Recommended Manufacturer Hang Height .. and why I think it’s wrong

Indoor Growing Lighting
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In almost all cases, I think moving the light closer is a bad practice and generally bad advice. However, I’d like some input from the audience, especially if you have a background in STEM or have gotten more than a few grows under your belt. A lot of people just like to yell “inverse square law” and stop there, but I think that’s only half the story. Here’s what I’m thinking:

Consider the thought of you holding a PAR meter at the top of a tree outside on a sunny day. Now hold that same par meter at ground level. Chances are the PPFD will not change because the sun is 93 million miles away and moving your meter a few feet up or down won’t do much to change the readings our devices can register.

Same experiment, now think about our grow tents. Imagine a situation where you can achieve a target PPFD of 500 (top of the canopy) at both 18” of hang height and at 36” of hang height with the same light. I’d argue that the situation where the hang height is 36” (while still giving off the same PPFD) is far better.

The reason for this is that in the 36” hang height situation, the light needs to put out more power to make up for the increased distance from in order to achieve the same PPFD as the 18” hang height. Because of the increased power need to achieve the same PPFD, all that excess light energy leads to better penetration into the canopy. Think back to my tree example and you’ll see that the further away the light source is, the less of a disparity in PPFD at top of the canopy versus bottom of the canopy.

To prove this, try it with your own meter. Replicate the same experiment, and once you achieve 500PPFD at each respective hang height, move your meter exactly 1’ down and take note of the reading. You’ll clearly see that your number plummets way faster at the 18” hang height, meaning it has decreased penetration into the canopy.

So…. Assuming your light is powerful enough to achieve the target PPFD at any height in your tent, why wouldn’t you hang it from the highest possible position?

Note 1: this situation does not account for efficiency, so in this experiment I’m purely observing the situation that would produce the best, most consistent light levels, not the one that would make my electric bill as cheap as possible (however, when you consider street / dispo prices of weed, I’d argue saving every penny on the light bill is like stepping over dollars to pick up pennies).

Note 2: this situation also assumes you have more than enough Watts of energy to achieve target PPFD from nearly any hang height. In a situation where your knob is at 100% and still not hitting target PPFD, then bringing the light closer is the only option.

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Nice OP.

I wouldn’t call myself an experienced grower; however, I have successfully grown multiple autos including some QP plants, all in organic soil, with only tap water. I run 18/6 seed-to-harvest, and only need my light (depending on plant heights) to max out at ~60% the whole grow at 20” away, otherwise I tend to cook them…so this is a pretty big cost savings over further away and turned up more for the same PPFD.

What are the chances that what I grow would be any better? I grow pretty good stuff but would be genuinely curious if you really think it’d make a difference for the cost increase/energy use?

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I haven’t thought it through quite as thoroughly as you have, but I’m a big fan of keeping my fixtures up higher above the canopy and on a stronger intensity because aside from the benefits you’re talking about it also spreads the light footprint out over a larger area. It helps when I mess up and overcrowd my grow spaces.

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I don’t think it would make the quality better per say, I think if anything the growth toward the bottom of the canopy that starts to look a little popcorn-y will be more full and well developed like the top of the colas. In other words, better yield due to the fact that the PPFD readings drop much more slowly when the light is higher versus when it’s very close. I think growing with the lights as close as possible at the lowest possible wattage to reach target PPFD at the canopy is the worst way to grow weed with respect to yield.

But again, I think everyone should try with their own meters to check and see for themselves. I didn’t fully believe it until I tried it myself because it goes against a lot of internet wisdom that says “bring the light closer.”

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The problem with this is that simply moving your light doesn’t really change your photosynthetic photon flux density, just the concentration at any single given point you’re measuring. If you have a 1m² and a light with ppf of 1000 umols/s, by definition the ppfd is 1000 umols/s/m² at 12" hang hight and it’s 1000 umols/s/m² at 36" hang height.

The reason one height may be better than another is because based on the size and layout of light emitting surfaces that light pattern will change. If light is too low the shape of light pattern could potentially not cover entire plant(s) or be too focused on a small portion of a plant(s). If light is too high the light pattern could be larger than canopy or hitting tent walls instead of canopy which will waste potentially useful photons.

This is better because in order to get a single point ppfd of 500 umols/s at 36" you need more light than you would to do it at 18". More light over same area is almost always going to be superior, up to saturation point anyway. This doesn’t really have anything to do with light penetration as light penetration will see the biggest increases be increasing the size of light emitting surface and canopy entry angles not increasing power. Light goes around leaves much better than it goes through them.

