Computing your light requirements

[Hermitian]

• Lumens are a measurement of luminous flux – the amount of light perceived by the human eye. Since light bulbs were originally manufactured for human vision alone it became the standard of measure for bulbs. It is still very important in architectural design and in civil building codes. Most governments require that the lumen output of a bulb be included on the label.
• Watts are a measurement of the rate of energy transfer per second, which in science and engineering we refer to as power. Watts / square meter or Watts per square foot in the context of lighting refers to radiant flux; i.e., the amount of energy being transferred per second to a surface area.
• Example. A popular 23 Watt compact-fluorescent bulb states on the label that it has the equivalent output of a 100 Watt incandescent bulb. This means that the compact fluorescent produces the same lumen output as the incandescent. It doesn’t do this by magically creating energy out of nothing! Instead, that particular bulb is more efficient at producing radiation in wavelengths that stimulate the human eye.
• Plants absorb light from the human visible spectrum, but (depending on the plant) in ratios of about 1/3 of the blue sub-spectrum, about 1/3 of the red sub-spectrum, and about 1/3 of the rest combined. Chlorophyll – whose central atom is Magnesium, is key in this process. For most plants, it is an error to grow them with only a single wavelength of blue and a single wavelength of red. It can work, but plant vitality is reduced. It is also an error to overdose your plants the longer wavelengths of red – the so-called infrared. Plants only utilize the infrared for heat content and too much will cause stress if not overheating.
• Effective output refers to the output power of a lamp in the target spectrum. For plants, incandescent bulbs have the least effective output. An HPS or MH bulb has a nominal effective plant light output of 78%, with the lower wattage bulbs (e.g., 400W) achieving less and the highest wattage bulbs achieving near 82%. The remainder goes mostly to long-wave and heat. So for example, a 900W incandescent bulb has an effective plant light output of about 725 Watts. In contrast, fluorescent full-spectrum bulbs in the 5000 Kelvin to 6500 Kelvin range have over 90% effective plant light output, with the T5 HO 6500K bulbs near 92%.
• Diffuse light is preferred by plants as opposed to coherent light. Fluorescent bulbs output diffuse light, where as lasers are extreme examples of coherent light. Incandescent bulbs, spectrum shifters, and polarizers produce a mixture of the two.
• The ideal radiant flux for Cannabis is between 50 Watts per square foot and 75 Watts per square foot of effective output – projected at a distance of 1 meter. If your bulbs are closer then less is needed, if you are farther than more is needed. The distance relationship is not linear. For example, if you half the distance you still need about 70% of the output.
• Projected area is the entire surface area illuminated by a bulb or series of bulbs. If your bulbs are uncovered (no hood) in a room 15’ x 15’ x 9’ high then the projected area is 990 square feet. If instead you have a robust mirrored hood that focuses upward and side output to the floor, then your projected area is about 225 square feet plus about 5% of the remaining 765 square feet for efficiency loss in the reflectance – a total of about 260 square feet.
• Calculate the wattage you need by multiplying the number of square feet your bulbs will project onto by the target number of Watts / square feet, and then dividing by the effective efficiency of the bulb. For example, to achieve 75 Watts / square foot in the 15’ x 15’ x 9’ example room using a reflective hood over T5 HO lamps, we’ll need: 260 s.f. x 75 W/s.f. / 92% which is about 21 kW.

 

[ttystikk]

Very helpful! How does this work when comparing different types of light sources, such as different HID lamps, or plasma? How does it account for the focused lenses that are part of most LED lighting design?

 

[Hermitian]

Very helpful! How does this work when comparing different types of light sources, such as different HID lamps, or plasma? How does it account for the focused lenses that are part of most LED lighting design?

It is effective plant spectrum output watts that matter. As mentioned in the first post, Cannabis plants are looking for a mix that is (in power) about 1/3 blue, 1/3 red (not infrared), and 1/3 of what we consider "visible" spectrum combined.

HID and plasma lamps are designed for human visibility. Take a look at the charts available on the following page and you'll find they produce an abundance of yellow-orange spectra, and in general are "upside down" for plants:


The focusing lenses on LED lamps do help direct output where it is needed. However, they do not amplify power. So for example, a complete 350 W LED system that is projecting onto a 3 ft x 3 ft area at a height of 1 meter is delivering an effective output of

(350 W) x (90% efficiency incl. lenses) / (9 sq.ft.) = 35 Watts / sq.ft.​

If instead the same system is only 1/2 meter above the plants and focused on the same 3 x 3 area, you have almost 50 W / sq.ft.

