Needed lighting concepts to develop LED grow lights

I start this with the purpose of join on a single thread all the basic information needed to develop and understand how LED lights work and to provide a solid base of what is known at this moment about effects of lights on plants.

There are many crap info about this topic on the net, so im trying to offer some basic solid info which allows people to discriminate whats the wrong info widely available at the net, which unfortunately is the most.

First off, its important to differentiate between the photosynthesis and other light driven effect of plants. Photosynthesis is the process by plants transform the light they receive into energy usable for the plant. Its the fuel which allows the plant to grow, and its tightly related to the amount of photons which absorb the plant.

Other effects of light are related to light quality, not to quantity. There are many of this effects, and its important to not forget them, but i let this question to the end. Probably some of this effects will appear when we talk about a photosynthetic issue, but i prefer to concentrate first on photosynthesis, which is the main task of light.

What is the light

Light is a electromagnetic radiation. We call light to the portion of the electromagnetic spectrum which is visible to humans, from 380 to 780nm, which is called the visible spectrum. Below it is the UV (Ultraviolet) and after it, the IR (infrared).

Plants use for photosynthesis a similar range than humans for vision. Both use mostly wavelenghts, which is the parameter which identifies different electromagnetic radiations, between 420 and 685nm, although photosynthesis still take place at noticeable amounts from 400 to 700nm, which is many times used too as a simplified visible range, due light below 400nm and over 700nm are very little visible.

This range, 400-700nm, is called the PAR (Photosynthetic Active Radiation) range. Very often, although is stated the opposite, when we talk about light, we are referring to the PAR range.

A nanometer (nm) is a 10^(-7) meters=1/10000000 m=0.0000001m.

Visible range is divided into the different pure colors:

Violet:380-430nm
Blue:431-480nm
Cyan:481-510nm
Green:511-565nm
Yellow:566-590nm
Orange:591-625nm
Red:626-780nm (its often splitted into near red (or just red), up to 700nm and far red, over 700nm)

(Note: Exact limits between colors arnt clearly defined, so you can find other limits for these colors, but its, give or take, correct.)

A very relevant characteristic of electromagnetic radiations, thus of light, is it behalf both as a wave and as particle. This is called "wave-particle duality". In the practice, this mean we can understand light in any of both ways at our convenience. So when we talk about optic watts of light, we are taking it as wave, and when we talk about photons, we are treating it as particle.

Generally, light is treated as wave when we study how it propagates and behave alone, but as particle (photon) when we study how it interact with matter (as when lights hit the plant).

There is an important practical consequence of using one or the other: energy which carries a photon is inversely proportional to its wavelength (wl). So a mol of photons will carry more energy as shorter is its wl, or the inverse: a watt of light carries more photons as longer is its wl.

Plants dont know nothing about watts, they sense and use light from the amount of photons they receive, independent of its wl. This is the deep cause behind plant uses better red photons (longer wavelengths) than others of shorter wl. And this is the reason botanist uses the number of photons to study how light affect plants, instead of watts.

The unit used to measure the number of photons is the mole, which equal aprox to 6.02*10^23 particles (Avogadro's number). Most times, its used its million diminutive, the micromol, which is 1/1000000 mole. For light, is commonly abbreviated to E (Einstein) or μE (microEinstein, often written uE for keyboard limitations). Although it isnt an official unit, ill use them for comfort.

As reference, a 400w HPS emits about 650 uE (per second) (in PAR, always its not stated in other way).

Light and Photosynthesis. Quantitative analysis

Through photosynthesis, plants gets the required energy to dissociate water molecules (H2O) into its components (H and O) and build, together with the C from the air's CO2 (carbon dioxide), the organic matter used to build the plants. Some other elements are required in small amounts, and they are uptaked by roots (they are called plant's nutrients). So a plant need mainly air (CO2), water and light to thrive.

The process of photosynthesis requires light, water and CO2 and produces O2 (oxygen). So its possible to measure accurately photosynthesis by measuring CO2 uptaken or O2 produced. Photosynthesis is limited for the required factor (light,water,CO2) which is depleted earlier. On indoor growing, it shouldnt be a lack of enough water, and the grower should provide enough CO2 by air renovation or directly CO2 supplementing if want to optimize the grow, so the main limiting factor is the light available. Its a must on a well designed grow room.

Photosynthesis is tightly linked with total amount of photons absorbed. This concept is the base of all, and it should be clear for any grower. So im going to analyze it deeper:

-Amount of photons. Not of watts, or lm. Plants use photons, so the number of photons is the essential figure to consider. The more the photons which reach the plant, the better (up to a limit).

