Introduction to LED Grow Light Terms: PPFD, PAR, and PPF

Several different terms are essential to understanding grow lighting and the optimization of plant yields across multiple climates, grow spaces, and plant types.

If you're thinking about getting into indoor growing, you'll need to learn some terminology to successfully measure the amount of lighting your plants will get. Terms like PPFD, PAR, and PPF should be in every home grower's vocabulary.

In this article, we'll explain what terms like PPFD, PAR, and PPF mean as well as walk you through the basics of using these terms in your growing process.

How are these Terms Related to LED Grow Lights?

Due to their versatility and long lifespan, light-emitting diodes (LEDs) have become increasingly valuable to growers over the past few years. In addition to adjusting a plant's growth across several color spectrums, they can also play an important role in adjusting PAR. As a result, LED lights can be perfectly matched to the right PAR throughout the plant lifecycle to ensure the right results for growers.

Due to the fact that PAR photons are equally distributed across the spectrum, the optimal LED grow light alignment would then balance the photons of the lamps according to the plants' spectral sensitivity curve to ensure the optimal light wavelengths. Adjustments like these can significantly impact plant health and quality, so light distribution needs to be carefully managed to ensure that all parts of the plant receive equal light at the right temperature and distance. Basically, a deep understanding of PAR, PPF, and PPFD can have an enormous impact on crop quality and yield success.

What are Photons?

Photons are tiny particles that make up light. Wavelengths distinguish photons from other particles. A light source's intensity is determined by the number of photons it emits. Photons also comprise light and radio waves and the force carrier for the electromagnetic.

Since light consists of photons, creating a photon-rich environment is essential. Growing houseplants requires light. Light is needed for plants to convert oxygen, water, and carbon dioxide into carbohydrates. Without this photon radiation energy, also known as light, plants cannot grow, bloom, or produce seeds.

What is PAR Light (Photosynthetically Active Radiation)?

Light in the 400 to 700-nanometer range is called photosynthetically active radiation (PAR), and it is how much light is available for photosynthesis. PAR varies by season, latitude, and time of day. Photosynthesis and plant growth requires photosynthetically active radiation. Higher PAR promotes plant growth, and monitoring PAR is important to ensure plants are receiving adequate light for this process.

Having PAR between 400-700nm is one of the most important sources of energy for plants. However, it must be consistently adapted to ensure that it isn't too high or too low as too high or too low PAR can negatively impact plant growth and health. Plants have evolved to continuously adjust to environmental changes by balancing the need to convert radiation energy with the need to protect themselves from photoinhibition.

How to Measure PAR Light?

PAR is measured in millimoles of light per square meter. The PAR value ranges from 0 to 3,000 millimoles per square meter. A silicon photovoltaic detector device is used to measure PAR. These detectors measure light between 400 and 700 nanometers in wavelength.

Some PAR sensors measure photosynthetically active radiation through their photon flux density (PPFD). Most plant biologists, horticulturists, ecologists, and other environmental scientists use this type of light sensor.

What is PPF (Photosynthetic Photon Flux)?

We use this term to indicate how much light, or more precisely, how many photons are emitted from a light source each second. There is no consideration for the direction or landing of the light in PPF. Basically, it describes the total amount of photons emitted by lamps or any other light source. The Photosynthetic Photon Flux, or PPF, represents the amount of light emitted each second by a light source in the PAR zone.

Such photons are measured by PPF and PPFD. It is vital to note that PPFD measures photon density on a particular surface, while PPF measures the number of photons released by a light source.

How to Measure PPF?

PPF is measured in µmol/s. As far as the total number of photons is concerned, both PPF and PPFD are used to count them. PPF is simply micromoles per second, but PPFD is micromoles per second per meter squared.

Basically, PPF measures the amount of PAR produced by a single light source. A lamp's PPF measurement can be used to compare its performance between lamps. Be aware, however, that the beam angle can affect the eventual PPFD reading depending on the product.

What is PPFD (Photosynthetic Photon Flux Density)?

The PPFD, or photosynthetic photon flux density, measures more than just the PPF; it measures the surface area as well. A PPFD measures how well those lights are working in terms of their output and how many of those crucial photosynthetic photons impact the growth area.

