Lighting Information (PAR, PPFD, umol/s, DIL)

Light is the single most important environmental variable concerning plant growth. Plants are autotrophs that evolved the ability to use light energy from the sun to produce a food source through the process of photosynthesis. The process is quite complex, and for the sake of this guide, we can use this simplified definition of photosynthesis: using light energy to split water (H 2 O) and fix carbon dioxide (CO 2 ) to form carbohydrates (CH 2 O) and oxygen (O 2 ) ( Figure 1 ). The purpose of this guide is not to discuss the biochemical light reactions of photosynthesis, but to discuss the different properties of light and their influence on photosynthesis. Quality (spectrum), quantity (intensity), and duration (photoperiod) are separate but related light properties that influence photosynthesis. The science behind each property will be defined, then the application of the properties will be discussed in regards to horticulture lighting systems

Spectral Light Quality

Photosynthetically active radiation (PAR) is the major driver of photosynthesis in plants. However, not all wavelengths of light are equally efficient at driving photosynthesis. Two plant scientists by the names of Dr. McCree and Dr. Inada performed several studies in the 1970s to determine the influence of light spectra on photosynthesis. This research resulted in the creation of a photosynthetic response curve that is now appropriately known as the McCree curve. If you refer to the McCree curve ( Figure 2 ), you will notice that red light (600-700 nm) is almost twice as effective as blue light (400-500 nm) at driving photosynthesis, with green (500-600 nm) light in between the two. Prior to this research, it was a common misconception that since chlorophyll absorbs light mainly in the red and blue parts of the visible light spectrum (leading to the green color of plant leaves) that green light was not used by plants for photosynthesis. However, precise and independent measurements of photosynthetic activity under different wavelengths by McCree and Inada demonstrated that light in the green spectrum (500 – 600 nm) is nearly as effective as blue light for a considerable number of plant species. The short explanation for this experimental fact is that higher plants have evolved both
biochemical and biophysical solutions (e.g. accessory pigments) to utilize green light. These accessory pigments (mainly carotenoids) can be thought of as storage molecules for photons that are not directly absorbed by chlorophyll.
Spectral light quality is a key component that goes into the design of horticulture lighting systems, especially in sole-source (absence of sunlight) lighting applications. Traditional high-intensity discharge (HID) lighting systems (high-pressure sodium and metal halide) have always had a limitation when it came to modifying the spectral light quality. Using light-emitting diodes (LEDs) for horticulture lighting systems allow manufacturers the ability to create custom spectral light qualities along with many other advantages over conventional lighting systems, including: high photoelectric conversion efficiencies, low thermal output, and adjustable light intensities. Light quality not only influences photosynthesis, but it also influences the morphology of plants, which is known as photomorphogenesis.

Light Intensity

The number of photons that are absorbed by specialized photoreceptors known as chloroplasts directly influences the rate of photosynthesis. As light intensity (PPFD) increases, so does the rate of photosynthesis, until a saturation point is reached. Every plant species has a different light saturation point where photosynthetic levels plateau based on the light environment they evolved in. Light saturation occurs at much lower intensities in plants that evolved in shade conditions than those that evolved under full sun conditions. However, light saturation normally occurs (especially in sun plants) when some other factor (normally CO2) is limited (Figure 3). Another important consideration regarding light intensity is known as the light compensation point. Plants have a minimum light intensity required to promote maintenance growth to keep plants alive. As you would expect, the light compensation point occurs at higher light intensities for sun plants than shade plants. Providing adequate light intensities with the correct spectral light quality is critical to promote new plant growth.

Horticulture lighting systems can be used in two ways to increase light intensities to promote photosynthesis. Supplemental light can be provided by lighting systems in greenhouse environments, generally during light limiting conditions (eg. winter months in northern latitudes, cloudy conditions, or a combination of the two), or as a sole-source of photosynthetic light in an indoor controlled environment (eg. growth chamber, warehouse, grow tent, etc.) where sunlight is not being used as a source for photosynthesis. One of the key benefits of using LEDs for either application is the low thermal output at the surface of the diode. Achieving very high PPFD with HID lights has always been limited by the distance that the lamps need to be kept from the crop canopy, since these lamps emit a high percentage of energy as infrared (IR) light. IR light is not photosynthetically active and significantly increases plant temperatures, therefore, one method to mitigate this response is to increase the distance between the light and the crop canopy (which results in decreased light intensities and limits the environments they can be used in to facilities with tall ceilings). With proper thermal management, LEDs dissipate most of their heat from the back side of the diode, therefore, lighting fixtures can be placed at much closer distances from the crop canopy, allowing very high PPFD levels (≥ 1000 µmol/m 2 /s) to be provided to plants.

