Horticulture Lighting for Growers
PAR, PPF, PPFD Explained

Since you are reading this article, you probably agree that the world of horticulture lighting can be intimidating. There are handfuls of acronyms used to rank and describe horticulture lighting systems, a variety of different types of lights, and a wide range of manufacturers to choose from. It can be difficult to know where to even begin! In this article we will deep dive on some of these key terms to help you understand the factors that are important in picking a lighting system.

Before we go into the specifics of horticulture lighting, let’s take a step back to the good old days of high school science and refresh our memories on how plant life works. If you paid attention in class, perhaps you recall that photosynthesis is the process in which a plant converts light energy (typically from the sun) into chemical energy that it uses to live, grow, and reproduce. When it comes to photosynthesis, some types of light are better than others. It was discovered in the 1900s that light behaves both as a particle and a wave; different types of light have different wavelengths, which impacts how the human eye perceives light as well as how plants absorb the light. For example, plants can more easily absorb red and blue wavelength light than other wavelengths such as yellow and green.

Photosynthesis is the heart of plant life, and it underpins all measurements in the world of horticulture lighting. When it comes to evaluating light systems, it is important to consider how much light in the photosynthesis spectrum the lighting system produces, and how much of that light actually reaches plants in the plant canopy, greenhouse, garden, or farm.

Now that we have jogged our memories on photosynthesis, let’s dive into some of the terms that are commonly associated with horticulture lighting. The first term is PAR, which stands for Photosynthetically Active Radiation – quite the mouthful! PAR is used to describe the bulk of range of light wavelengths that plants can use for photosynthesis. The majority of the spectrum of light that plants can absorb ranges from about 400 to 700 nanometers – anything outside this spectrum is less effective for photosynthesis. Specifically, light at either end of the PAR spectrum (red and blue wavelength light) stimulates photosynthesis best, while light towards the center of the spectrum (yellow and green wavelength light) is less effective in generating photosynthesis. 

So, when choosing a horticulture lighting system, it is important to remember that the light must fall within the PAR spectrum to be useful for photosynthesis. The main thing to keep in mind with PAR is that it describes the intensity of light within a specific range. For example, a 400 watt metal halide light, depending on the bulb, may put out around 140 PAR watts of light.  Many LED systems only emit specific light in this spectrum by utilizing specific diodes. That’s why LED grow lights have a purple like glow, they are using blue and red diodes. Because LED lights are selecting diodes at specific spectrums, they can utilize less energy to grow plants. However, they can’t seem to grow a plant as big and as dense as a high quality double ended HID grow light, like the 1000 watt DimLux.

The next three terms we will discuss – lumens, lux, and foot candles – are closely linked. You may recognize the term lumen, because it is commonly used with household light bulbs to represent the total quantity of visible light emitted from a source, based on the sensitivity of the human eye to certain wavelengths. It is important to note that lumens are based on light visible by the human eye, not necessarily light that is useful for photosynthesis (PAR). Lux and foot candles are measures for the intensity of light over a certain area. Lux are measured in lumens/m2 while foot candles are measured in lumens/ft2.

While lumens, lux, and foot candles are useful units of measurement for choosing a light bulb for a living room lamp or an office light, they are not particularly useful for measuring horticulture systems. As we discussed earlier, red and blue wavelength light is most effective for stimulating photosynthesis. However, the human eye best observes light that is towards the center of the visual spectrum, which is yellow wavelength light. Thus, because plants and the human eye prefer different wavelengths on the PAR spectrum, it is not useful to use lumens, lux, and foot candles, which measure light in relation to the human eye, to evaluate lights for the purpose of stimulating photosynthesis. Doing so would be like evaluating which fork is the best for eating soup!

While lumen, lux, and foot candle are not particularly informative in the world of horticulture lighting, PPF certainly is. PPF stands for photosynthetic photon flux and it measures the total amount of PAR produced by a light in a given second, expressed in micromoles per second (μmol/s). PPF does not tell you how much light will actually land on your plants, but it will clue you in on how efficient the light is at creating PAR. The metric is similar to lumens, but for PAR light, since it measures total amount of light produced. Since PAR helps plants undergo photosynthesis, lights with higher PPF are more desirable.

It is important to note that there has been some confusion and disagreement amongst light manufacturers, lamp manufacturers, and industry experts surrounding the true definitions of PPF and PPFD. When checking a company’s horticulture lighting products, ensure that PPF is measured in micromoles per second (μmol/s) and PPFD is measured in number of PAR photons per meters squared per second (μmol/m2/s).

