The metrics used to quantify horticulture lighting can be confusing at times. If you have been researching horticulture lighting systems, you have likely been bombarded with a variety of metrics that manufacturers use to market their products.
Some terms and acronyms you are likely to see include watts, lumens, LUX, foot candles, PAR, PPF, and PPFD. While all of these terms are related to lighting, only a select few really tell you the important metrics of a horticulture lighting system.
If you are looking to optimize your growing conditions or researching what lighting system is best for your operation, a firm grasp on the nuances of lighting metrics is the foundation to cultivation and financial success.
Horticulture LED lighting technology is still in its infancy and there is an abundance of misinformation, unlike other industries that have established clearly defined metrics to assess the performance of a solution.
Take, for example, the automotive industry. We all have a general understanding of horsepower, miles-per-gallon, and even torque. These are all critical metrics used to determine whether a vehicle can perform a specific application (i.e. towing a trailer or racing in a quarter-mile) and to evaluate options when looking to purchase a vehicle.
Metrics in LED horticulture lighting should provide the same type of insight; they should accurately and consistently answer the questions, “Can this light perform the function needed, and, more specifically, is it optimized?”
While there is currently a lack of industry standards for LED lighting in horticulture—and some manufacturers take advantage of this by misreporting or inflating performance claims—many lighting companies in the industry are encouraging the implementation of formal industry standards to ensure performance claims can be verified. Lighting technologies like high-pressure sodium and metal halide have had these standards in place for years.
Still, the start to standardizing the answers to these LED performance questions is with a firm grasp on the metrics by which horticulture lighting is measured. While other measurements like the daily light integral are important, for the purpose of this article we will focus on photosynthetically active radiation and photosynthetic photon flux.
It should also be mentioned that growers should not use a foot candle/LUX sensor when taking light measurements, since these follow the sensitivity curve of the human eye and give wrong information when comparing light sources with different spectrums. Instead, a quantum sensor, which is designed to follow the sensitivity curve of plants and measures the number of photons between 400 and 700 nanometers (nm), should be used.
Photosynthetically Active Radiation
Let’s start with photosynthetically active radiation (PAR). It’s s a much used, and often misused, term related to horticulture lighting. PAR is the spectrum of light (specifically 400-700 nm, which are the primary wavelengths of light used to drive photosynthesis). Knowing the spectrum of a lighting system is extremely important and can easily be achieved by requesting a spectral power distribution chart from a lighting manufacturer.
Spectral distribution will have significant implications on the growth and development of plants, along with the overall energy efficiency of a lighting system. However, the amount of PAR delivered to a crop is an equally (if not more) important metric to focus on. Photosynthetic photon flux (PPF) and photosynthetic photon flux density (PPFD) are two metrics used to measure the amount of PAR.
Photosynthetic Photon Flux
This measures the total amount of PAR that is produced by a lighting system each second. This measurement is best taken using a specialized instrument called an integrating sphere, which captures and measures essentially all photons emitted by a lighting system. The unit used to express PPF is micromoles per second (μmol/s).
Photosynthetic photon flux is a very important metric if you want to be able to calculate how efficient a light is at converting electrical energy into photons of PAR. If the PPF of the light is known along with the input wattage, you can calculate how efficient a horticulture lighting system is at converting electrical energy into PAR.
However, it is important to note that PPF does not tell you how much of the measured light actually arrives to the plant canopy. While it’s nice to know the PPF of a lighting system, PPF alone is often useless information to a grower.
Photosynthetic Photon Flux Density
This measures the amount of PAR that actually arrives at the plant, or, as a scientist might say, “the number of photosynthetically active photons that fall on a given surface each second.” 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).
If you want to find out the true light intensity of a fixture over a designated growing area, it is important that the average of several PPFD measurements at a defined height (typically, the top of your crop canopy) are taken. Lighting companies that only publish the center point PPFD directly below their lighting fixture grossly overestimate the true light intensity of a fixture.
A single measurement does not tell you much, since horticulture lights are generally brightest in the center, with light levels decreasing as measurements are taken towards the edges of a coverage area. It is easy for lighting manufacturers to manipulate PPFD data if buyers are not educated on the proper way to measure PPFD.
To ensure you are getting accurate PPFD values over a defined growing area, the following needs to be published by the manufacturer: the maximum, minimum, and average measurement; measurement distance from light source (vertical and horizontal); number and location of measurements; and the min/max ratio if only an average PPFD is provided.
“PPF” is commonly used to describe PPFD by academics in peer-reviewed journals, and debate continues among plant scientist and engineers on which usage of the term is correct. One way to avoid ambiguity when reading a journal article or product brochure is to focus on the unit of measurement.
If the unit includes m2, then the unit is referring to the light intensity at the surface of a plant canopy (PPFD). If m2 is not included, then the unit is referring to the total PAR emitted from a light source (PPF). Understanding the correct metrics to use will not only allow you to make better purchasing decisions of horticulture lighting systems, but will ultimately make you a better grower.
For more information
regarding this topic, please see Horticulture Lighting Systems: Understanding The Numbers.