Light Duration

Light duration (photoperiod) is the length of time a plant is exposed to light during a 24-hour period. The length of a photoperiod can influence the overall light intensity that a plant receives in 24 hours, which in turn influences overall growth. This is described as daily light integral (DLI), which is defined as the cumulative PPFD delivered during 24 hours, and is expressed in mol/m2/d. Photoperiod also influences the transition from vegetative to reproductive growth in several plant species. However, it is actually the dark period (skotoperiod) and not the photoperiod that determines when certain species will transition to reproductive growth. The photoreceptor phytochrome is mainly responsible for signaling the transition to reproductive growth in photoperiodic crops (To learn more about this click here to read our photomorphogenesis guide). Short day (long night) plants flower when the phytochrome perceives an uninterrupted long night (generally ≥ 12 hours). Long day (short night) plants flower during short nights (generally ≤ 12 hours). Alternatively, several plant species are day neutral, where photoperiod does not influence flowering.

Horticulture lighting systems can be used to provide photoperiodic light to extend the day length to either promote flowering of long-day plants or suppress flowering of short day plants regardless of the season or climate. Traditionally, HID, incandescent, or fluorescent lights have been used to provide photoperiodic lighting in greenhouses. However, these technologies are relatively inefficient at converting electrical energy into PAR. Fluence lighting systems convert electrical energy into PAR more efficiently than these lighting technologies. See this publication from Utah State University for more information.


The work of Drs. McCree and Inada was fundamental in understanding the influence of spectral light quality on photosynthesis; however, the study of photobiology is still in its infancy and rapid advances in LED technologies have allowed researchers the ability to expand on the work of previous plant scientists. There is still a great deal of work to be done in the study of photobiology, and Fluence Bioengineering is working with world-renowned plant scientists and commercial growers to continue to explore the interaction between life and light.

We have known that light is responsible for driving plant growth via photosynthesis for many years; however, the influence of light on plant development has only become well understood in the last century. The color of light (spectral light quality) is not only an important variable for photosynthesis, but also acts as a packet of information to signal light mediated developmental responses in plants, such as: seed germination, stem elongation, and flowering. The term used to describe these responses in plants is photomorphogenesis (photo = light, and morphogenesis = the process that causes an organism to develop its shape). Plant morphology (plant architecture) is extremely important in controlled environment agriculture where vertical or horizontal growing space may be limited. Depending on the plant architecture you desire, there may be other aspects of horticulture lighting systems to consider beyond providing a source of photosynthetic light.

Photosynthetically active radiation (400 – 700 nm) is mainly used for photosynthesis , however, plants can sense wavelengths ranging all the way from UV-C (260 nm) to far-red (730 nm) using separate photoreceptors that are not utilized for photosynthesis. These photoreceptors direct an adaptive response in plants under changing environmental conditions to regulate key stages of plant development which depend strongly on the spectrum of light, and in some cases timing, periodicity, and the overall exposure. The latter is usually called fluence, and is measured in micromoles of photons per square meter of surface. There are very low, low, and high fluence responses, with the corresponding sufficient light levels ranging from those of star light (for very low) to direct sunlight (for high). The purpose of this article is to describe photomorphogenic responses in plants to help you consider

Red And Far-Red Light Responses

When it comes to photomorphogenesis, the most understood developmental processes are those controlled by red and far red light (for the purposes of this discussion, we will refer to red ® light as the spectral region around 660 nm and far red (FR) light around 730 nm). In order to better understand the influence that these two spectral regions have on plant development, you need to first understand the significance of the pigment known as phytochrome, which is responsible for R and FR light mediated responses.
Phytochrome is a pigment protein which exists in two interconvertible forms – a red light absorbing form (P r ) and a far red absorbing form (P fr ). Phytochrome converts from one form to another upon absorbing the corresponding light until an equilibrium is established (phytochrome photoequilibrium), with the relative amount of each form depending primarily on the ratio of R to FR light in the light spectrum. To put this another way, when P r absorbs R light it is converted into P fr , and when P fr absorbs FR light it is converted into P r (There is some overlap in in the spectra of both forms, and phytocrome does absorb some blue light as well, but for the sake of this guide, this will not be discussed). The prevalence of one form or the other (which depends on the R/FR spectral ratio) can stimulate or inhibit a number of developmental processes such as: seed germination, leaf unrolling, chlorophyll formation, and stem elongation. Additionally, phytochrome is the controlling factor of promoting (or suppressing) flowering in photoperiodic plant species. For the sake of brevity, and to discuss important applications related to horticulture lighting systems, we will focus on the influence that phytochrome has on flowering and stem elongation.