The next term we will discuss is PPFD, or photosynthetic photon flux density. After reading that name, aren’t you glad they came up with acronyms? PPFD is similar to PPF, but it measures the amount of PAR that actually lands on the plant, rather than the PAR emitted from the lighting system. Scientifically speaking, PPFD is a measure of photons that reach a given surface (ideally a plant) each second. PPFD measurements are specific to certain locations in your greenhouse, farm, or garden, since PPFD is impacted by the plant’s distance from the light. Therefore, when evaluating different lights, it is important to look at PPFD measurements at different vertical and horizontal distances away from the light source. To confirm you are getting a truly accurate PPFD measurement, ensure the lighting manufacturer provides the vertical and horizontal distance from the light source that the measurements were taken, along with the quantity of measurements included in the data. It is easy for light manufacturers to misconstrue PPFD numbers, so it is critical that you look at how the company calculated PPFD, not just the PPFD value itself.

Another important consideration regarding PPFD is that the amount of PPFD a plant needs will depend on a number of factors, including what stage the plant is at in the plant lifecycle, the age of the plant, the type of plant, and temperature, among other things. When it comes to PPFD, you essentially want to choose lighting that mimics what the plant would prefer if it were in a natural sunlight environment. For example, plants that love intense, direct sunlight, such as lavender and yarrow, will require high levels of PPFD to survive and flourish, while their counterparts who prefer shaded environments will grow best with less PPFD. Where the plant is in the plant lifecycle also has an impact on their optimal PPFD levels. For example, younger plants that are still growing will survive better with lower PPFD, while plants that are flowering or fruiting will require higher PPFD levels.  

The good news about PPFD levels is that they are relatively flexible. If your plants are in a stage where they require lower levels of PPFD, you can dim the lights or raise them up higher away from the plants. Conversely, if your plant requires higher PPFD levels, you can make the lights brighter or bring them closer to the plants. Your plants will also adapt to PPFD levels on their own, too  – how cool! Plants that are receiving less PPFD than they need will spend energy growing stems and leaves in an attempt to absorb more light. They may also inject CO2 into the growing space, a process called CO2 enrichment, to increase the amount of photosynthesis occurring. The moral of the story is that PPFD is an incredibly important, yet flexible, metric to keep in mind when you are purchasing a horticulture system. Do your due diligence to choose PPFD levels that are right for your plants and your growing environment, but remember that you can (and should) manipulate PPFD levels by controlling distance of your lighting from the plant canopy and overall brightness or dimness of your lighting system.

The last technical term that we will discuss in this article is photon efficacy. Photon efficacy describes how efficient a light is at converting electrical energy (measured in Joules) into photons of PAR. Essentially, photon efficacy is measuring how well your lighting system converts electrical energy into light that is useful for your plants’ photosynthetic processes. Higher photon efficacy, measured in µmol/J, means that your lighting system is more efficient. If the lighting manufacturer does not provide photon efficacy, you can easily calculate it by taking PPF and dividing by electrical watts.

Now that you have all of the tools in your horticulture lighting toolbox, we can discuss what factors you should consider when evaluating a horticulture lighting system. As we discussed earlier, it is not important to consider lumens, lux, or foot candles when evaluating horticulture lighting, as those measurements relate to the visible light spectrum, not the photosynthetic spectrum. However, it is absolutely critical to consider PPF, PPFD, and photon efficacy when choosing a horticulture lighting system.

When evaluating your different options, the first step is to ensure that the lighting manufactures provide PPF, PPFD, and photon efficacy measures. PPF and photon efficacy are relatively straightforward metrics, but with PPFD it is essential to understand how each company calculated the metric. If a lighting company only uses the central point of their measurement area to calculate PPFD, they are likely overstating the PPFD value, since the center point of the surface will receive the highest amount of PAR.

An example of best practices in horticulture lighting metrics is DimLux. You’ll see we provide the PPF for both CMH (Ceramic Metal Halide) and DE HID (Double Ended High Intensity Discharge) lights. We supply bulbs of the highest quality bulbs available with our light systems. As we learned, different light bulbs and lamps provide varying levels of plant usable light. DimLux uses specially designed ballasts to drive their bulbs optimally for the life of the product. Additionally, the reflectors are scientifically shown to distribute light more evenly, and reflect more light, than any other manufacturer. DimLux’s is the most energy efficient light system available today, so it can help you achieve energy and costs savings as you grow your plants. As we all know, not all bulbs and lamps are created equal, and DimLux has emerged as the leader of the pack.

When it comes to choosing a horticulture lighting system that is right for your plants, just remember that metrics are the key! Now that you know the terms involved in the business, you have the tools to make the right decisions. Always keep PPF, PPFD, and photo efficacy in mind when choosing a lighting system. If the manufacturer does not make those metrics available and accessible, they are probably not the manufacturer you want to go with. Happy growing!