If you are looking to buy an LED grow light for indoor plants, you will be overwhelmed with the range of metrics that lighting manufacturers use to promote their products. Some of these terms and acronyms include watts, LUX, foot candles, PAR, PPF, and PPFD, as well as lumens and photon efficiency. While many of these terms do pertain to lighting for indoor horticulture, only a handful, such as PPF, PPFD, PPE, and PAR, are more crucial than you must learn. This article is aimed at helping you get a complete understanding of what they mean, their differences, and which best suits your indoor planting.
Definitions of PAR, PPF, PPFD and PPE
PAR is the acronym for Photosynthetic Active Radiation, which refers to the effective spectrum used by crops for photosynthesis, with a wavelength ranging between 400 nm and 700 nm. PAR light is often misused. This is because it is not a metric unit like kg or inches, but rather a type of light needed for plant growth. The quantum sensor is used to detect and measure the PAR light intensity.
PPF is the acronym for Photosynthetic Photon Flux. The unit is µmol/s. It is the best measurement of how much PAR a light fixture can emit. Simply put, PPF is the number of photosynthetic photons that a growing light fixture can emit per second.
PPFD is an important metric to measure the performance of LED grow lights. PPFD is the acronym for Photosynthetic Photon Flux Density, and the unit is µmol/m2/s It displays the PPF inside one square meter, which is the number of PAR photons falling on the surface of your crops per second. When you know the PPFD, you know the number of PAR photons hitting your crop at each point.
PPE or Photosynthetic Photon Efficacy, is an important metric to quantify the efficiency of a lighting fixture. In LED grow lighting, PPE is a statistic of how much PAR light is produced by a grow light from its input power. The higher this value of the PPE is, the more efficient a lighting grow system is at converting electricity into photons of PAR.
Dr.Bruce Bugbee explains the differences between all the definitions more specifically:
How Are PPF, PPFD, and PPE Measured?
PPF is assessed using a integrating sphere test. The light fixture is put in a spherical sphere and the total number of photons of PAR light radiated by the fixture in all directions is measured. The sphere test tracks the number of PAR photons each second, and the results are represented as micromoles per second, which is written as µmol /s.
The best way to compare LED grow light fixtures is to examine their PPF light output — not lumens or watts. Remember that this is the total number of PAR photons a light fixture gives out in all directions—not the number of photons that will reach your plants.
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Usable PPF is the measurement that matters to growers since it describes the number of photons that are available for photosynthesis. The Usable PPF is the number of PAR photons that hit the chlorophyll of the plants.
The Total PPF is the total amount of light emitted by a fixture. Total PPF is measured with a device called an "Integrating sphere" which analyzes all the photons produced by the fixture. However, in most cases, 10–15% of these photons will be lost to radiance or reflection. As a result, the total PPF is always higher than the usable PPF.
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The PPFD measurement also depends on the height of the light source. The longer the distance of the grow light, the wider the coverage area of the usable PPFD for your crops, but the lower the intensity. Therefore, it is vital to know from which distance the light source was measured. Only by measuring the overall PPFD value over the entire lighted area and at different distances above the tree canopy can a useful comparison be formed between grow lights. The PPFD, as of today, is the best available measure to compare grow lights on the market.
Here's how Dr.MJ Coco measure the PPFD of fixtures:
PPE is the PAR photon output of the light source divided by the input power to light. Therefore, if you know the PPF and the power of a fixture, you can just divide them to find its PPE. In essence, PPE=PPF/POWER. Understanding the PPE of LED grow lights is crucial for making prudent choices and getting excellent yields.
Which Metric Should Be Considered While Choosing Fixtures?
The PPE measures a fixture's lighting effectiveness, whereas the PPFD measures the amount of PAR photons that hit your crop. These two measurements are the top metrics to consider while purchasing an LED grow lamp for indoor plants.
Take the PPFD MAP of Mini Sun-2 (150 Watts Version) as example:
The numbers of the chart meaning if you place a Minisun-2 in a 2'x2' tent and hang it up at 14.7" height with full power, it could provide up to 1193µmol (PPFD). For marijuana plants,700µmol would be the best.
How much PPFD for cannabis in every stage? (Seedling, Vegetative, Flowering)
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Indoor plants, such as cannabis plants require 16 hours of light during their vegetative stage and 12 hours at the flowering stage. You can either supplement CO2 or not. But, knowing the ideal PPFD for your cannabis will help you know how to grow a cannabis plant best.
According to experts, for regular cannabis growing, the PPFD for the seedling stage is 100–300 µmol/m2/s; the PPFD for the vegetation stage is 400-600 µmol/m2/s, and the PPFD for the flowering stage is 800-1,000 µmol/m2/s.
With CO2 supplement in Cannabis growth, the PPFD in the seedling stage is 100-300 µmol/m2/s, while the vegetative stage requires 800-1000 µmol/m2/s PPFD and 400-800 ppm CO2. The flowering stage requires about 1,000-1,500 µmol/m2/s PPFD and 800-1400 ppm CO2.
However, you can also consult other expert cannabis growers before supplementing with carbon dioxide when growing cannabis indoors. Sometimes, the additional inputs due to the use of CO2 may not justify the outputs you desire for your cannabis plants.
To invest in the best LED grow light for indoor plants, you need to examine all the PPF, PPFD and PPE while picking horticulture lighting fixtures. However, these three measures should not be used as the sole variables to make the decision. You must also examine other characteristics before selecting lightings, such as energy efficiency and light lifespan, as well as waterproofing and heat dissipation.