Understanding the scientific metrics that are used for horticultural lighting is the first step to measure and evaluate grow lights. This Grow Light Primer explains the key metrics and why they are important. Learn about PAR, Micromoles (µmol), PPF vs PPFD and more. We give straightforward definitions of the grow light terminology to demystify grow light science and allow you to make better grow light purchase and management decisions.
Grow Light Metrics
Photosynthetically Active Radiation (PAR)
Photosynthetically Active Radiation (PAR) is the region of the electromagnetic spectrum that plants use for photosynthesis. PAR wavelengths are between 400 nanometers and 700 nanometers. When we measure PAR, we are counting the number of photons in the PAR wavelengths. It is those photons that are absorbed in the leaves of the plant and power photosynthesis. In this way, measuring PAR is very different than measuring Lux or Lumens.
Lumens and Lux
Lumens and Lux are both measures of the brightness of the light. Lumens measure the total brightness of a fixture, and lux measures the intensity of brightness at a particular distance and position. The brightness of light is important for humans to be able to see, but it is not the measurement that is important to plants. With many spectrums of light, it is possible to estimate PAR by measuring Lux. However, Lux and Lumens alone are not very relevant to horticulture. We are interested in PAR.
PAR values are expressed in micromoles (µmol). A micromole is a standard unit for counting particles (e.g. atoms, molecules, ions and electrons). In this case, we are counting the number of photons in the PAR wavelengths.
Micromoles are used to express two different measurements of PAR: Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD). It is important to understand the difference.
Photosynthetic Photon Flux (PPF)
Photosynthetic Photon Flux (PPF) is a measure of the number of PAR photons per second. PPF values are properly denoted as micromoles per second (µmol/s). PPF describes the number of photons or “the amount of light”.
PPF is used to describe the total number of PAR photons that a fixture can produce per second. However, not all the photons will successfully reach the canopy. Therefore, there are two PPF values that are important. The Total PPF that a fixture can produce, and the Usable PPF, which is the number of photons that are directed to the canopy and can be used for photosynthesis. I describe each in more detail below.
Calculated, Total & Usable PPF
PPF is the correct metric to evaluate the amount of light that a fixture will produce. However, there are different ways to estimate or measure PPF. In order to make an accurate evaluation, it is important to understand the different types of PPF data.
Most manufacturers who publish PPF data about their lights report “calculated values”. These values are based on the efficiency ratings of the LED diodes that are used in the fixture. Calculated PPF values assume 100% efficiency and ignore the reality of thermal and other efficiency losses. Manufacturers report calculated values because they are easy to produce, and they make the fixtures appear more powerful and more efficient than they actually are.
We have analyzed dozens of fixtures from different manufacturers and found that the reported PPF figures are typically almost 20% higher than the true Total PPF measured in independent laboratory testing. There is a consistent pattern in the data, so it is possible to estimate the true Total PPF from the Calculated PPF. However, estimates and comparisons made using calculated values are less reliable than other forms of PPF data.
Total PPF describes the total number of PAR photons emitted by a fixture. In order to measure Total PPF, fixtures are placed in a device called an “integrating sphere”. An integrating sphere captures all the photons produced by the fixture and directs them to a sensor for precise measurement.
An integrating sphere is an excellent way to measure the total number of PAR photons that a fixture produces, however, it is not representative of the conditions that we grow in. Even in an ideal grow situation, 10-15% of the photons that a fixture creates do not reach the plants. Therefore, integrating sphere testing measures more photons from a fixture than we will ever be able to use for photosynthesis. What is important to us as growers is that portion of the Total PPF which reaches the canopy and is available for photosynthesis. We call this the “Usable PPF”.
Usable PPF describes the amount of PAR photons that are produced by a fixture and arrive to the canopy of the plants. It is the number that we really care about because it describes the amount of light that will be available for photosynthesis. Usable PPF is less than the Total PPF because some photons that are produced by the fixture are lost in radiance and reflection. The best way to determine the Usable PPF produced by a fixture is with carefully controlled field measurement. Field measurements use a PAR sensor in a setting that simulates a grow tent.
Accurate measurement of usable PPF depends on careful testing with specific protocols. Hanging height, test area size, measuring equipment and the presence or absence of reflective walls can make a significant impact on the usable PPF that will be measured in a field setting. Read our Grow Light Testing Protocol to learn more about how we measure Usable PPF in our Field Measurement testing.