Note however that LED systems produce light in very narrow spectra and for the most part skip the general 1/3 background spectra that plants need for vitality. So in reality you have

350 W LED system at 1 meter focused on 3' x 3' area ==> 23 W / sq.ft.

350 W LED system at 1/2 meter focused on 3' x 3' area ==> 33 W / sq.ft.​

 

[desertmedgrower]

Hey man... this thread is awesome and exactly what I am looking for.

I am currently running 3 lights and trying to determine how far to set up each light from my canopy.

the lights on either end of the 10x5.5 growing area, are both air cooled HPS 1000 Watters in a Raptor and a Great White reflector (AN bad ass bulb and Digilux on the other) . In the middle I have a 600 w (true wattage) LED with 8:1 Red : Blue LED .

I am trying to decide what the best distance to hang the LED would be mainly, I have a pseudo- scrog / lst system going as to maximize bud growth around the canopy... but the plants get a decent amount of light down low (didn't crowd the canopy too much) and haven't lollipopped yet



I was thinking of making my canopy look like a plateau in the middle so that the plants in the middle could encroach on the LED and get hit from the diagonals from the HPS...

What are your thoughts , as your knowledge in Lumens and lighting far exceed mine. (I'd love to hear the math behind LED lighting and raising the light too Thanks!!)

 

[Hermitian]

(I'd love to hear the math behind LED lighting and raising the light too Thanks!!)

In engineering terms, the "reference value" for full-sun outdoor agricultural crops is 1200 Watts / square meter (total) at a displacement (distance) of 1 meter. The distance of 1 meter is chosen because it is easy to calculate from, and the value of 1200 W/m^2 is what is most commonly observed between 15d north and 15d south about the equator. Narrowing in on the power of the solar sub-spectrum that is utilized by plants we arrive at approximately 550 W/m^2 in humid environments, 750 W/m^2 in arid environments -- which converts to the commonly quoted range of 50 W/s.f. to 75 W/s.f.

As stated in the first post, it is power (Watts in this case) that is the important measure. Radiated power obeys an inverse-square law with distance. If you divide the distance by 2 (cut the distance in 1/2), then to obtain the equivalent power of the "reference" you'll only need sqrt(1/2) ~= 0.7 = 70% as much.

The "reference" measure of 50-75 W/s.f. is for effective radiated power in the plant spectrum -- which is a mix of 1/3 blue spectrum (not just a single wavelength), 1/3 red spectrum, and 1/3 of the minor range (for plants) of 450nm to 600nm. LEDs do not do this. An LED system gives you a narrow portion of the blue, a narrow portion of the red, and typically zero of the minor range. A very generous argument would say that an LED system only provides 2/3 of the plant power spectrum, a more exacting analysis comes in around 2/5.

... the lights on either end of the 10x5.5 growing area, are both air cooled HPS 1000 Watters in a Raptor and a Great White reflector (AN bad ass bulb and Digilux on the other) . In the middle I have a 600 w (true wattage) LED with 8:1 Red : Blue LED .

Let's be generous and suppose your reflectors are nearly ideal and you only get 5% loss on "half" of the emissions. This infers that your effective output from the two bulbs is 2 x (500 + 95%*500) x 82% HPS efficiency ~= 1600 W. Let's be equally generous and say that your LED provides 2/3 of full spectrum, so its normalized output is 400 W. (In reality, its probably more like 300W.) This gives you a total effective plant output of about 2000W at the nominal distance of 1 meter.

Your area of projection is 55 square feet. At a distance of 1 meter you have a radiated flux of 36 W/s.f. If you half the distance and focus the light on the same area, you will increase the flux by a factor of sqrt(2). This works out to about 51 Watts/sq.ft. -- which is just within the range you want provided there is not high humidity in the environment to further dampen the light.

 

[desertmedgrower]

Your area of projection is 55 square feet. At a distance of 1 meter you have a radiated flux of 36 W/s.f. If you half the distance and focus the light on the same area, you will increase the flux by a factor of sqrt(2). This works out to about 51 Watts/sq.ft. -- which is just within the range you want provided there is not high humidity in the environment to further dampen the light.

This was exactly what I was looking for. Now... define high humidity? my humidity is in the 40 % - 50 % range in my flower room...

 

[Hermitian]

This was exactly what I was looking for. Now... define high humidity? my humidity is in the 40 % - 50 % range in my flower room...

