How To Set Up Your LED Grow Light
Distance affects light intensity and light footprint, as well as how heat is generated.
Learning how to set up your LED grow light is highly relevant to anyone who has bought a grow light or is considering buying one.
In a previous article, we discussed what to look for when buying an LED grow light. We covered terms like PPFD (PAR), light footprint, spectrum and efficacy, so we won't be focusing too much on these features in this article.
If you've just bought a grow light or are about to pull the trigger, this one's for you. We'll go over how to hang the lamp, at what distance and how determine light intensity (PPFD, or "PAR"). We'll also discuss heat and heat management.
The latter topics may be especially useful to newly converted HID growers, as LED lamps typically produce less heat, which allows them to be hung closer to plants’ canopies.
An Intro to Grow Lights
Grow lights come in all shapes and forms. HIDs have been used for decades while more recently, LEDs are becoming a popular alternative. Within the LED category there are different types of lamps, fixtures and bulbs. There are the standard LED grow light fixtures that typically have a metal or plate chassis, a couple of built-in fans and drivers, and sometimes a heat sink.
In recent years, we've seen COBs (circuit on board) as well as the "quantum boards" (QB), which are essentially an LED panel with a driver and sometimes a heat sink, rise in popularity. As we make technological advances, we achieve more and more efficient bulbs and diodes, which means less energy (watts) is consumed to produce the same amount of light output.
Still, it's the consumed or drawn watts that determine how much heat the light source generates as wattage is proportional to BTU (British Thermal Unit).
A second variable that is universally applicable for all grow lights is the relationship between distance and PPFD (light intensity). The shorter the distance, the more intense the light (higher PPFD). However, at the same time, the light footprint (coverage area) also decreases in size.
When growing indoor plants, cannabis included, the plants will need different light intensity during their grow stages. Clones and seedlings need much less intense light than mature plants, which are producing buds or fruit. Adjusting the lamp up from the canopy will decrease the intensity of light (decrease PPFD). Adjusting the lamp closer towards the canopy will increase light intensity (increase PPFD).
It's also important to understand how light spreads over an area; a lamp's so-called "light footprint."
Preparing the Light To Be Hung
This one is simple. Unpack the LED grow light and find its hanging points. Typically, a grow light will come with a metal wire hanger that screws or clips onto the lamp's body. The metal hanger hangs from a rope ratchet (typically not included with grow lights) from a grow tent's or grow space's ceiling.
The metal wire hanger can vary in length. If you realize that the included hanger with your light is a bit too long for your tent or grow space, shorten it by guiding the metal wire around the frame of the grow tent.
Heat from a grow light, regardless if it's an LED or an HDI/HPS, travels upwards. The more wattage a lamp consumes, the more heat it'll generate. For example, if you just switched from a 400W HPS to one or two 100W LED grow lights you'll see a drastic reduction in heat produced.
This can be a good or bad thing.
For most growers, at least for those who live in a warm location, switching from HPS to LED could reduce or eliminate the need to run an AC unit that would otherwise be necessary in order to cool the grow area. On the other hand, those who grow in colder climates may need the increase in temperature to give their plants ideal growing conditions.
Assuming you grow in a small and confined area, like a grow tent, with an ambient temperature of 72°F o (22°C), any type of grow light will result in a heat increase. The more watts, the more heat generated. A 100W grow lamp in a 2x2x6' tent will increase the temperature by about 10-15°F (5-8°C).
This is a rather insignificant difference and can easily be managed with one or two small USB table fans where the warm air at the top of the grow space is circulated out of the tent.
Getting the Distance Right and Understanding PPFD (PAR) Values
This is where it gets interesting: A lamp that is too close to a leaf could burn it but a lamp that is too distant from a leaf leads to a less-than-ideal amount of light received and could hinder growth.
Plants require a certain amount of light, or rather, light with a certain intensity.
A high number of photons in an area equals intense light. A low number of photos equals low intense light. The amount of photons in a specific area is determined by the total output of photons (light) from a lamp; whether the photons are angled with covers, reflectors, or lenses; and finally, the distance between the light source and said area.