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Funny this is being mentioned. I was just talking to Cory at HLG about this same thing. The cone of light that emits downwards over the plants. I called them because when i was calibrating the Photone app again, after a recent update, i was unsure of exactly what heighth i should have my lights to get the best reading. Also, I’m using 3 of their lights in a 4×4 tent. I was trying to picture in my minds eye how each lights cone would overlap anothers and what would be the optimal height for each to get maximum overlap and light deapth into the plants. I believe in hanging the lights as high as possible also. This is the way to go i believe. From all the data I’ve been studying and using plain common sense tells me this. I dont need to be an experienced Cannabis grower to understand this. Synthetic lighting has been around for alot of decades and its not hard to understand that having the light hanging far enough away will increase the amount of light that spreads out deeper over the entire grow space. Instead of having it closer to the plants where its more of a concentration of a beam of light. We are not growing under flaslights. We are growing under plates of light panels that simulate the Sun. Its just how i was able to figure this out, by comparing it to a flashlight. The closer i move it to the object, the less light that shined over the area but became more intense but just in one area. The further it moved away the brighter it got and created more shadows. The more shadows, the more light.
Its interesting to learn that the further the light the deeper it penatrates the canopy down into the smaller budsites… this is helpful in my quest to find the optimum heights i need for my lights…

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@dbrn32 @MartyMarajuanaseed I’m going to perform an experiment tonight at various hang heights and report my findings. Hopefully we can generate some good discussion once I have some actual numbers to work with. Light is is the component I’d say I need the most education in, so I really wanna understand how this stuff works a lot better since I’m getting nutrition, watering, and some of the other basics down a lot better.

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I really like a lot of the explanation and thought here. I’m certainly no expert so I’m glad some of my theories seemed to track with what you and @dbrn32 have mentioned. I think about this stuff WAY too much when I’m smoking :rofl:

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Alright y’all the results are in. Experiment was conducted with a Mars Hydro FCE6500 in a AC Infinity 48x48x80.

Scenario 1 was a 12” hang height and 12” canopy (meaning bottom of canopy 24” from the light)

Scenario 2 was a 48” hang height and 12” canopy (meaning bottom of canopy 60” from light). I intentionally made this one way past any manufacturer to definitively test my theory in a grow space.

In both scenarios a PPFD of 500 was achieved at the top middle of the canopy as the reference point. The light was dialed up or down accordingly to reach this number. Obviously, scenario 2 required more power.

In scenario 1, average PPFD levels dropped by 30% in the top corners and a whopping 47% in the bottom corners (yikes).

In scenario 2, average PPFD levels only dropped by an average of 18% while the bottom corners only dropped by an average of 30%.

Definitively, higher is better if you have unlimited power from your light source. Of course, in situations where you cannot get to target PPFD even at 100% on the knob, then bringing the light closer is the only contingency.

Scenario 2 easily adds an ounce of two in the corners of your tent where larf would normally be.

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Interesting find… so, the further the lights are from the canopy, while still maintaining the required PPFD, the more intense the light will be in the corners and through the canopy.
I was noticing in my tent, that there would be a significant drop in DLI at the corners. Where i had a DLI of 45 at the center of my tent at canopy level, the corners would drop to the teens! I thought how could this be and shrugged it off as a faulty meter. But, it is because i have my lights too low. From this test you conducted, it shows me that i need to raise my light to get better light distribution deeper into my canopy and corners of my tent. Thank you for sharing this information. It does male sense.

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Not necessarily. There could and probably will be a point where most lights could be too high and you would start to see this swing back the other way some. And potentially see the overall photon availability drop too. Tent walls are reflective to help combat this, but nothing is 100% efficient at reflection and some of these photons would be gobbled up as heat losses.

This goes back to my earlier comment about the physical size and characteristics of each light playing into the ideal height above canopy some. Someone used flashlight as example and that’s very good way to look at it. If you’re trying to cover a 10’x10’wall with flashlight you can’t have it right up against the wall. But you don’t want to be 50 yards away either. Depending on the flashlight you have the ideal distance might be 15’. If you got a flashlight with larger beam from different reflector or used 4 of the same flashlights then this distance would obviously change.

Long story short, there will be ideal height ranges for each light in each canopy size.

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I definitely agree with you. This is just a hunch, but wouldn’t the scenarios that would lead to this probably be uncommon, at least for the typical home grower? 48” hanging height seems like the upper end of what most people could reasonably achieve in a grow tent, especially when you consider a five gallon pot or DWC system probably adds another foot to the plants’ overall height.

Outside of theory and focusing purely on practical application however, I still think “higher is better when not limited by wattage” is nearly 100% true for most home growers because I’d venture to guess the majority of people aren’t growing in a tent that’s significantly taller / larger than 48x48x80”.

For the effects you mentioned to be a major concern, wouldn’t you need a tent that is way taller or a light that’s comically small (like a single TS1000 in a 4x4)?

I still want to test the flashlight analogy you mentioned because i think it has tremendous value… what you said about the footprint of your grow space exceeding the effective footprint of the light source has me thinking. It makes 100% sense logically. I’d be curious to see how an underpowered light (which will serve as my “flashlight”) compares to a strong enough light that’s sitting right over the canopy.

Which is worse I wonder? I guess the reason I’m interested and invested now is because sometimes people want to grow their own medicine, but only have a modest budget to work with. Racking and stacking these different configurations from best to worst might help some growers make the most out of a less-than-ideal situation, such as falling for the “2000W light that’s only 200W” trap on Amazon.

When I made that same mistake (on top of kicking myself for wasting money) it definitely would have been helpful to know how to optimize the performance of an underpowered light in the meantime while I saved up for a proper light.