Photosynthetic Photon Flux Density (PPFD)
Photosynthetic Photon Flux Density (PPFD) is a measure of the density of PAR photons that strike a target per second. PPFD values are expressed as micromoles per meter squared per second (µmol/m2/s). PPFD is a measure of the density of photons at a particular distance and position.
PAR meters and “PAR maps” both report PPFD values. It is the correct measurement to use when you are determining the intensity of the light at canopy level and the proper hanging height for grow light fixtures. However, PPFD alone cannot be used to evaluate the overall size or efficiency of different light fixtures.
PPF vs PPFD
PPF measures the number of photons and PPFD measures the density of photons hitting a target. PPF (µmol/s) is a quantity measurement while PPFD (µmol/m2/s) is a quantity per area measurement (density). Often manufacturers report PPFD values and try to present them as measurements of quantity, size, or power. However, PPFD does not describe the power of the fixture, it is simply a spot measurement of photon density.
As we show in the diagram above, PPFD is significantly affected by how far the measurement is taken from the light fixture. If we take a measurement very close to the fixture, then we will record a high PPFD. As we move away from the fixture the density of photons decreases. Because of this, PPFD alone does not tell you how much light the fixture can produce or how much area it can cover. The fact that a light fixture can produce a high density of photons (High PPFD) does not actually mean that it produces a large quantity of photons (PPF). A small light fixture can deliver a high PPFD but only to a small area.
Both PPF and PPFD are reported as µmol
PPF is properly µmol/s and PPFD is µmol/m2/s, however, both PPF and PPFD values are often reported simply as “µmol”. Thus, when you see values reported as “µmol”, you need to know whether it is a measure of density (PPFD) or a measure of the quantity of light (PPF).
Converting PPFD Measurements into Usable PPF
In our grow light testing we take multiple PPFD readings over an area divided up into a grid. We then multiply the average PPFD by the size of the testing area in square meters. This converts measured PPFD (µmol/m2/s) into Usable PPF (µmol/s). Average PPFD (µmol/m2/s) x square meters = Usable PPF (µmol/s).
Photosynthetic Photon Efficiency (PPE)
Photosynthetic Photon Efficiency (PPE) is a measure for how efficiently a fixture converts electricity into photosynthetically active radiation (PAR). It is determined by dividing the PPF by the wattage used. Using both PPF and PPE as abbreviations can get confusing, so we refer to this value as “Photon Efficiency”. Photon Efficiency is the single most important value for comparing and evaluating different grow lights.
Photon Efficiency ratings are labeled as “µmol/w” or “µmol/j”, which are equivalent measurements. However, the ratings may be based on Calculated PPF, Total PPF, or Usable PPF. Most manufacturers publish Photon Efficiency based on calculated values. Some reputable manufacturers base their efficiency ratings on a measurement of Total PPF. In our guide, we publish Photon Efficiency ratings based on Usable PPF. This means that our efficiency ratings are lower than manufacturer’s published data. Our ratings reflect how efficient the fixture is at delivering usable PAR photons to the plants.
To learn more about how we calculate grow light data, read our article, “Calculating Grow Light Efficiency, Coverage & Harvest Potential”.
A Coco for Cannabis and Migro Collaboration
Our Grow Light Guide is a collaboration among Dr Coco and Dr Photon from Coco for Cannabis and Shane Torpey from Migro Lighting. At Coco for Cannabis, our mission is to help growers maximize the success of their cannabis crops by providing scientifically accurate information and sharing proven growing practices. We are excited to collaborate with Shane because he has earned a well-deserved reputation for doing fair and accurate grow light testing. We created this guide to share reliable grow light data and empower home growers with the best information about grow lights.
Grow Light Guide Articles
- Grow Light Calculator
Our grow light calculator provides an accurate way to analyze and compare different grow light fixtures. You can enter your own grow light data or select preloaded fixtures.
- Calculating Grow Light Efficiency, Coverage & Harvest Potential
We explain how to use the Grow Light Calculator and the science behind it. Learn how we use Usable PPF to calculate our efficiency ratings, coverage estimates, and benchmark harvest targets.
- Grow Light Testing Protocol
Field testing produces the most accurate measurement of Usable PPF, but it must be done correctly. In this article, we explain our testing protocol and the science behind it. Learn about test area size, the importance of reflective walls, and the scientific way to determine optimal hanging height.
- How Much Light (PPF) Do You Need for Indoor Cannabis?
We review the science of cannabis photosynthesis and explain the optimal quantity (PPF) and density (PPFD) of light for indoor cannabis. We include a Grow Space Calculator which shows the optimal light and harvest potential for any grow space.