PPFD and Light Intensity

PPFD, PPF, PAR and Lumens

Today we are gonna look at PPFD and how we measure light in a garden. In the past, we used lumens, but unfortunately that wasn’t telling us the whole story. As you will read below lumens only measure visible light, now this information is valuable when designing lighting for a warehouse or chemical plant, or even a classroom, but plants use a larger spectrum than we can see. Photosyntheticlly Active Radiation is how we typically measure intensity from a grow light today but is in usually represented as PPFD, this information tells us the intensity at a given distance over time. Most growers today use these numbers to dial in the intensity of a high yielding crop and maximize the canopy space. A wise man once said:

“Lighting is only as powerful as your A/C.”

 What this gentleman was implying was that with typical HID lighting a substantial amount of heat is generated by 1000W lights, ~4,000BTU’s, meaning gardeners are limited to the number of lights in a garden and how much air conditioning they can provide. Large commercial gardens with new DE lighting have switched to using High Temperature Refrigeration units to cool these large grow rooms.

First, lets look at what wikipedia has to say about all these word we are using to measure just how bright a grow light is.


“The lumen …is… a measure of the total quantity of visible light emitted by a source per unit of time.”


PAR (Photosynthetically Active Radiation)

Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place…

PAR measurement is used in agriculture, forestry and oceanography. One of the requirements for productive farmland is adequate PAR, so PAR is used to evaluate agricultural investment potential. PAR sensors stationed at various levels of the forest canopy measure the pattern of PAR availability and utilization. Photosynthetic rate and related parameters can be measured non-destructively using a photosynthesis system, and these instruments measure PAR and sometimes control PAR at set intensities. PAR measurements are also used to calculate the euphotic depth in the ocean.

In these contexts, the reason PAR is preferred over other lighting metrics such as luminous flux and illuminance is that these measures are based on human perception of brightness, which is strongly green biased and does not accurately describe the quantity of light usable for photosynthesis.


DLI (Daily Light Integral)

“The daily light integral (DLI) is the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day. It is a function of photosynthetic light intensity and duration (day length) and is usually expressed as moles of light (mol photons) per square meter (m−2) per day (d−1), or: mol·m−2·d−1.

DLI is usually calculated by measuring the photosynthetic photon flux density (PPFD) in μmol·m−2·s−1 (number of photons in the PAR range received in a square meter per second) as it changes throughout the day, and then using that to calculate total estimated number of photons in the PAR range received over a 24-hour period for a specific area. In other words, DLI describes the sum of the per second PPFD measurements during a 24-hour period.”


Next, lets look at what our leading manufacturer a Light Emitting Diode technology has to say about how these terms apply to our garden.

Horticultural Lighting Metrics

“If you have been researching LED horticulture lighting systems for your plant growth facility, you have likely been bombarded with a variety of metrics that lighting manufacturers use to market their products. Some terms and acronyms you are likely to see include: watts, lumens, LUX, foot candles, PAR, PPF, PPFD, and photon efficiency. While all of these terms do relate to lighting, only a select few really tell you the important metrics of a horticulture lighting system. The purpose of this article is to define these terms and acronyms, correct some common misunderstandings, and help growers understand which metrics are applicable to horticulture lighting systems, and which ones are not.

Photosynthetic light response curves chart | PPFD, Horticulture lighting metrics, How to measure PPF, PAR LightFigure 1: Photosynthetic light response curves


Plants and people perceive light very differently from one another. Humans and many other animals use something called photopic vision in well-lit conditions to perceive color and light. Lumens are a unit of measurement based on a model of human eye sensitivity in well-lit conditions, which is why the model is called the photopic response curve (Figure 1). As you can see, the photopic response curve is bell shaped and shows how humans are much more sensitive to green light, than blue or red light. LUX, and foot candle meters measure the intensity of light (using lumens) for commercial and residential lighting applications, with the only difference between the two being the unit of area they are measured over (LUX uses lumen/mand foot candle uses lumen/ft2).

Using LUX or foot candle meters to measure the light intensity of horticulture lighting systems will give you varying measurements depending on the spectrum of the light source, even if you are measuring the same intensity of PAR.

The fundamental problem with using LUX or foot candle meters when measuring the light intensity of horticulture lighting systems is the underrepresentation of blue (400 – 500 nm) and red (600 – 700 nm) light in the visible spectrum. Humans may not be efficient at perceiving light in these regions, but plants are highly efficient at using red and blue light to drive photosynthesis. This is why lumens, LUX, and foot candles should never be used as metrics for horticulture lighting.