There are long-day plants (which require short nights to flower) , short-day plants (requiring long nights), and day-neutral plants which have no specific requirement for the photoperiod. This dependence on the photoperiod is referred to as photoperiodism, however, it is actually the length of the dark period (skotoperiod) that regulates flowering of photoperiodic plant species. In the absence of light, P fr slowly converts to P r , and as the skotoperiod increases, so does the relative amount of P r . Long day plants (which have a short skotoperiod) will not flower if P fr converts to P r during the skotoperiod, while short day plants (which have a long skotoperiod) will only flower if P fr converts to P r during the skotoperiod. Photoperiodic phytochrome responses occur in the low fluence range (as low as 1 µmol/m 2 ), so it can only take a short flash of R light to during a skotoperiod to revert P r back to P fr . For example, flowering of a long-day plant may be induced by night interruption, using a series of short flashes of red light with photon flux levels as low as a few µmoles/m 2 /s. Conversely, short-day plants may be induced to flower by a single flash with pure FR light at the very beginning of the dark photoperiod, after turning off all other lights. This effectively adds a couple of hours to the dark period for the purpose of flowering, which can be used to extend the light period for growth and optimize plant yields as a result. Switching the above methods for plants with opposite photoperiod requirements would delay flowering, which may also be desired sometimes (e.g. to provide the best quality flowers on schedule for certain holidays).
A good energy-saving (and thus, cost-saving) strategy is to use one set of lights for growth and another for photoperiod control when necessary. Since phytochrome response is in the low fluence range, the number of fixtures needed for photoperiod control may be much smaller than that of fixtures needed for growth. In addition, the operating time needed for photoperiod control can be much shorter, such as only minutes at a time. Since FR light is only partially photosynthetically active, its use in horticulture lighting is often limited for reasons of energy efficiency.

Shade Avoidance Response

Another important R and FR photomorphogenic response important to horticulture lighting systems is called the shade avoidance response. Far-red light is transmitted through leaf tissue more so than red light, which causes an enrichment of far-red light, relative to red light, for plants grown under canopies. When a low R:FR ratio is perceived by phytochrome pigments, a shade avoidance response is activated to elongate hypocotyls or stems in an attempt to out-compete neighboring plants. This is very important when it comes to the spectral light quality of horticulture lighting systems. Photoperiodic lights that provide a low R:FR ratio to promote flowing may also induce a shade avoidance response in plants, which may result in an undesirable growth habit (especially if a compact growth habit is preferred).
##Blue Light Responses

Two important blue light photreceptors are cryptochromes and phototropins. Blue light is important for a variety of plant responses such as: suppression of stem elongation, phototropism (bending towards a light source), chloroplast movement within cells, stomatal opening, and activation of gene expression (some of these are morphogenic and others aren’t). Stomatal opening and height control are of particular relevance to horticulture lighting systems. A low overall content of blue light in the growth spectrum (e.g. less than 10% of the total photon flux) can lead to leaf edema (swelling of the leaves) and developmental problems in several plant species. The absolute content of blue light has a progressively stronger effect for plant height reduction. This may be desirable in some cases (e.g. to produce more compact seedlings and reduce transportation costs) but generally leads to a lower photosynthetic efficiency of the light with respect to energy consumption. A high relative content of blue light reduces the plant leaf area and may be undesirable for that reason. Near UV light has an effect similar to blue light, with further reduced photosynthetic efficiency, especially below 400 nm (although the other effects may be stronger by comparison). It also affects the biosynthesis of compounds responsible for the flavor of certain fruits, increased anthocyanin concentration, as well as that of other compounds which are not directly produced by photosynthesis alone. Whenever the use of near UV light is necessary to control a corresponding sensory mechanism or the production of a specific molecule of interest by the plant, an overall efficiency trade-off may have to be reached, similarly to that for the use of far red light.