Its important to note that same energy (for example 1 watt) of blue (450nm) have 33% less photons than of red ones (670nm) (450/670=0.67 : as noted before, energy carried by a photon is inversely proportional to its wl) if we take the amount of red photons as base. If we take the amount of blue photons as reference, then 1 watt of red ones carries 49%, near half, more photons (670/450=149). So very often, producing as more red photons possible is the most effective way of using artificial light for growing plants (if the efficiency of producing 1 watt of each are similar).

This effect is what does that plants have adapted their systems to use 670nm photons the more efficiently, as sunlight reaching Earth's surface has higher number of then than of any other wl, while the higher energy (watts) received is of green light.

-Absorbed. Plants absorb differentially photons of different wl, and it depends too slightly of the light density they are receiving. One of the ways of improving plants lighting is by optimizing the light level and the spectrum in order to get the max absorption (and less reflection) of photons possible.

This way is how cannabis reflect different wl:


Note that although the higher reflection is of green (around 550nm), and thats why see the plant green, just a small fraction of green is reflected back, as 15%, and not all as you can read many times on the net.That plants reflect back all the green light is a false statement.

This false statement is found very often linked to other false one, that plants dont use green photons for photosynthesis. Plants reflect back more green photons than of other wl, as they use them with lower efficacy, but as max its used at half the efficacy of red ones. Lower efficacy of green, yes, but its not wasted at all. That green light is wasted is another false statement. Ill analyze this topic deeper later, as its qualitative and not quantitative analysis.

Quantitative analysis II. Photosynthesis and irradiance

Irradiance refers to the light falling on a given surface. It measures the light density at a given point. The unit used is uE/m2 (per second). Its often found as PPFD (Photosynthetic Photon Flux Density).

Its the radiometric equivalence to the more known iluminance (photometric, for humans, as lm) which is measured on lm/m2=lux. Light concepts preceded by a "i" are referred to the lighted object or surface, and not to the light's source.

The term light's "intensity" is too used for irradiance, but i prefer to avoid using it, due there is other light "intensity" concept which is referred to the light source, which is measured in Cd (candles) or W/sr (optic watt per steroradian). It could lead to confusion, thus ill only use the irradiance term, which not only is clearly referred to the lighted surface, but its too a radiometric concept: radiometric units refers to physical entities (watts, photons) while photometric units refers to how human sense the light (lm, cd), thus they are very misleading when used for plants lighting.

There is a relatively simple way of measuring irradiance with a standard luxometer (light meter) knowing the spectrum of the light: its explained on the "Bulb Comparison" thread which is absolutely complementary to this thread.

Irradiance depends strongly on the point where its measured. It drops sharply with the increased distance and its very affected by how the reflector distribute the light. A concept very similar is when we calculate the average light available, by dividing lm by sq meter (or sq ft). If instead of using the photometric lm, we use number of photons and divide it by sq meters where it's distributed, we get an average uE/m2 figure. Irradiance is too given on uE/m2, but it relates to a point with a given position and distance to the lamp while the calculated uE/m2 is just an average of the light thrown to a given space, without taking into account position or distance.

Sorry for the long introduction to the term irradiance, but its a very bad understood concept which is very important to know how light affect plants, thus i want it very clear. Please ask later any doubt about it: optimizing the lighting of a grow is a task mainly of improving irradiance distribution along the grow room so the better you understand it, the better you will can improve your lighting.

(Im trying to condense on a few post what is often studied along a year. If you dont have previous knowledge about this topic, is very probable you dont understand all on a first read, neither on a second one. But trying to understand it worth, and its not as complex as it seems, just take your time and ask for explanations

Photosynthesis behaves on a typical way depending of the irradiance level al leaves, as this graph shows:



(From The photosynthesis 'light response curve')
(Read too for more in deep graphs and explanations on Eutrophication - light and growth)

There is a first part of the curve which is near linear, meaning that equal increase in irradiance level lead to a equal increase of photosynthesis. The slope of this line determine the maximum photosynthetic rate of the plant. This is called the "light limited" part of the P-E curve (photosynthesis(P) vs irradiance(E)), because what limit the photosynthesis is the amount of light. Along levels of irradiance of this part of the curve (the lowest), more light produces more photosynthesis.

But there is a point where the curve goes flattening, called max Photosynthesis rate point, often noted as A. This is the part of the curve called "CO2 limited", because is internal CO2 concentration in leaves what limit P. Plants is using more CO2 than its able to absorb from the air, thus part of the light cant be used for photosynthesis. The higher the irradiation from this point, the more light is wasted. For cannabis, this point is about 300 uE/m2 at ambient CO2 concentrations: at higher CO2 levels, this point happen at higher irradiance, as well as the flattening of the curve is less pronounced, because plant is able to keep internal CO2 higher.