Many LED lights focus their light with either 120-degree or 90-degree lenses or reflectors. This results in photons being more concentrated in the area underneath the lamp but also makes for a smaller light footprint (coverage area). Most lenses and reflectors absorb or block a certain amount of photons, which means a loss in the total amount of light that could reach the plants.
Read: 10 Mistakes to Avoid When Using LED Grow Lights
All photons have a natural tendency to move about as they are beamed out of the light source. A lamp with its diodes facing down towards a plant's canopy will emit the photons towards the plant. However, as the photons are beamed from the diodes towards the plant, the photons spread. If there is no focusing lens or reflector, they could theoretically spread in a 180-degree direction.
As the photons spread and become less dense, the intensity of the light decreases. The further away the lamp is from a certain point, the less intense (fewer photons) the light hitting that point will become.
Here's an illustration:
Light footprint and intensity changes based on the distance between the flashlight and the table.
Above is another illustration of how the photons emitted by a grow light spread as they fall from their source. We see how the density of photos, which equal light intensity, decreases the further the photons fall. In other words, the longer the distance between the light source and the plant, the less intense the light received by the plant will be.
If the distance between the plant and the lamp would be zero, or very close to zero, the plant would essentially receive all of the photons emitted by the lamp, such as very high intense light.
Now that we understand the theory behind increased distance equals lowered intensity, we are ready to adjust the lamp's distance to achieve desired intensity (PPFD) at canopy level.
PPFD Levels for Growing Weed
Cannabis plants will need approximately 100 PPFD during their clone stage, 100-300 PPFD during seedling stage, 300-500 PPFD during vegetative stage and 600-800 PPFD during flowering. During the flowering stage, a cannabis plant could potentially take even higher PPFD levels if all other conditions (nutrients, CO2, temperature, humidity) are ideal.
For beginner growers, 600 PPFD is a good number to aim for.
Read: A Beginner's Guide to Buying LED Grow Lights for Less Than $200
PPFD is measured with a quantum sensor and the unit is in umol/m2/s (micromoles [of photons] per square meter per second). Exact quantum meters, such as the Apogee MQ-500, do not come cheap and are rarely a worthwhile investment for a small scale grow. Without a quantum meter one has to rely on data supplied by the manufacturer of the grow light.
Even if these numbers aren't always exact and could possibly be inflated from time to time, it's usually the only data to go on.
A video-based PPFD test is much more trustworthy than data sheets but few manufacturers are transparent or thorough enough to record PPFD tests.
This is what a video based PPFD test looks like:
The point? Use whatever data you have at hand but be critical and know that not all data is completely trustworthy. If you have already made your grow light purchase, get the PPFD sheet from the seller. If you haven’t yet made a purchase, try to find a seller that has both a grow light that meets your requirements but is also able to supply exact and transparent PPFD tests.
Below are two examples of how PPFD or PAR values are presented online. The first example is for our own Z5 LED grow light. The second is a random LED grow light from Amazon.
Example 1: LEDTonic Z5 LED Grow Light
- Price: $169
- Power draw: 112W
- Efficacy: 1.3 PPE (umol/J)
Here we see PPFD measurements for various distances between the light source and sensor. We see that the PPFD has been measured at 12" (~30 cm), 18" (~45 cm), and 24" (~60 cm). These values are detailed with their own respective color on a light footprint map.
When the lamp is hung at a 24" distance, the center spot underneath the light receives 343 PPFD. The outer edges of the 3x3' tent receive about 100 PPFD, which is what clones need.
This means that a suitable place for clones would be in the outer edges of the tent when the Z5 is hung from 24". Alternatively, if the lamp is hung even higher, clones could be moved further towards the center of the light as the intensity (PPFD) would decrease.
Plants in their seedling stage will need 100-300 PPFD. By looking at the footprint map, we see that this intensity is reached about a foot (30 cm) from the center spot at a distance of between 18-24".
Once the vegetative stage has been reached, cannabis plants will need more intense light so the lamp needs to be closer to the canopy. We're aiming for 300 PPFD during early vegetative stage and 500 during the late vegetative phase.
This means the Z5 needs to be hung at approx. 18". At this distance, the intensity is ~400 PPFD in a 1.5x1.5' area underneath the lamp.