Green Light Responses

The least understood spectrum related to photmorphogenic responses in plants is green light (500 – 600 nm). The control effects of green light are generally opposed to those of red and blue light. For example, green light has been shown to reverse blue light induced plant height reduction and anthocyanin accumulation. The phytochrome and cryptochrome photoreceptors mentioned earlier are also responsive to green light, though to a significantly lesser extent than to red or blue light. So far, all efforts by researchers to find photoreceptors responding primarily to green light have given no definitive results. However, it should be mentioned that the addition of green light into the spectrum of horticulture lighting systems has demonstrated to be beneficial to the growth of several plant species. Similar to far-red light, green light penetrates deeper into leaves and canopies than red or blue light, and can significantly increase the rate of photosynthesis. The addition of green light also significantly improves the color rendering index (CRI) of a horticulture lighting systems, which allows growers to effectively monitor crops for disease or nutrient deficiency/toxicity symptoms, without the use of specialized glasses.


Generally we all worry about the wattage of our lamps, but they can’t all be measured in the same scale, for example, efficiency of COBs in comparison to HPS. PAR on the COBs is way higher than it is on a HPS lamp with the same wattage. I know there are guidelines as to how much light is enough, what the minimum required, in PAR, PPFD, umol-s or even DLI… Anybody here know?

I’ve seen mention of up to 65 mol/d and according to purdue university studies on different plant species, most other light loving plants like sunflowers and tomatoes are recommended for a DLI between 22 and 30mol/day.

So for cannabis, bottom threshold for optimal growth and photosynthesis is a DLI of DLI of 22 would be:
24/0 schedule: 254.6 micromoles/m2/s-1
18/6 schedule: 339.5 micromoles/m2/s-1
12/12 schedule: 509.25 micromoles/m2/s-1

For Cannabis, the Top threshold for optimal growth and photosynthesis is a DLI of 65 moles per day.
***extremely important notice, only go up to these amounts if you are using supplemental CO2, do not go this high if you are not using supplemental CO2 as you will actually slow down photosynthesis and waste energy.

24/0 schedule: 752.31 micromoles/m2/s-1
18/6 schedule: 1003.08 micromoles/m2/s-1
12/12 schedule: 1504.6 micromoles/m2/s-1

The generally accepted guidelines for artificial light PPFD in flowering are this:
in a 12/12

PPFD of at least 510 micromoles/m2/s-1 for the low end of optimal intensity
PPFD of at least 800-1100 micromoles/m2/s-1 for perfect optimal lighting without additional CO2.
PPFD of at least 800-1500 micromoles/m2/s-1 for perfect optimal lighting WITH additional CO2.

So, apparently the longer the light is on the less of umol/s you need and viceversa!