Finally, there is a point when further increase of irradiance dont get any increase on P, wich is called the "saturation" point. If we still increase irradiation, plant finally protect itself from damage due to excess light and P decreases. There are different ways used by plants to do it, but at really high irradiance plants deactivate photosynthetic systems and chlorophyll is retired from the leaves, producing the effect known as "light bleaching" (because leaves becomes white), which is irreversible (permanent damage).

There are many practical consequences of this pattern of the P-E relationship:

-Light use is higher as closer to the max P rate point (A) the irradiance we use, thus higher productivity of light (g/uE).

-The most efficient use of light is when we can distribute the light on a way all leaves works on similar irradiance, close to A. This mean zenital lighting (from top) isnt desiderable ideally, as it produces high irradiance at the top of the plant (thus, wasting light) and low irradiance at the bottom (wasting the ability of those leaves to produce more photosynthesis if they have more light available). Although is desiderable a slightly higher irradiance at top than the bottom, because old leaves are less efficient doing P than new ones, in general we must try as even lighting along all the grow volume (in 3 dimensions) as we can. This is one of the main advantages of LEDs over other ways of lighting: small sources of light may be distributed along the grow, without the requirement of use light from top, as with HIDs, due to heat. NASA has achieved up to 35% higher yields using same light just by moving part of the LEDs arrays from top to side lighting. With our plant, doing it not only increases light productivity, but allows to harvest fat buds from the bottom part of the plant instead of small ones.

-CO2 enrichment is little useful at low irradiance levels, but very useful as higher the irradiance used.

This article models C3 plants P-E behavior on each part of the curve based on the limitating factors very deep. I only recommend reading it with strong botanic background, its intended as a model for predicting P-E after genetic engineering.

PS: P-E curve of C4 plants is different due the different way of keeping internal CO2 concentration, but i wont enter on that as Cannabis is a C3 plant

Qualitative analysis. Photosynthesis and spectrum

We understand light quality for the distribution of wl that emits the light. The graph which show it is called SPD (Spectral Power Distribution), many times abbreviated as spectrum.

Reading other threads about lighting, it seems that spectrum is the only parameter which determines the efficacy of light for growing. But its not true, what determines mainly the efficacy is the energy efficiency of the bulb, which determine the amount of PAR watts delivered. But spectrum affect the efficacy by two ways: changing the amount of photons that carries each PAR watt and because there are wl which are more efficients promoting photosynthesis than others.

Opposite to general belief, differences in efficiency promoting P are relatively small, and almost always is below a 20% for wide spectrum lights (white of different tones). Using LEDs is possible to get somewhat higher enhancement, but its up to 50% in best cases (meaning an optimized spectrum may reach 50% more photosynthesis for the same amount of photons than for example, an HPS). But its very important to note than this enhancement may be applied to the number of photons, which, again, is the base of all. If you use half photons with an average 50% more efficacy promoting P you get a final efficacy of just 75%.

If you use half photons, dont mind how good is the spectrum, you never get as much photosynthesis (thus, plant growth). This is very often forgotten by LED grow lights designers, which largely overstates the importance of spectrum alone. This mistake is due many people use "action spectrum" curves, which are curves build from laboratory absorbency of photosynthetic pigments, and not from measuring P promoted by each wl with live plants.

Main general studies about this topic were performed by McCree and Inada in the 70's of last century. Their results have been repeated many times for specific plant species, so their results are widely accepted as valid. Both performed the studies by irradiating the plants with narrow bandwidths of light (2nm) and measuring the photosynthesis (by O2/CO2 changes).

The most used today is the McCree curve, wich shows P for each mol of photons absorbed. You may find it many times plotted as background of horticultural lamp's SPDs:



The curve of Inada, however, shows the P promoted by watt of incident energy, and not of absorbed photons. Thus, it gives together the effect of photon's absorbency and its efficacy promoting P:



1 is the average of 27 herb plants and 2 the average of 7 trees.

Studying both arises some conclusions:

-Maximum photosynthetic efficacy is achieved by red photons. And differences between different red wl are small. The max is at 670nm, but all the range between 610 and 670nm gets very good efficacies. There is a sharp drop in efficacy at 685nm, so is important to try not use LEDs emitting part of the light there.

-Blue and green photons efficacy is similar.

-There is a drop at the end of the blue range, where is the minimum efficacy, and not in the green. 470nm blue leds are the worst, but Royal Blue leds emitting at 450nm or sightly less are very good.

