knna

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 wich allows people to discriminate whats the wrong info widely avalaible at the net, wich unfortunatelly is the most.

First off, its important to differenciate 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 wich allows the plant to grow, and its tightly related to the amount of photons wich 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 phothosynthesis, wich is the main task of light.

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knna

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

Plants use for photosyntesis a similar range than humans for vision. Both use mostly wavelenghts, wich is the parameter wich identifies differents electromagnetic radiations, between 420 and 685nm, although photosynthesis still take place at noticiable amounts from 400 to 700nm, wich 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 refering 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 caracteristic 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 materia (as when lights hit the plant).

There is an important practical consecuence of using one or the other: energy wich carries a photon is inversely proportional to its wavelenght (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 wavelenghts) 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, wich equal aprox to 6.02*10^23 particles (Avogadro's number). Most times, its used its million diminutive, the micromol, wich is 1/1000000 mole. For light, is commonly abreviated to E (Einstein) or μE (microEinstein, often written uE for keyboard limitations). Although it isnt an official unit, ill use them for confort.

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

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knna

Throught photosynthesis, plants gets the required energy to disociate 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 (oxigen). So its possible to measure accurately photosyntesis by measuring CO2 uptaken or O2 produced. Photosynthesis is limited for the required factor (light,water,CO2) wich 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 avalaible. 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 esential figure to consider. The more the photons wich 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 differents wl, and it depends too slighty 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 absortion (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.

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knna

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) wich is measured on lm/m2=lux. Light concepts preceded by a "i" are refered 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 wich is refered to the light source, wich is measured in Cd (candles) or W/sr (optic watt per steroradian). It could lead to confusion, thus ill only use the irradiance term, wich not only is clearly refered 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 Comparision" thread wich 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 avalaible, 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 throwed 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 wich 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 indeep graphs and explanations on Eutrophication - light and growth)

There is a first part of the curve wich is near linear, meaning that equal increase in irradiance level lead to a equal increase of photosynthesis. The slope of this line determine the maximun 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 irradiances 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 irradiances, aswell 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 differents ways used by plants to do it, but at really high irradiances plants desactivate photosynthetic systems and chorophill is retired from the leaves, producing the effect known as "light bleaching" (because leaves becomes white), wich is irreversible (permanent damage).

There are many practical consecuences 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 irradiances, close to A. This mean zenital lighting (from top) isnt desiderable ideally, as it produces high irradiances at the top of the plant (thus, wasting light) and low irradiances at the bottom (wasting the ability of those leaves to produce more photosynthesis if they have more light avalaible). Although is desiderable a slighty higher irradiances 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 requeriment 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 irradiances used.

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

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

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knna

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

Reading other threads about lighting, it seems that spectrum is the only parameter wich determines the efficacy of light for growing. But its not true, what determines mainly the efficacy is the energy efficiency of the bulb, wich 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 wich 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 differents 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 enhacement may be applied to the number of photons, wich, again, is the base of all. If you use half photons with an average 50% more efficacy promoting P you get a final efficcacy 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, wich largely overstates the importance of spectrum alone. This mistake is due many people use "action spectrum" curves, wich are curves build from laboratory absorbance 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 bandwiths 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 enrgy, and not of absorbed photons. Thus, it gives together the effect of photon's absorbance and its efficacy promoting P:



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

Studing both arises some conclusions:

-Maximun photosyntetic 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 emiting 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 minimun efficacy, and not in the green. 470nm blue leds are the worst, but Royal Blue leds emiting at 450nm od slighty less are very good.

-Use of photons almost all the PAR range is similar. The curves are pretty flat, with the minimun 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 wich 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 chlorophill b than those grown using sunlight, in order to use better the yellow light of them. So we always may expect that after plants acclimatation 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 spectrums than to a nearly monochromatic ones. 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, wich is still unknown.

In order to show the importance of this task, i strongly recomend to read OPTIMIZATION OF LAMP SPECTRUM FOR VEGETABLE GROWTH. It shows how tomato, wich 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, wich need way more green to perform fine.

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



Wich point out that higher content of blue light may worth when using high irradiances. It aswell shows how perfomance 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 coments about leds are clearly obsolete (its from 1994), but most of the lighting concepts are completely valid and very accurate.

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knna

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

-Blue light requeriment

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 sucessfully under LPS, wich are yelow 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 strickly 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 aspect wich advice to use it:

-Phototropism. (Check the full page, excelent 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 unespected 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 lenght. This effect is exponential at low doses, and gets estabilized at arradiances about 30 uE/m2 of blue light. At 40 uE/m2 near the minimun internodal distances are achieved. As total irradiances levels of 400-500 uE/m2 seems to be the most adecuate for MJ growing, the blue fraction to keep internodes 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 consuption, 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 irradiances). 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 dilema of what is better: if use more blue than required, with lower photosynthetic efficacy, or enrich with CO2, wich is costly too but allows to use redder spectrums. This dilema 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 refered to a given CO2 level.

- Near Red/Far Red ratio

This ratio affect many biological parameters, through Phytochrome sensing. It affect strongly phonotype, 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 spectrums 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 irradiances and high R/Fr promotes small but thick leaves, with high chlorofill concentrations, while low irradiances and low R/Fr promotes large but thin leaves with low chlorofill density. Its called respectivelly sun and shade adaptation. Shade adaptation works better at low light levels and sun one works better at high irradiances.

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, wich allows to faster growth. While on flower clearly is better a sun adaptation wich allows to use higher irradiances 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) photostationary equilibrium, wich is what in the last instance determine these effects and wich 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 unkonwn and seem a promisory 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

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