That's low, semi-arid, and probably ok for many strains. High is 80%+.

Careful with those HPS close to your plant tops -- they put out a lot of long-wave red that your coolers can't control. Give some consideration to keeping the height near 1 meter and replacing the LED with 2 HPS. That would give you 72 W/s.f. at 1 meter.

I'm sure you noticed that I'm a fan T5 HO 6500K bulbs. You don't need a refrigeration unit to keep your growing area from overheating, and they are very efficient. However, if you've already invested in HPS technology then it is probably cheaper to add-on than replace.

 

[desertmedgrower]

I live in an extreme heat desert environment and that room is on an outside wall... so any way I look at it I pretty much have to cool that room down. I wasn't aware that it was possible to cram enough flouros into that 10x6 Area haha ;)

Quick question... you mind checking out the 600W LED i got from proledsystems and tell me what kind of efficiency I am really getting out of it.?.. It sounds like you are able to cut through the BS much more efficiently than I am haha

OH, and another question about light penetration... I have heard the 1000W hps is much better at penetrating the canopy so should that be taken into effect when setting up your lights and calculating what is needed? Can a flouro and LED that is not of equal wattage penetrate as greatly?

 

[Hermitian]

I live in an extreme heat desert environment and that room is on an outside wall... so any way I look at it I pretty much have to cool that room down. I wasn't aware that it was possible to cram enough flouros into that 10x6 Area haha ;)

Side-by-side at 2 inch spacing will get the desired power density.

Quick question... you mind checking out the 600W LED i got from proledsystems and tell me what kind of efficiency I am really getting out of it.?.. It sounds like you are able to cut through the BS much more efficiently than I am haha

I'm sure their engineers and lawyers are careful with the labeling so that all their claims can be justified. Of course you know that the LEDs themselves are mass-produced for use in fiber optic communication? The actual cost of diodes in bulk is an eye-opener.

But from the engineering standpoint in plant sciences we are thinking of the power-spectral response curve for plants and how well does a light source "fill" that up. The argument can be made (witness the advertisements) that a single wavelength blue diode "fills up" the capacity for the blue spectrum. People who do careful studies will tell you this is not the case, but the advocates of the 1st position will say it is good enough for annual plants. Even if this is true, then the system is still missing minor spectra which are definitely used by plants. This is why I say we can be generous and consider the LED Blue/Red systems to be 2/3 efficient.

OH, and another question about light penetration... I have heard the 1000W hps is much better at penetrating the canopy so should that be taken into effect when setting up your lights and calculating what is needed? Can a flouro and LED that is not of equal wattage penetrate as greatly?

Light penetration is a function of power, coherency, wavelength, and the context of "penetration". If the subject were the use of light underwater to determine what kind of features lay ahead, then we would want a highly coherent light (a laser) with a wavelength that can travel effectively underwater, and enough power to get it to the desired range.

But the kind of "penetration" we desire here is bright indirect light which comes from incoherent; i.e., diffuse light at sufficient power. Sunlight and fluorescent bulbs are diffuse. Incandescents and HIDs and LEDs are partially coherent. To compensate for this, I recommend the 1 meter distance because there will be some interference of emissions across the displacement and somewhat mediate the partial coherency.

So to sort of answer your question: Suppose I have a 1000W HPS and a 1000W T5 HO system at identical distances from identical sets of plants. The HPS will likely deliver more power to straight-line non-obstructed targets at the farthest distance from the source. But in comparison to the T5, it will also create the more strident shadows.

 

[ttystikk]

Hermitian, this is extremely informative. Thanks again for your insights. As many farmers here may already know, I've been hard at work on what I feel to be an improved version of the light rotators that were all the rage up until maybe 10 years ago. While the older gear did a great job of spreading light around, and in fact also mimicked the sun's movement to some degree and helped counteract the effects of leaf shading, people have by and large stopped using them due to a perceived lack of sufficient average light intensity expressed over time as the light moves around its full track.

I am in the midst of addressing this problem with several strategies. For a little background, please visit my thread on the rotator, at Advanced Techniques and Problems, 'Built my own light rotator; Wanna See?'.

Some specific design parameters:

1. I am still moving the light in a continuous circle a bit less than 4' in diameter, like the designs of old. It's placed some 12-15" above the canopy, and moves at about 1/3 rpm.