-Use of photons almost all the PAR range is similar. The curves are pretty flat, with the minimum being near 65% of the max. Enhancement due to optimized spectrum maybe nice, but it may be up to an 25% against white sources in best cases (25% of the max: if we take the lower efficacy spectrum as baseline, as that of HPSs, it may be up to 50%. But in the practice is going to be very difficult get enhancements over 20%). Claims of LED sellers of 8x (800%) or higher enhancements are completely off base, as many people has checked, unfortunately.

These curves have a severe limitation: dont take into account possible synergies between differents wl. a famous synergy is known as the "Emerson effect": if a plant is irradiated with 660 and 700nm light at same time, P produced is higher than if its irradiated with them separately. Its possible there are more synergies like that we still dont know. However, there have been studies that has calculated the P of white light by adding the P promoted by each wl showing than measured results are between a 7% error margin of calculated values using the McCree curve. Its a decent error margin which confirms the validity of the method.

But things are more complex, due photosynthetic systems of plants are very adaptable, and they change to use the light quality they are receiving the best. Plants grown under HPS may have up to double chlorophyll b than those grown using sunlight, in order to use better the yellow light of them. So we always may expect that after plants acclimatization to a given light quality (after about a week under it), P promoted is going to be higher than calculated. This effect still reduce further the improvement margin by spectrum optimization.

Another effect to keep in mind is that most plants have shown better results when exposed to wide spectrum than to a nearly monochromatic one. There are exceptions, as wheat, so we must check it with cannabis, but my personal results point toward cannabis liking wl along all the PAR range, still being a little demanding plant specie in terms of light quality. The main task of LED grow lights researchers is to find the best wl distributions for cannabis, which is still unknown.

In order to show the importance of this task, i strongly recommend to read OPTIMIZATION OF LAMP SPECTRUM FOR VEGETABLE GROWTH. It shows how tomato, which is known to be a very little demanding specie about spectrum (as cannabis), still produce more when is grown under decent amounts of blue and green light (for the same energy used), but it produces more when using higher red proportion than with cucumbers, which need way more green to perform fine.

Other article of that page shows how plants adapted to lighting of different colors perform different than those grown under sunlight. It shows too how different quality of lights get saturated at different irradiance:



That point out that higher content of blue light may worth when using high irradiance. It as well shows how performance under different light qualities is affected by leaves age:



All the articles on the same page worth the reading: International Lighting in Controlled Environments Workshop. Some of the comments about leds are clearly obsolete (its from 1994), but most of the lighting concepts are completely valid and very accurate.

Qualitative aspects of lighting (non photosynthesis related)

Until now, ive talked mainly about photosynthesis. But light affect plants on other ways that arnt related to photosynthesis. Im going to resume the most known:

-Blue light requirement

Most plant's species requires some amount of blue light in order to grow healthy. Cannabis not seem to be one of them. Ive read of MJ grown successfully under LPS, which are yellow monochromatic lights, although with poor productivity, but health. Ive never seen it, so im not sure of this, and i never find any article about it. But it seems cannabis isnt specially demanding in terms of light quality. Not requiring blue strictly not mean MJ wont benefit of using it, of course, as it probably does.

But, independent of it required or not, blue light play a role on some aspects which advice to use it:

-Phototropism. (Check the full page, excellent botany resource online). The movement of plant towards the light. 450nm light (blue) has by far the stronger effect over phototropism. It may be a concern if strong blue light is delivered laterally to plant producing unexpected reactions. Anyway, phototropism effect is stronger on the taller tip of the plant, where auxins concentration are higher.

-Internodal distance control. Blue light has an strong effect reducing internodal length. This effect is exponential at low doses, and gets stabilized at irradiance about 30 uE/m2 of blue light. At 40 uE/m2 near the minimum internodal distances are achieved. As total irradiance levels of 400-500 uE/m2 seems to be the most adequate for MJ growing, the blue fraction to keep internode distances short is relatively small, about 10% of total or still less. But providing some blue light is a must to avoid stretching.

-Stomata aperture. Blue strongly promotes stomata opening, while red light has the opposite effect. Stomatas aperture increases transpiration, thus water consumption, but help the plant keeping internal CO2 concentration high enough. And this strongly affect photosynthetic efficacy (we saw how low CO2 internal concentration is the main limitation of P at moderate-high irradiance). So when growing with LEDs, or any reddish spectrum, there is only two ways of avoiding low internal CO2 concentrations: compensate it with enough blue light or grow in CO2 enriched environment.