During flowering, 600 PPFD (or even a bit higher with added CO2 and good plant management) is recommended. We see that at 12", the PPFD is approximately 740 on average (even higher in the center) in the one square foot directly underneath the lamp. Raising the lamp with 2" to about 14"/35cm would reduce the PPFD to around 600-650 and increase the coverage area slightly (these are rough estimates and not actually pictured in the image).
An area of 1x1' or even 1.5x1.5' is pretty well covered at this distance.
In actual plants, this means one large plant or two, three or possibly even four small plants. Depending on how you train and manage your plant and also in what you grow in (pot size, medium), the plant will grow to a certain size. We've seen people grow in everything from small plastic solo cups to massive bathtubs.
Depending on the area that your grow covers, you'll need a light or several lights to illuminate the area your plants occupy.
Example 2: Generic LED grow light from Amazon
- Price: $155
- Power draw: 260W
- Efficacy: No data
Here we see a flat, 2D, PPFD (PAR) chart from a popular brand on Amazon. We could not find any video documentation of the PPFD test so the accuracy of these numbers could be questioned, but it'll do as an example.
The first thing we notice is that the price is about the same as the Z5 but the drawn power is more than twice as high. Yet the light output (PPFD) is roughly the same in the center values under both lights. This means that the generic light is significantly less efficient at converting electricity into light (PPFD), seeing as its power draw is twice as high but the output is about the same.
As a result, the generic lamp will draw more electricity per emitted photons, which also means it will generate more heat than the LEDTonic Z5.
We also see that the light output chart is a vertical 2D illustration and doesn't show a square area. The chart only shows center spot values at various distances and at 24" it shows PPFD values in one direction from the center. As the lamp is rather rectangular (15.7x8.4" according to the listing), the light spread will not be even in all directions from the center spot.
This means that the PPFD chart at hand is not giving us very useful data.
According to the chart at 24", the light produces 549 PPFD under the center, low 400s one foot (30 cm) from the center and low 200s two feet (60 cm) from the center.
We learned above that marijuana requires about 600 PPFD during the flowering stage, so this particular lamp would need to be positioned at about 22" from the canopy. However, the light footprint (coverage) is poor at 24" so the footprint will shrink further at 22" meaning that only about a 1x1' (30x30 cm) - possibly 1.5x1.5' - will get adequate light.
This is the theory behind adjusting the distance between the LED grow light and plant canopy while taking the plant's growth stage into account. Understand what stage your plant is in, find out the PPFD data of your grow light, adjust the distance so the PPFD emitted at a certain height matches the PPFD required by the plant.
If a plant receives too little light (low PPFD), it will stretch toward the light source. A less-than-ideal amount of light will limit photosynthesis and total mass production will not reach maximum.
If a plant receives harmful, intense light, the tip of its leaves will "burn" (turn yellow). This should also be avoided.
Tip burn, however, is a good indicator that your lamp is too close to the plants. If you have no tools or data to measure PPFD, move the lamp closer and closer towards your plants until you notice a faint tip burn, which usually happens after 12-24 hours after adjusting the lamp.
As soon as the tips start to become yellow, you know that the lamp is a bit too close to the canopy. Increase the distance by two inches (5 cm) or so, then the light should be at a good height. Keep in mind that as the plant grows taller, the lamp needs to be raised further.
We want to stimulate our plants’ photosynthesis as much as possible. It is through photosynthesis that plants grow and bear fruit (buds). The right light intensity and light quality have a big impact on photosynthesis so it’s important for every grower to understand how his or her grow light should be hung to give plants the best possible conditions for, well, maximum yield.
As practically all grow lamps out there have different output, one distance cannot be generalized for all lamps. As we see in the examples above, even two lights that have vastly different draw power can have similar output and coverage. To optimize your grow, you need to make the calculations for your specific requirements.
Use data from the grow light manufacturer to determine your lamp's output, then adjust the height of the lamp from your plants’ canopy based on the plants’ grow stage.
Written by Max Hallberg | Lead Product Developer at LEDTonic
Max Hallberg is the lead product developer and indoor growing enthusiast at LEDTonic. Max has tested, modified, and improved LED grow lights to achieve ideal growth and fantastic harvests for all types of indoor gardens. Knowing both hardware and plant biology, Max helps people get the most out of their grow.