How do I hang a LED grow light? How far should the lamp be from my plants and canopy? Should the distance between grow light and plants change during a grow cycle?
These are all fair and commonly asked questions in the indoor growing community. Let’s address them.
When it comes to LED grow lights, sadly, there is not one rule or distance that fits all lamps. With our old and beloved HID/HPS lamps, the light output was very similar between the different bulb and ballast brands. This cannot be said for LED grow lights. Even two LED lamps that draw the same amount of power (watts) usually have vastly different light output and light coverage.
To understand what distance a lamp should be hung at, we need to know how much light said lamp puts out. Light quantity is measured in PPFD (sometimes also referred to as PAR). The PPFD value tells us how many photons hit a specific area, a canopy, for instance, and we measure this value with the unit umol/m2/s.
All light consists of photons. The more intense the light, the higher is the number of photons. With a quantum meter, an Apogee MQ-500 for instance, we can measure just how many photons hit a specific area.
There are, of course, other brands of quantum meters. Some meters are incorrectly marketed as quantum meters even though they are actually just a light meter (measuring lumens) that converts the result into umol/m2/s. Not very reliable. The point I’m trying to make is that if a seller is showing a lamp’s PPFD values, it helps if the seller also reveals how the PPFD values were measured. How reliable is the information?
Set up your lamp to deliver the right PPFD
You cannabis plants will need roughly these levels of PPFD:
Seedling/clone: 100 umol/m2/s
Early veg: 200 umol/m2/s
Late veg: 400 umol/m2/s
Late flower: 600 umol/m2/s
The higher the PPFD value (umol/m2/s) the more intense is the light. A mature plant can absorb and handle more light than a tiny little sprout.
What’s important to remember is that the PPFD values decrease with distance. How? Why?
PPFD is, as mentioned above, light intensity/quantity. Imagine standing an inch from a bright lamp and looking into it. It would be a struggle for the eyes. However, if we take a couple of steps back and look into the lamp again, the light is much less intense to our eyes. The closer we are to a lamp, the more of its emitted photons will hit us, or any surface for that matter. If we move further away from the lamp, the emitted photons will instead spread over a larger area, i.e. they become less dense, thus the light will not feel as intense. Same goes for plants. This might make more sense with a few examples.
If we know what PPFD levels we need to give our plants, we can find a lamp that delivers this light output. Lamps’ PPFD output should be specified by the manufacturer, preferably as a light footprint map or a PPFD chart. Let’s take a look at a few models.
- HLG 65:
HLG is one of the big players in the industry and this is the light footprint map of their HLG 65 lamp. It’s priced at $99 and while it’s very efficient, it’s total light output is good but not great. Although understandable as the lamp only draws 65W.
We see two maps, one measured when the lamp was hanging 12” above and one where the lamp was hanging 18” above. Same lamp, same draw power, different PPFD levels based on height.
Looking back at the PPFD levels that cannabis plants require, we see that at a 12” distance, this lamp doesn’t fully stimulate a mature cannabis plant. The plant would certainly grow, but it could take a bit more light than this lamp is able to put out. However, if the lamp is only meant to be used for the veg cycle, we see that the lamp delivers more than enough light.
At 18” we see that the 2×2’ area underneath the lamp is well covered with PPFD levels from 140-290 umol/m2/s.This would be good for early and mid veg.
2) Viparspectra VP600:
Here’s VP600’s light footprint map. This lamp costs $149 and draws 265W. Although the distance between lamp and sensors, according to the map, is 22”, the spread of light is rather poor for a lamp drawing 265W. The PPFD value in the center, 719 umol/m2/s, is high enough to stimulate photosynthesis in flowering weed plants, but the light coverage area is rather poor. We see that the PPFD values fall from 719 (highest) to 409 (lowest) even within the center 1×1’ square. At the 2×2’ square we see PPFD values as low as 142 umol/m2/s.
This tells us that the 1×1’ area directly under the lamp when it’s hanging at 22” is okay to flower in but the outer edges of a 2×2’ are not getting enough light to ideally stimulate photosynthesis.
The takeaway is that this lamp would probably flower a 1.5×1.5’ area well at 22”.
As the manufacturer does not show any other measuring data, we don’t know what height we should set the lamp for if we’d be growing clones, seedlings, or plants in the veg phase.
My guess would be that at 30” the PPFD in the center would be around 400-500 umol/m2/s which would be good for late veg or early flowering.
3) Bloomspect 1000W:
Here’s a different type of light footprint map compared to the previous two. This one is one dimensional and we only see one single PPFD measurement which is in the centre spot right underneath the lamp, but at various heights.
A false conclusion would be to assume that the center spot value us also valid for different spots off-centre, i.e. one foot to the left. This is far from true. As the lamp is rectangular the emitted light won’t even spread in an even fashion so estimating the lamp’s actual light footprint becomes even more difficult. We simply have no idea how good the reach is. The values a foot from the centre could be half, or a third, or less. Without certain information, setting up this lamp would be mere guesswork and the results would likely be affected.
It appears that the centre spot PPFD value at approx 20” would be around 700 umol/m2/s, which is a good PPFD value for late flowering, the left out information simply doesn’t make this lamp a reliable tool.
We strongly advise against purchasing a lamp without a clear light footprint map.
This manufacturer also has a different but seemingly very similar lamp.
It has almost the same draw power as the previous one 268W compared to 278W and has the same number and size LEDs.
Yet, if we take a look at both PPFD charts, the results are VASTLY different.
At 18” inches, the second lamp has almost TWICE as intense light as the previous model. At 30”, this lamp has more than twice as high PPFD value than the previous model.
Why? Optical focusing lenses. These lenses can be fitted right after the diodes and will focus the light. Instead of the photons spreading as the fall, they will instead be beamed downwards in a very straight line. While this isn’t necessarily a bad thing, it will create for a MUCH smaller light footprint. As the photons are “organized” to fall straight down by the lens, the coverage area will decrease dramatically. If the goal is to only illuminate a small area underneath the lamp, then focusing light can be a good thing. However, it makes absolutely no sense whatsoever to focus light to get 2650 umol/m2/s or even 1680 umol/m2/s. Frankly, even 1110 umol/m2/s is over the top. This lamp would burn everything that is put directly underneath it but give no, or very little, light to plants a bit off-centre.
The seller doesn’t show a 2D light footprint which, according to me, can only mean that he wants to hide all values that are off-centre.
Conclusion – what distance is right for me and my LED grow light?
As demonstrated above, there’s not one single magic distance that works for every stage of the plant’s stage or for all lights. You must do a bit of research and find a lamp that fits for you. To understand if a lamp is a good fit, you need to know what PPFD (also referred to as PAR) levels it emits. Wattage, lumen, cost, etc are not enough to judge a lamp. You need to know its PPFD output and preferably over a 2×2’ or 3×3’ area as well as measured at several distances. Then you can set it up at the correct distance for whatever stage your plants are in.