2. I'm currently using either one or two 1000W HID bulbs in standard xtrasun style sealed reflectors- these allow me to vent the heat from the hoods outside the room. The disadvantage of these reflectors is that they do not allow the light to shine out to the sides much at all, hence my nickname for them; 'flashlight hoods' lol

3. The next phase of my research- it's primitive and not rigorously instrumented, to say the least- will involve replacing the hoods described above with one large Adjust-a-Wing brand reflector, with one 1000W MH, since the unit is currently in a vegetative space.

3a. I'll set up the reflector and bulb so that the wings are as wide as possible and the bulb is as low in its adjustable mount as possible so that it can shine furthest to the sides.

3b. The bulb will also be positioned just 15" above the plants- but keeping in mind that it moves around its circle at about 1/3 rpm. The circle diameter is just under 4 feet. While I haven't held a radar gun to the hoods as they zip by, I'm guessing they're moving on the order of 4" per second.

4. I'm currently using standard mylar sheeting placed vertically around the edges of the canopy to reflect light. I realize its inefficiencies, so I'm planning to replace this with a rigid reflective material- a specular type, like polished aluminum sheet- set at an angle to the vertical of about 20-25 degrees, so that light is reflected downwards to the plant tops near the edges.

5. The entire circle- or 8 sided stopsign, if you prefer- enclosed by the sheeting will be roughly 6.5 feet across. The idea here is to utilize the 'sideshine'- the direct light from the bulb that shines out nearly horizontally from the hood under the 'wings'-in conjunction with the angled canopy edge reflective material to boost the light intensity at the edges of the canopy, even that part furthest from the bulb.

My intention is to provide at least adequate light to the canopy at all times, no matter where the bulb is. The area of greatest concern here is the 'dark crescent', furthest from the bulb, where the direct light intensity may attenuate to a level inadequate for strong growth.

Conversely, the area directly under the bulb will be getting an intense blast of light, perhaps even greater than the sun's maximum intensity, but never for very long since the hood is continually moving. Also, the movement of the hood means that plants get excellent penetration and leaf shading is no longer an issue.

My early results are promising, in that even simple and relatively inefficient versions of the canopy edge treatment and the hood still create an apparently even pattern of growth across the canopy, judging by the growth of the plants themselves.

In the past when I used a similar setup for flowering, I got very good yields per watt, and the intermittently high light intensity contributed to high resin output on the plants...

How might I go about using some math to get a more precise picture of what I'm doing, and perhaps some instrumentation to gain a clearer picture of the lumen flux through time?

My gut feeling- and that's all it is at this point, since I admit I don't yet have hard data I can point to for someone else to replicate, is that while the grams per square foot of production may fall somewhat, it will not fall by the 50% one might expect when doubling the square footage of growing surface. Also, I feel the grams per watt will surge over that of stationary setups, perhaps as much as double. How might I go about getting some more precise and accurate descriptions of what I'm doing?

 

[Hermitian]

ttystikk -- I understand your light rotator is a labor of love and many years hard work. So I'm feeling remorseful about giving you negative feedback. Please accept my apology.

Plant growth is due to accrued energy input. When the light moves away, you introduce a delay at best, and for annuals a reduction of growth. Plants do not have a quick-acting metabolic system like humans. When a moving light returns, it takes a plant several minutes (hours if we include the roots) to have all its processes functioning at full potential.

Another misconception about "moving lights" is due to confusion between lumens and watts. When you see a moving light system in action, it appears that a lot of light is being spread around. Well, that is true if we are considering illumination. But if we measure the power (for example, in watts / s.f.) then the situation looks poor. The fact is, if the "moving lights" systems actually worked as advertised, we would put solar panels instead of plants under them and sell the excess energy that we magically create!

 

[ttystikk]

You know, if the results I see with my own eyes squared with what you just said, I'd be much more inclined to believe you.

The trouble is, they do not. So now what?

 

[Hermitian]

There have been many studies of moving light systems, both amateur and academic. None of these have resulted in "acceptable results" for a crop. No successful grower in agriculture is using a moving light system.

From an energy standpoint alone, you are in denial. Like I said, if what you are proposing actually worked we would put solar panels instead of plants under the moving light(s) and sell the extra energy we magically create.

 

[ttystikk]

There have been many studies of moving light systems, both amateur and academic. None of these have resulted in "acceptable results" for a crop. No successful grower in agriculture is using a moving light system.

From an energy standpoint alone, you are in denial. Like I said, if what you are proposing actually worked we would put solar panels instead of plants under the moving light(s) and sell the extra energy we magically create.

