Indeed, the efficacy with which a horticultural light fixture converts electrical energy into usable light for plant growth is critical to the success of any controlled environment plant growth facility - often called CEA (controlled environment agriculture). Fig. 1 shows a vertical farm that is one example of CEA. Realize that the efficacy consideration is necessarily far different for lighting tuned for plants compared to lighting tuned for humans.

FIG. 1. The ability to deploy lighting solutions within inches of a crop canopy is a breakthrough for vertical farming applications. Properly designed LED solutions enable higher yields per square foot compared to poorly designed LED solutions and other lighting technologies such as HPS and fluorescent.
Metrics for horticulture
We will start by defining the proper metrics used for horticultural lighting applications to add context for the rest of the article. Plants primarily use wavelengths of light within the visible range of 400-700 nm to drive photosynthesis (Fig. 2), which is why this range is also called photosynthetically active radiation (PAR). Photosynthetic photon flux (PPF) measures the total amount of PAR that is produced by a lighting system each second. This measurement is taken using a specialized instrument called an integrating sphere that captures and measures essentially all photons emitted by a lighting system. The unit used to express PPF is micromoles per second (μmol/s).

FIG. 2. The graph depicts the average plant response to photosynthetically active radiation (PAR).
Photosynthetic photon flux density (PPFD) measures the amount of PAR that arrives at the plant canopy. PPFD is a spot measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second (μmol/m2/s).
Last, we will discuss photon efficacy, which refers to how efficient a horticultural lighting system is at converting electrical energy into photons of PAR. If the PPF of the light is known along with the input wattage, you can easily calculate photon efficacy for a horticultural lighting system. Given the unit for PPF is μmol/s, and the unit to measure watts is joules per second (J/s), the seconds in the numerator and denominator cancel out, and the resulting unit becomes μmol/J, which is the unit of measure used to represent efficacy. The higher this number is, the more efficient a lighting system is at converting electrical energy into photons of PAR.