Measuring Light: The important differences between PPF and PPFD
There exists some industry inconsistencies between how PPF and PPFD is used when describing their lamps
output. For those basing their decision on which lamp is best suited their garden it’s important to know what
these different terms mean.
PPFD (Photosynthetic Photon Flux Density) μMol/M2S is certainly well defined, accepted and
consistently used. However, PPF is used inconsistently by nearly every source referencing it. We
have seen a quantum sensor manufacturer indicate that the terms PAR, PPF, and PPFD are all the
same having units of μMol/M2S. We have also seen many refer to PPF in various ways having units of
μMol/M2S. We have seen industry leading light manufacturers refer to PPF as the total radiant light
output in the PAR region of a lamp in units of μMol/S. So the question is: If PPFD is well defined and
uniformly accepted, why then are so many within the industry using PPF to have the same meaning?
The likely answer is that there is no good solid standard for the industry to work within. So taking a look at area
lighting and their value definitions the best conclusion we can draw is that the units for PPF should be
μMol/S. We think the reason for this confusion results from a disconnection between the lamp and sensor
manufacturers, particularly since lamp specifications should be based on PPF and field measurements
based on PPFD. Another issue contributing to the confusion is that so many manufacturers, both lighting and
measurement, do not consistently show the complete proper engineering units in their marketing, operating,
and specification documents.
PPF as μMol/S is the value that a grow lamp should specify for its total light output (Radiant Watts would also
be appropriate), this would be similar to using Lumens for a regular area light. PPFD is the primary
measurement taken by a Quantum PAR meter and is expressed as a density over a unit of area, this would be
similar to using Lux or Foot-Candles on a regular light meter. We do believe these to be the proper definitions
as originally intended, but have been unable to find any industry standard or document to substantiate that.
For your consideration:
PPF (Photosynthetic Photon Flux) issued by (or should be) manufactures as the lamps total emitted number of
photons per second in the PAR region. Units of measure: μMol/S
PPFD (Photosynthetic Photon Flux Density) represents a field measurement and is defined as, the number of
photons in the PAR region emitted per M2 per Second. Units of measure: μMol/M2S
In standard commercial/industrial lighting terminology the use of the term “flux” generally applies to the light
energy on a per time basis which in grow lamps would be the lamps PPF value. By definition, Energy per time
or energy transfer rate is defined as Power. In lighting terms, light power is generally defined in terms of
Radiant Flux or Luminous Flux.
The point of this discussion is that the term “flux” used in the absence of density applies to energy or power
emitted, but not over a specific area. The term “Density” generally applies to the energy or power per a
volume of space, in the case of lighting, it generally applies to an area rather than a volume, since light striking
a surface area is more applicable than light within a volume.
Lamp manufacturers should rate the lamps overall output in PPF (μMol/S) or PAR radiant Watts rather than
Lumens which are strictly for human vision. Ideally in the future we may have a generally accepted PAR
sensitivity standard that can be applied to give results as Yield PPF or Yield Radiant Watts which would be a
more equivalent measurement to Lumens, but would be plant specific
If a lighting manufacturer, not a lamp manufacturer, were to use PPFD as a specification for a particular
fixture/lamp combination they would need to include all of the following information for the user to properly
interpret the information being given:

  1. Provide the height from which the measurement was taken.
  2. Provide multiple locations across the illuminated footprint at meaningful points allowing for
    interpretation of consistency of intensity and footprint limitations.
  3. Provide multiple sets of PPFD measurements at different heights.
    To publish in a PPFD value various considerations with this approach would be:
  4. Measurements must be taken with reflectors in place to achieve accurate results.
  5. For HID lamps since the lamps are sold separate from the reflectors there can be many different
    combinations of lamps and reflectors. Any measurements taken need to be specific to a particular
    lamp/reflector combination.
  6. At the close height that grow lamps are used their light distribution patterns will not change in a regular
    expected manner. The use of a reflector in the design requires that a certain minimum measurement
    distance is used for values to stabilize and act with an expected regularity for varying heights. This
    height in many cases will be greater than the typical height used for an indoor grow, but would likely be
    good for a greenhouse supplemental application. Basically we are referring to relationship of Light
    Intensity as a function of 1/d2, where d is the measurement distance from the light source. Until this
    certain height is achieved, you will not see measured values following this rule at all.
  7. As for LED panels, these measurements will not stabilize until a height is reached for which the beam
    patterns of the LEDS have interlaced enough to be somewhat homogenous, both with respect to
    intensity and mixture of different LED wavelengths.
  8. Interpreting the results of overlapping light from adjacent lamps in multiple lamp systems can be a
    difficult task, but would be necessary to determine the true quantity of light intensity at any particular
  9. It will likely still take some field experimentation with proper measurement equipment and different
    configurations of the lamps to come to the optimal lamp placement, particularly in a multiple lamp
    This is all great information but how do I use it in the field?
    As indoor gardeners we are trying to optimize all the conditions in the garden which include an even spread of
    light at enough intensities to meet our crops daily light needs. It’s important to have accurate canopy values as
    it relates to the amount of CO2 that is necessary to meet crop flowering demands. So while this may seem like
    a lesson in advanced mathematics, it’s really good for the gardener to approach how much light they expect to
    see at the canopy based on this approach:
  10. Take the PPF value that the manufacturer provides for total PPF output.
  11. Reduce that output by ~20% as a reasonable loss associated with light not making it to the canopy.
  12. When you have that number divide it by the canopy surface area in M^2 and that would represent what
    intensity you would expect over that surface area.
  13. Use that value to calculate how much CO2 your plants would require based on RH between 50-70%.