Your grow light manufacturer should provide you with accurate PPFD data covering the entire light area. If you want to buy a light, consider factors such as its distance from the light source, the average measurements, and the minimum/maximum ratio.

How to Measure PPFD?

PPFD is measured in micromoles per square meter per second (µMol/m2/S), which establishes how many PAR photons are landing on a specific area. Unlike lumens which will only measure the brightness of a light source, PPFD measures the number of photons within the PAR region that reach the target each second.

An effective amount of PPFD for indoor plants can be anything between 600-900 PPFD. This amount is great for the flowering, fruiting, or budding stage of plants.

Importance of Measuring Light Output of LED Grow Lights

To grow and develop, plants rely on light as their primary source of energy. Photosynthesis, or the process by which plants turn light into chemical energy, is accomplished by using Photosynthetically Active Radiation (PAR), a type of light. If you're using LED grow lights, it's important to know how to measure the light output and intensity for healthy crops!

Grow lights are evaluated by the area they cover, which provides a good indication of their power. Another way you can measure the light output of LED grow lights is by looking at the PPF or PPFD of the light source.

A LED grow light's wattage can be used to determine its intensity. As the wattage increases, the light intensity increases as well. However, we don't recommend using the wattage as a means to compare LED grow light intensity or strength.

How to Compare LED Grow Lights

Rule 1: Don't Compare Grow Lights Based Lumens

Humans perceive light intensity as lumens. In light of this, total lumens may be a dead metric since it does not correlate with photosynthetic growth rates. A horticulture lighting system should not be sold by someone trying to promote lumens because this measurement has nothing to do with the effectiveness of grow lights.

Rule 2: Don't Compare Grow Lights Based by Electrical Watts

To compare horticulture lighting systems, people often use total electrical wattage, dollar per watt, or watts per square foot, but these metrics are 100% useless and will most likely lead a consumer to make a poor purchase decision because plants do not grow on electricity.

Among the most popular LED fixtures on the market today, radiometric efficiency (how much light it emits per watt of electricity) can vary by up to 200%. Therefore, since plants grow by light and not by electricity, you should consider how much light the fixture emits.

Don't forget that you're not buying watts. In order to compare the performance of horticulture lighting solutions, a quantitative measurement of light output and the efficiency of the system is critical, which can be seen through PAR, PPFD, and PPF measurements.

Rule 3: Looking at the Single PPFD Measurement Will Give You Nothing

A single PPFD measurement isn't very useful unless you're growing a small plant directly under the light source. Manufacturers can easily demonstrate an extremely high PAR measurement directly under the fixture by placing the LEDs closely together and using narrow beam optics. In any case, unless you are only cultivating one plant in this specific location, you should have an idea of how much PAR is being emitted throughout the entire growing area. Most LED lighting systems have small fixture footprints that contain the LEDs, thus producing very high PPFD levels directly under the fixture. You will see a significant reduction in these light levels if you move the PAR sensor just a short distance from the main light source.

Rule 4: Don't Forget about PAR

Remember, higher PAR promotes plant growth, and monitoring PAR is important to ensure plants are receiving adequate light for this process. To optimize your growth, it is recommended that your plants receive 500-1000 µmols of PAR light for every m² (PPFD). Less than this, and growth rates will be low. Higher PAR intensity for your plants than this and the increase in growth rates do not justify the extra energy cost.

So, if you're thinking about purchasing LED grow lights for your indoor grow setup, make sure that your LED lights have proper PAR measurements, as well as PPFD. Higher PAR LEDs will be more effective for indoor growing.

To Sum Up!

As a refresher, PAR (Photosynthetically Active Radiation) is a region of the electromagnetic spectrum (400 to 700 nm) that promotes photosynthesis. PPF (Photosynthetic Photon Flux) is a critical metric that tells us how much PAR a light source emits. A PPF does not measure PAR at a specific location (e.g., your crop canopy), but it tells you how many photons are being emitted from a light source every second. On the other hand, the PPFD (Photosynthetic Photon Flux Density) calculates the number of photons that are emitted in the PAR region at a specific location (e.g., how much light penetrates your canopy) every second.

You must be able to tell the difference between these LED to grow light terms in order to be successful at getting high yields from your homegrown crops!