Probably this is the main reason to use moderate amounts of blue when growing with LEDs, as previous cited factors advising to use blue only requires it at minimal amounts. On a CO2 enriched environment, probably percentages of blue around 10% are enough, but if not, percentages at least double that are required to keep internal CO2 high enough. This issue places a dilemma of what is better: if use more blue than required, with lower photosynthetic efficacy, or enrich with CO2, which is costly too but allows to use redder spectrum. This dilemma shows too that there is no a only way of building LED grow lights, but there is different optimal spectrum distributions depending of the setup.

This issue must be kept in mind when trying to find optimal spectrum distribution for MJ growing, so it must be always referred to a given CO2 level.

- Near Red/Far Red ratio

This ratio affect many biological parameters, through Phytochrome sensing. It affect strongly phenotype, by some ways:

-Internodal distance. The higher the ratio R/Fr, the shorter internodes. This is complementary with the effect of blue light, although R/Fr effect is weaker. Strong far red spectrum promotes stretching (incandescents, for example).

-Affects branching. Again, as blue light. More blue or higher R/Fr promotes branching.

-Determine leaves morphology, together with irradiance level. High irradiance and high R/Fr promotes small but thick leaves, with high chlorophyll concentrations, while low irradiance and low R/Fr promotes large but thin leaves with low chlorophyll density. Its called respectively sun and shade adaptation. Shade adaptation works better at low light levels and sun one works better at high irradiance.

On early veg, makes sense to use a lower R/Fr ratio to promote a fast covering of ground to reach an higher light capture, which allows to faster growth. While on flower clearly is better a sun adaptation which allows to use higher irradiance and results on more compact plants. But an interesting question for what i dont have any answer is if this adaptation affect to resin production and tricomes density.

R/Fr ratio strongly affect the Phytochromes (Phy) photo stationary equilibrium, which is what in the last instance determine these effects and which affect too to photoperiod sensing of plants. Short day plants as cannabis has proven to require up to 2 and a half hour less of dark period to continue flowering when exposed to strong Fr environment at the last hour. Manipulation of Phy sensing is still very unknown and seem a promissory field of experimentation on cannabis.

I thought to continue with the technical aspect of building LED arrays, but now im thinking its probably better on a own thread. What do you think? (this mean the thread is now open )

Hope this thread help you understanding light and plants better.

knna

knna, great and compact info, I started to learn about this stuff a couple of weeks ago, you did a great job on collecting this info from many sources!

Are you doing any experiments on wl mixing for cannabis? Or you know about anyone that is doing it? I know about some experimentation Growing Marijuana with LED, Growshow, nothing else.

But then, wouldn't a white light source complete the spectrum, thus enabling some sinergy to happen? Or sinergy beheivor is suposed to happen then only a set of wavelenghts are used?

As I can see, manipulation of light in order to manipulate plants internal mechanismis is still a relatively unexplored field, am I right? It would be great to know how how to enhance bloom quantity and quality by light driven events.

Hi Gauss, welcome to the board

You are right, possibilities of lighting is a very unexplored field. Until the release of LEDs, availability of different spectrums was very limited, thus studies were focused of finding the effects of the lights actually used (HPS, MH, fluorescents), all of them wide broadband spectrum sources which induces effects very similar: differences between them are relatively small.

Now that both users and lighting manufacturers have the possibility of tune the spectrum on very different ways, we are starting studying the possibilities. But most is still unknown, we only have general guidelines, which I posted in this thread.

I have studied it for years and still I dont have answers for many things. And the answers I have can only be applied on some conditions. Plants are very complex and final effects depends on the whole response to different interrelated inputs. For example, reaction to a given spectrum depends strongly of other parameters, as the irradiance and CO2 level, and of other degrees of other parameters, as humidity (or in general, of VPD).

Actually Ive found some other synergies, but checked only for cannabis. Indeed, as you noted, best results are achieved (at least, on bloom) when using some white light as background of the other narrower wl added (red, mostly).

Ive been always been a fan of keeping things simple. A deep understanding should serve to implement the simplest solutions. We can complicate things a lot when researching, in order to isolate responses to narrow wavelength, but in the practice, its difficult to beat the efficacy of an spectrum composed just for white and red LEDs. That solution, mounting white and red LEDs on different circuits and adjusting their power separately allows to get excellent efficacies both for veg and flowering phases.

We are still trying to find the lighting more effective for producing more with a given amount of watts. But for sure that we must find too the spectrums and irradiances that produces the best quality product and if they vary from the best producing, find the best compromise between both. I believe its a task of years.

I really wish I had read this sooner than find it all out the long way. Very cool Knna, great read for sure.

knna

Thank you so much! I too am glad red this the whole way thru. Perhaps a couple more times to sink in!