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Not really, but you are most likely just thinking from perspective of your equipment. Most of lights currently marked are rip-off’s of the same few models from larger companies, but there are still some high powered lights that are relatively small and large lights that aren’t necessarily all that powerful. I have a 360 watt lighy that is about the same size as ts1000 and uses much better components. There would surely be significant differences of having each in 4x4 tent. You have to remember that something like a 1000 watt hps light easily fit into a 2x2 tent. While it makes no sense to do that, the principal is the same.

This is pretty easy. Put in a dark room and start something like 6" above floor then put piece of tape at edge of light pattern. Then raise and repeat. The pattern, cone, beam or whatever you want to call it will change, but number of photons from light wont.

If you are a guy you should be familiar with the term “there’s no replacement for displacement”. Same thing applies to grow lights, only the equivalent is your ppf. That is photons on your canopy. If the choice is having the right amount of light vs other features you would much rather have the recommended amount of light in a less desirable configuration than not enough light in a more desirable configuration.

This also has multiple ways of looking at it. I wouldn’t say there is a way to maximize having an underpowered light, but you usually have some options to consider. Running the light higher will increase throw of light allowing you to cover more area which is typically going to yield more. But if light density is low your buds will be a little more airy than they would covering more properly sized canopy. Whether you end up with more weight either way will probably vary on some other factors. But I’ve seen qp’s inside of quart sized ziplock bags and ounce of fluff that filled more than half a gallon bag.

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I think we’re getting into the weeds here (no pun intended) with theory vs practical application and getting away from the main point.

My original theory was that in a grow tent, higher hang height is better if you have unlimited watts. By better, I mean more consistent PPFD through a notional 12” canopy as the light gets brighter and moves further away. I still haven’t seen any real numbers, examples, or evidence that disproves this (so long as you have enough wattage). Sure, we can sit and list off unlikely scenarios that would show there are some rare, narrowly tailored exceptions to this, but that isn’t reflective of reality or practical application.

A light can be as small as a quarter, or large enough to fit the whole grow area like a bar-style fixture. Neither of these two things change the fact that higher is better with unlimited watts.

*note: by unlimited wattage I mean your light has enough power / efficiency to reach target PPFD from any height in the grow tent.

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Well then I disagree. Watts and single point ppfd measurements are useless without context. And you can’t simply say having a light higher is better as a blanket statement and have it be accurate

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What do you disagree with with? I’d be happy to run this by my physics department head and see where I’m wrong. At the end of the day I need numbers my man, I provided them and evidence. But I’ve yet to see the contrary.

I’m not trying to be argumentative but why does the cannabis community seem to be so resistant to new ideas? It’s like the unspoken rule that going against years of internet and basement science is sacrilege in this hobby. I’m just trying to find a scientifically sound answer to better help myself and others who come to me for guidance. I don’t like being the guy talking out of my butt, which is why I so rigorously question and test a lot of these age old beliefs in cannabis growing.

It’s not to be difficult with people, it’s just a “well then show me the evidence” kind of deal.

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I’m not saying that, I’m saying that is true when you have unlimited watts. Theres a big difference.

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I still disagree. On one part because watts don’t really have much to do with anything, on another because there will always be a limiting factor to how much power you have available, and the last because wasting light would be silly.

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You are essentially saying keeping the light further away and turning it up will cause less light fall off?

Yes this is correct and this is how energy works. You will see less loss from the top of the canopy to the bottom of the canopy, you have more driving force.

Just like speakers, ear buds are loud when they are close but a few inches away and it has lost its effectiveness. Take a larger speaker and it can be heard 100 yards away and a few feet will not make a difference.

A bigger light further away is always the better option, better coverage and less lighting drop off.

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I looked at this a little while back with one or two of my fixtures & noticed that I will get a stronger & more even pattern if I run at 100% & hanging higher vs. 75% & hanging lower to maintain the same ppfd at the center.
Just keep in mind that a given photon doesn’t gain or lose energy with a power change at the fixture, & that a given photon won’t lose energy due to having to travel further in order to hit something. The fixture runs like a strobe light & you are increasing or reducing the amount of photons produced over time by regulating the strobe effect with a power change. So you have to look at the spread patterns of the individual LEDS, the spread pattern of the overall fixture, & the reflectivity off of the enclosure walls to see what causes the ppfd to stay the same at the center, & to increase outwards if you raise the fixture & the power to keep the ppffd the same at the center. You’d be getting the same amount of photons landing beneath the center of the fixture, just from more sources. You’d probably be getting less photons landing below the center from the leds directly overhead the center of the plant, but more photons at the center of the plant from the leds around the edges of the fixture. The ppfd also might stay at or closer to the highest number going further out from the center of the plant due to a better overall spread pattern, & the edges & corners can gain if they are in a better overall spread pattern. Reflections off of the walls should get better also. Hanging too high also moves you out of the ideal spread pattern & you start sending more & more photons into the walls alone. I would guess that stuff below the canopy should benefit if you’re in the ideal spread pattern of the fixture & throwing more photons down.

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