This is an example of how you would use the PPF to get an expected PPFD value;
Lamp PPF Output: 1,100 μMol/S
Grow Area: 4’ x 4’ = 16 sq-ft = 1.49 M2
Grow Losses off Canopy: 20% = 80% used
Light Actually Used = 1,100 x .8 = 880 μMole/s
Sample Average PPFD Formula: 880 μMole/S ÷ 1.499 M2 = 591 μMole/M2 -S
Summary Conclusions: PPF and PPFD certainly have their place when discussing grow lamp output values
and how those values may then be measured in the field. However, there does exist inconsistencies on how
lamp and sensor manufacturers use these terms and for the average gardener who may lack advanced
degrees in Physics and Engineering, these are not going to be terms they can easily come to grips anyway.
As such we felt introducing an alternative method, which was easier to comprehend, would be in order.


NOTE: If you are not familiar with the differences between PAR, PPF and PPFD – please read our article about horticulture lighting metrics first to get the most out of this article.

Read article about horticulture lighting metrics

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. PPF does not measure PAR at a specific location (e.g. your crop canopy), but it tells you how many photons within the PAR region are coming out of the light-source every second. PPFD (Photosynthetic Photon Flux Density) measures the amount of photons within the PAR region at a specific location (e.g. the amount of light delivered to your canopy) every second. If you have a PAR meter, it is reporting PPFD (μmol/m2/s) measurements. You must understand the differences in these metrics before you can compare various horticulture lighting systems. Many manufacturers realize this can be a confusing topic, so it is very easy for companies to mislead potential customers with exaggerated marketing claims, misleading information, and by showing a limited set of (or using blatantly wrong) metrics. However, once you understand the differences in these metrics, you will be able to cut through all the ‘marketing’ and ‘hype’ and simply ask manufacturers to provide the data you need to successfully compare lighting fixtures.

In order to explain the correct method for evaluating a horticulture lighting system, let’s first highlight some of the metrics used today that you need to avoid:

  • RULE NUMBER 1: Don’t use electrical watts to compare grow lights
  • RULE NUMBER 2: Don’t use lumens to compare grow lights
  • RULE NUMBER 3: Don’t be fooled by a company that claims to have a magical growth spectrum
  • RULE NUMBER 4: Don’t just look at a single PPFD measurement directly under the fixture
  • RULE NUMBER 5: Don’t focus on the wattage of the LEDs used in the fixture (1W, 3W, 5W, etc.)

In general, if you see a company using any of the above items to promote their horticulture lights, run away and don’t look back. None of these metrics, nor their derivatives, tell you anything about the performance of a horticulture lighting system.

Rule No. 1: Don’t Use Electrical Watts To Compare Grow Lights

Many people use total electrical watts, dollar/watt or watts/square foot to compare horticulture lighting systems, but these metrics are 100% useless and will most likely lead a consumer to make a poor purchase decision. Why? Simple. Electricity doesn’t grow plants. Furthermore, radiometric efficiency (how much light a fixture emits per watt of electricity) can vary by up to 200% amongst the popular LED fixtures on the market today. Hence, since light (not electricity) grows plants, you need to ask how much light a fixture emits. It sounds simple, but 99.9% of horticulture lighting companies do not advertise this metric. Instead, they focus on electrical watts. Why? Because it is very hard to design an efficient lighting system (measured in μmol/J) that delivers high light levels, but it is very easy to build an inefficient lighting system that consumes a lot of electricity. High efficiency LEDs, power supplies and optics cost more than less efficient components, and many manufacturers use lower quality components to increase profit margins
Remember…You are not buying watts. You are buying a system that delivers light to grow your plants, so a quantitative measurement of light output and the efficiency in which the system produces that light is the critical metric you should use to compare the performance of horticulture lighting solutions.

Rule No. 2: Don’t Use Lumens To Compare Grow Lights

This one’s easy to explain. A lumen is a rating of how bright a light appears to the human eye. However, since human vision is not correlated to photosynthetic grow rates, total lumens is a dead metric. As a rule, if someone is trying to promote lumens for a horticulture lighting system, they should not be selling horticulture lighting systems.