PAR is photosynthetic active radiation. PAR light is the wavelengths of light within the visible range of 400 to 700 nanometers (nm) which drive photosynthesis (Figure 1). PAR is a much used (and often misused) term related to horticulture lighting. PAR is NOT a measurement or “metric” like feet, inches or kilos. Rather, it defines the type of light needed to support photosynthesis. The amount and spectral light quality of PAR light are the important metrics to focus on. (To find out more about spectral light quality click here). Quantum sensors are the primary instrument used to quantify the light intensity of horticulture lighting systems. These sensors work by using an optical filter to create a uniform sensitivity to PAR light (Figure 1), and can be used in combination with a light meter to measure instantaneous light intensity or a data logger to measure cumulative light intensity.

Three important questions you should look to be answered when researching horticulture lighting systems are:

How much PAR the fixture produces (measured as Photosynthetic Photon Flux)?

How much instantaneous PAR from the fixture is available to plants (measured as Photosynthetic Photon Flux Density)?

How much energy is used by the fixture to make PAR available to your plants (measured as Photon Efficiency).

The three key metrics used to answer these questions are:


PPF is photosynthetic photon flux. PPF measures the total amount of PAR that is produced by a lighting system each second. This measurement is taken using a specialized instrument called an integrating sphere that captures and measures essentially all photons emitted by a lighting system. The unit used to express PPF is micromoles per second (μmol/s). This is probably the second most important way of measuring a horticulture lighting system, but, for whatever reason, most lighting companies don’t list this metric. It is important to note that PPF does not tell you how much of the measured light actually lands on the plants, but is an important metric if you want to calculate how efficient a lighting system is at creating PAR.


PPFD is photosynthetic photon flux density. PPFD 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 lamp over a designated growing area (e.g. 4’ x 4’), it is important that the average of several PPFD measurements at a defined height are taken. Lighting companies that only publish the PPFD at the center point of a coverage area 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 the coverage area. (Caveat Emptor: Lighting manufacturers can easily manipulate PPFD data. To ensure you are getting actual PPFD values over a defined growing area, the following needs to be published by the manufacturer: measurement distance from light source (vertical and horizontal), number of measurements included in the average, and the min/max ratio). Fluence always publishes the average PPFD over a defined growing area at a recommended mounting height for all of our lighting systems.

Photon Efficacy

Photon Efficacy refers to how efficient a horticulture lighting system is at converting electrical energy into photons of PAR. Many horticulture lighting manufacturers use total electrical watts or watts per square foot as a metric to describe light intensity. However, these metrics really don’t tell you anything since watts are a measurement describing electrical input, not light output. 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. As a reminder, the unit for PPF is μmol/s, and the unit to measure watts is Joule per second (J/s), therefore, the seconds in the numerator and denominator cancel out, and the unit becomes µmol/J. The higher this number is, the more efficient a lighting system is at converting electrical energy into photons of PAR.


In order to invest in the proper horticulture lighting system to meet your cultivation and business goals, you need to know the PPF, PPFD, and photon efficiency to make informed purchasing decisions. However, these three metrics should not be used as sole variables to base a purchasing decisions. There are several other variables such as form factor and coefficient of utilization (CU) that need to be considered as well.

All factors need to be used in combination to select the most appropriate systems based on your cultivation and business goals, and the take home message is that PPF, PPFD, and photon efficiency are the proper metrics used by scientists and industry leading horticulture lighting companies. If a company does not provide you with the correct metrics used for horticulture lighting, they should not be selling horticulture lighting systems, and you will not be able to verify the true efficacy of their system. Fluence Bioengineering always publishes these metrics in product literature and is one of the leaders in photosynthetic photon efficiency as verified by Rutgers and Utah State University.”


*Working with folks like Fluence Bioengineering(formally BML) has been a big step in our education and the education of our customers, so why not just provide you with the information to read yourself. It has been through cooperation and discussion with manufacturers all over the horticultural industry we are able to try and keep up with all the rapid growth and development in the this industry. When we first learned about PAR we often wondered what effect this had on the plants and what, if any, parameters needed to be changed with a brighter light? As it turns out, YES, elevated levels of Carbon Dioxide are a must when dealing with a high intensity grow light, such as the fixtures from Fluence Bioengineering. *Total Dissolved Solids, or Nutrient levels, may also need to be increased and adjusted as plants grow and develop.
**Our belief is that genetics play a ~90% in a garden and the remaining ~10% is all environment, nutrients, and technique. Our next blog post will talk more about this and how you can dial in that last 10%, or maybe, even send you on the hunt for new genetics once you feel confident in your garden.
Thank you for reading and learning with us today, as always, if you have any questions always feel free to call the store during CST business hours. Thank you.

3 Replies to “PPFD and Light Intensity”

  1. Sagar Dhakal says:

    How PAR value can convert the PPFD?

    1. I found this.

      …and this.

      “Some researchers use the approximation 1 W/m2 ≈ 4.6 μmole.m2/s”

      Par seems to be a value of a given nm range, where as PPFD is a total output of photons over time. It doesn’t seem like an apples to apples comparison.

  2. It’s a great source of knowledge; I think it will be helpful for lot of people who are looking for learning more about ppfd . Thank you very much for sharing this article.

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