Hey, I'm not married to the idea, I just see good results when I use the rotator. I still think the way people set up and use rotators have flaws, so I'm interested in where I can find some of these studies you mentioned. If they're set up under different assumptions than the one's I'm using, I can easily see where they might get poor results.

Now, I'm not talking perpetual motion machines here- I am not thinking I'm 'creating' any extra watts. Hence the solar panel wisecrack is funny but off base, lol. Besides, the damned things are only 20% efficient or so... I'd have to have a LOT of 'magic light' to make that one come out ahead!

The advance that I'm researching has nothing to do with placing or keeping plants in partial darkness, at least not in any different way than others are shutting the lights off for 12 hours to induce flowering. That is a common misconception but I already know THAT doesn't work. It is erroneous to equate shutting lights off with the work I'm doing. The two are not the same in any way.

Two big differences stand out about the work I'm doing vs. the research I've seen and the discussions I've had with others. One; With the light down so close to the plant, it would burn them if it was stationary. Therefore, the plants are intermittently getting MORE lumen flux than they would in a standard stationary lighting setup. Two; the design of the canopy edge, the reflector and the careful management of distances within the system combine to ensure that plants do not fall below the minimum threshold for good production at any time. The idea that they need constant intense light to properly do their business is also questionable- after all, plants do fine on partly cloudy days, and always have.

I appreciate your opinion, and the fact that many share it with you. Yet, what I asked you for was not a blanket pronouncement, but rather some assistance with the math involved so I can gain a clearer mathematical picture of what I'm trying to do.

Just to make one last point absolutely clear; if you think I'm laboring under some misguided idea that I am creating light where none existed before, I'm here to tell you that you're seriously misunderstanding what I'm doing. I am working to boost the efficient utilization of the same light, so that it gives better results. Nothing magic about that, unless you think jet engines are magical in their ability to better convert fuel to thrust than internal combustion engines.

 

[ttystikk]

You know, if the results I see with my own eyes squared with what you just said, I'd be much more inclined to believe you.

The trouble is, they do not. So now what?

To be more specific about the improvements I see when using the light rotator approach;

1. increased resin prodution, especially in the path of the increased light intensity

2. better quality bud production, even lower down on the plant's structure where if the light weren't moving, the light would not penetrate

3. better and more even light coverage, with much smaller or even no differences in grow rates and quality from center to edge of the growing canopy

If there are other potential causes for these phenomena, I'm interested in knowing what they are so I can test them to see if they're the cause of what I'm seeing, or if I can rule them out as alternate explanations.

Again, I've seen research on intermittent lighting. That's not what I'm doing, so it's not a fair comparison.

I've also seen many moving light setups and most of them have serious flaws, which can be traced back to a situation where the plants beneath them suffer from inadequate lighting at least part of the time. I've addressed this flaw, so again it's not a fair comparison.

I have yet to see where a stationary light can deliver a very high lumen flux, that is, higher than broad daylight, and not adversely affect the plants. They always seem to be burned somehow.

If I'm creating anything with the light rotator system as I've set it up and am working with it, it is that I'm able to intermittently boost the light intensity beyond what the plants can handle on a continuous basis, and they seem to respond positively to to this input.

 

[Greeninthumb]

watts per square meter is an archaic term. Micromoles is what you need. HID lights are not for humans, all lights can be measured in micromoles so use micromoles and you'll be fine. I work with some very large growers and one that tested LEDS recently said his potency decreased because of the lacked spectrums. Research labs are also looking at this. LEDs are fine for propagation but if LEDs where all the rage why are almost ALL commercial growers and MJ growers in Colorado and other locations using them? I tested an LED and it failed in less that 250 hours of use. This is not the norm, but it happens. Anyone that can produce an LED is doing so to grab some sales. IMO I would stick with HID like the professional guys do.

 

[wobbly goblin]

you be schooling crickets homie
because all these cats are no longer here

fwiw

 

[Greeninthumb]

you found it alright :)

 

[caveman4.20]

Lol.....so consensus is plants do photo synthesize fast enough for moving light within the same recommended distance not if distance is doubled like most experiments,
For example
Mover 1 bulb inside 4x4 = dope improvement

Mover same 1 bulb inside 4x8 = failed experiment unless you apply my patented secret ninja technique ....jk

 

[Greeninthumb]

The one good thing a light mover WILL do is let the light hit different parts of the plant possibly making a bushier plant, but I agree stretching a fixture doesn't do much good IMO.