Rule No. 3: Don’t Be Fooled By “Magical Growth Spectrums”

Many scientific papers have confirmed that all wavelengths from 400 to 700 nm (the typical PAR range) will grow plants. However, there is a myth that is widely propagated on the Internet that claims plants do not use green light. Many companies promote their magical growth spectrum by publishing the commonly-referenced Chlorophyll A and B absorption spectrum chart. Armed with this chart, they mention that plants are green, so plants reflect green light from the full-spectrum light source. Have you heard this one before? Without going any deeper into this topic, it is important to note that there is no magical spectrum that is going to allow a 50W fixture to replace a 1000W fixture because it only uses the “wavelengths that plants need.” While plants certainly have numerous pigments and photoreceptors across the PAR range, nothing will trump the need for delivering the required levels of light (PPFD) to your plants. Spectrum has a very real effect on plant development, but be cautious of a company that spends too much time talking about their special spectrum (especially if they do not spend equal effort in publishing their delivered PAR measurements). There is a short list of companies who manufacture commercial-grade LED fixtures for the professional horticulture industry, and none of them market the number of LED ‘bands’ in their fixture.

Rule No. 4: Don’t Look At A Single PPFD Measurement

Let’s take a quick look at Rule 4. Unless you are growing a small plant directly under your light, a single PPFD measurement doesn’t tell you much. By clustering the LEDs closely together and using narrow beam optics, it is very easy for a manufacturer to show an extremely high PAR measurement directly under the fixture. However, unless you are only growing one plant in this exact location, you need to know how much PAR is being distributed across the entire canopy. Since most LED lighting systems centralize the LEDs into a small fixture footprint, these systems naturally produce very high PPFD levels directly under the fixture. However, these light levels will drop significantly as you move the PAR sensor just a small distance from the main fixture housing. If you are growing over a 4’ x 4’ area, you need to review the PPFD levels over the entire area to calculate the average light level the lighting system is providing. If you only had a center point measurement you may assume a fixture is extremely powerful. However, you would need multiple measurements across the 4×4 grow area to calculate the average amount of PAR that is provided by the fixture. Light uniformity across the grow area varies greatly from fixture to fixture, and unfortunately, most manufacturers do not publish complete PAR maps. It is easy to produce high PPFD numbers directly under the fixture, but it takes a very powerful and well-designed light to deliver high (and uniform) PPFD values across an entire canopy.

Rule No. 5: Don’t Focus On The Wattage Of The LED’s

Do you use 1W, 3W, 5W or 10W LEDs? We are asked this question on a frequent basis, but the wattage of the LED does not tell you anything meaningful about the lighting system’s performance. Since LED and fixture efficiency varies widely, the wattage of the LED is not a meaningful metric. Remember, the LED wattage is a system input, and growers care about the system output. Hence, the LED wattage doesn’t tell us anything about the system’s ability to deliver light to your plants.

As a simple analogy, the LED inside a lighting system is equivalent to the engine in a car. By itself, the horsepower rating of the engine doesn’t tell you how fast the car will go. Pair a high-horsepower engine with a poorly designed transmission, and the car will not go very fast. Hence, as far as the driver is concerned, the relevant metrics for a car are related to the overall performance (e.g. 0-60 mph time, top speed, miles per gallon). Any reference to a component inside the car is irrelevant to the driver. It is the same situation with lighting systems. The amount of light delivered to your grow area (PPFD), the electrical watt consumption, and the light distribution pattern are the important metrics to focus on, so ask for more information if a manufacturer wants to focus on the type of LED they use

Note: Since LED quality varies by a very wide margin, it is important to know the brand of LEDs used in the lighting system. There are a handful of world-class LED manufacturers, so make sure you find out what brand of LEDs are used in the lighting system. Assuming the fixture manufacturer has developed a reliable fixture design, higher quality LEDs should last longer if they are not being over-driven to achieve higher light levels.

Again, you are buying light to grow and develop your plants. In our opinion, you want to buy a lighting system that delivers the required amount of light to your plants for the lowest initial cost, while consuming the fewest electrical watts possible. Ask the fixture manufacturer to provide the following pieces on information: PPF, input watts, and PPFD maps for your intended coverage area. With this information, you can calculate: PPF/$, μmol/J, light distribution patterns, and uniformity levels.


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.

Humans Use Lumens

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/m 2 and foot candle uses lumen/ft 2 ).

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, 99.9% of 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/m 2 /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 Efficiency

Photon Efficiency 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 .


I am gonna need a little bit of time to reply to this… :wink:

Ok, @Dewb, in your own words, how do you like the Vero29v7 COBs?


