LED light spectrums what is beneficial and not


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Dec 25, 2014
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I came across this thread by LEDwizzard and thought it may be benificial to those considering purchasing an LED light system. I wish I had read this before buying my led. This thread was posted 2 months back. If anyone is interested I can post the link.

I get a lot of questions about "is this LED light good?" or "what is the best ratio of color1/color2?" and I see a lot of posts where people still vehemently believe IR and UV are necessary, or that you can grow using nothing but color X, or they flat out dismiss a whole chunk of the color as "wasted energy." So for what it's worth, here's my primer on the entire of light from UV all the way down to deep IR. The reason I feel this is worth the time is that people need to know, either as they build DIY LED setups or buy pre-fabricated setups, exactly what is valuable light and what isn't.
UV (200~430nm): One of the most virulent examples of perpetuated hearsay in the grow world is the claim that THC-laden trichomes exist, or that they increase in number/density, as a plant defense mechanism against UV light. The claim follows that hitting a plant with UV during flower or near harvest will boost production. I always ask "why would the plant only protect itself during late flower? Further, there aren't really any examples of other Rosales or even Eudicots with a UV-flower-defense mechanism, but if anyone knows of one I will stand both corrected and amazed. People often say "well it might help, so why not do use some UV-B anyway?" Well the answer is Thermodynamics. When a plant absorbs a high energy UV photon, it has to burn off a lot of energy as waste heat to convert it down to a usable PAR photon. Not only are UV LEDs inefficient to begin with, but the UV photon itself is wasteful. This is why CFLs cannot compete with LEDs: they start with a mercury-vapor UV photon of 254ish nanometers and the phosphor burns 50% of that energy immediately converting it into a visible light photon.
Blue (430~485nm): Blue LEDs are really efficient at producing photons. In fact virtually all "white" LEDs are simply a blue LED with a phosphor coating. However just like with UV photons, plants have to down-convert blue photons to a lower (red) energy level to use them in photosynthesis - wasted heat energy. Blue photons do have one very important feature though, and that is they inhibit auxin synthesis. Auxin is a plant hormone that causes elongation, so if you don't want stretchy plants, some blue wavelength light is a must. An important caveat is that auxin and its blue-wavelength-reactive buddies are important for flowering, which is why it makes sense for people switch to a "warmer" color spectrum during flower. The increase in auxin and the decrease in light from switching to 12/12 both explain why a plant grows 2X to 3X in height during the first few weeks of flower.
Green (500~565nm): There's this HUGE misconception, based on chlorophyll absorption charts, that somehow plants only need red and blue photons to grow. There are two problems with this. The first problem is that it assumes the only light-dependent activity in a plant is photosynthesis, which is flat out false. The second problem is that those charts of chlorophyll absorption assume you've isolated a choloroplast out of a leaf, or even isolated cholorophyll molecules in solution. What happens in a leaf - a complex, three dimensional structure - is much more interesting. Here's a fantastic article that explains that green photons are actually MORE efficient than red at driving photosynthesis. It's well worth the read. The problem with green LEDs is they are very inefficient at producing photons. A typical well-designed green LED will produce 50 lumens/watt, while a red from the same manufacturer with similar specs will produce the same lumens for 75% the power. In fact, some white LEDs produce more green photons per watt than green LEDs. Go figure.
Amber/Orange (580~610nm): Pretty much the exact same thing as green. Plants can and do use these wavelengths of light. One of the reasons HPS is still the gold standard for indoor growing is because it very efficiently produces a lot of green/amber/orange photons, which plants are quite excellent at using. Amber (not warm white, mind you, but true amber) is an extremely inefficient LED, and not worth the trouble, especially considering reds are cheaper and more efficient for the LED and for the plant.
Red (620-660nm): It's your main photosynthesis driver, duh. PSI and PSII photosynthesis pathways require two photons, one 680nm and one 700nm. Red LEDs typically come in a nanometer peak around 620-630, which means you don't have to waste much photon energy as heat in order to get usable photosynthesis energy. The only downside of red LEDs is they cost more (than whites) if you are building a DIY setup. Red LEDs are pretty rad because they run at a much lower voltage than white/blue/green/odd color, so for ~12V and 1 amp, you can often run 5 red 3W LEDs in series where you'd only be able to wire 3 white 3W LEDs in series. You can grow monster plants with setups that utilize only red LEDs with a little white thrown in to cover the rest of the spectrum.
Deep Red (660nm peak): It would be so freakin' handy if deep red LEDs were efficient. The photons are already very close to the peak photosynthetic efficiency wavelengths. Unfortunately, far red LEDs aren't as efficient as normal reds, and the trade-off in photosynthetic efficiency doesn't match the trade in chip emission efficiency, so its cheaper per watt (unless far red LED efficiency improves) to feed plants with cheap-and-easy 620nm reds.
Far Red (740nm) Don't waste your time. Like I mentioned twice before, photosynthesis needs one 680nm and one 700nm photon. Plants cannot convert a photon upwards in energy. To do so would violate the law of conservation of energy. There are no known instances in biology of a mechanism to fluoresce to a higher energy state. So all of these photons are wasted as heat.
Infrared (800+nm): Another horrible misconception is that plants somehow use this level of energy for growth purposes. Come on folks, IR is just heat. If a planted is heated, it respirates to stay cool. Like all other things on this planet, IR can be absorbed and re-radiated. What a plant cannot do is absorb an IR photon and somehow convert that photon to useful energy. To do so, a plant would actually be converting heat to something else and would rapidly freeze and die. Any LED fixture that features Infrared LEDs is literally pouring your money (via your electric bill) into the ground. Or to put it another way, the IR chips on an LED array are simply helping keep your grow tent warm.
White (100% PAR): I'm not going to bore you with explaining the in's and out's of color temperature or that cool white = veg, warm white = flower. You can read all about that in a million posts in this sub. A white LED is just a blue chip with a phosphor coating. The coating converts blues into a chemically-controlled spectrum of photons across the entire visible spectrum. "Warm white" LEDs have a short 460 blue peak and a broad slope from green to red. "Cool white" LEDs have a tall 460nm blue peak and a very similar slope from green to red.
Full Spectrum (typically advertised as a series of 6-12 peak wavelengths): UV is a waste, IR is a waste. Blue is okay, white is better, Red is best. If it had 3 reds for every 1 white/blue, in my opinion this is ideal.
That's about it. If you have science that disproves what I've said above, let me know!

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