I think when copy and pasting things due credit is deserved to the original source.


I figured it had something to do with Fluence (BML), because they are making studies for this, but i ended up buying COBs because they couldn’t offer a compatible dimmer from the factory, they referred me to some website for me to wire the dimmer… I want something that is 100% ready, specially for the price tag…

And taking the bars on and off, it’s only gonna make the equipment become lose and all the wear and tear involved in putting it on and off, not to mention the fact that it wasn’t gonna have the same foot print spread. There was nothing they could say that could convince me back then. I dunno now.

I love the videos, they make really good advertisement…

1 Like

I’ve had my eye on one for awhile, but the price tag is definitely making me hesitate. Especially with my Canadian monopoly money. Light is very complicated so I won’t even pretend to know everything there is to know but what I do like is that they try to teach people the proper metrics and tools for measuring light instead of trying to confuse to make sales.

I’ve seen quite a bit of grows on Instagram that use the Spyder X Plus and the general consensus seems to be that they are pretty great. There’s also some videos of large commercial greenhouses that use them. I have a feeling the weird shape it’s in has something to do with spreading the light more evenly instead of having a hot spot in the middle and the outsides being significantly weaker. I don’t remember BML older lights having the reputation that the re-branded and improved ones do today. I haven’t heard many people with bad things to say, and cannabis isn’t the only thing being grown with them.

I am going to continue to sit on the fence and daydream of better equipment, but I think I’ll have to learn more about the science of lighting before I’ll be able to confidently make an expensive decision like that. Hopefully more and more videos and finished happy grow reports continue to come out eventually enticing me enough to pull the trigger.


The only thing that threw me off really was the fact that they are not dimmable. The price is also a factor, i bought a 600W Vero29v7 and a 200W Citizen CLU048 for the price of one SpydrX Plus… So…


@MadScientist I have no personal experience with cobs, but I have seen people killing it with them.

1 Like

i thought i had stated it was copy/pasted. my bad
yes this info is from fluence bioengineering. @neogitus


Yes, it should be stated with at least Source: URL link in the footer of the text… Thanks for adding that!


the thing that makes cobs awsome is the fact that they put off the same ppfd as a hid at half the power so the plant gets the same amount of photons as a high power light without the power usage which is why they get about the same weight but the efficiency of the cobs is much much higher an as we all know efficiency=$ an prolly depending on the quality of cob or led prolly might put out more than a 1000 watt



I dont know if i can.
Best price ever, and not Chinese copy



How is that not a Chinese copy?

hi Uncle
Like i bought that (on Aliexpress)
all parts are official
Dismantle as soon as it arrives! To be certain

Like a dozen others articles (all official)

China has understood (for some) that everything is manufactured down there!
They are no longer obliged to make counterfeit, given their cost of manufacture.

My Monoroue was

  • in my country delivery 48 h + Warranty 1400 €
  • Aliexpress 3 weeks waiting for delivery - guarantee 840 € (its worth the cost to wait)

Must inquire about sellers, customer reviews, and most importantly NOT be pressed / the one testing!

Search on forums Fr you will find numbers of people who have ordered and tested!
keywords (COB, DIY, mount, LED) google gives me it first



1 Like

I buy everything in China. I went two years ago to see for myself. I was impressed. the technology is crazy. I asked about the Chinese “stealing ideas” he just laughed and said, " No, we don’t need to steal ideas. company’s pay there best engineers to come here and show us how to build there products for them" I got to go see for myself a few factory’s. top notch shit. problem is that there are several quality’s of the same product. people go, and think they are going to get some good products for a super cheap price. But, even en China quality costs more. So, when the buyer realizes the good products are out of budget, they are forced to buy the poor quality and try to pass it off as good. This is not the Chinese fault. Also, like @Lancien says, do some homework, read the reviews. I had the chinese make me D.E 1000 watt Wing style reflectors. most d.e lights are for greenhouses and have to be very high up.

all is said



As far as I can see, (no pun intended), it seems that the only equipment for measuring plant-available light intensity is either a shitty hydrofarm unit that will break upon opening(acc’d to amazon reviews)…or some stuff that exceeds my frugal budget. I suppose the ghost farmers of old are slapping their fore-heads, insisting that a few cents worth of any seed should indicate “good, meh, or crap”, right?

How do you measure light, senseis?



1 Like