LED grow lights... input please.

[Curiosity]

I was told that as long as the mcd rating is high at high angle, they are MORE powerful than a 15 angle.

Example.
660nm, 4000mcd 15 degree angle.
660nm, 4000mcd, 100 degree angle.

This means that I am still getting 4000mcd spread further across the plants. Not in that narrow angle, so in theory those light a more than 4x stronger.

Lumens = Candelas*2pi*[1â??cos(theta)]
That makes it 0.85 vs. 29.5 lumens... about 30 times better.

Curiosity Reviewed by Curiosity on 04-29-2006. LED grow lights... input please. Hello guys, I'm exchanging eMails with a representative from a LED company. I asked him if they could produce a custom light for growing, and here's what he told me: Our options are as follow: 940nm, 850nm, 660nm, 630nm, 610nm, 590nm 570nm, 530nm, 510nm, 470nm, 460nm, 400nm and a total of 64 LEDs in the bulb. Rating: 5

[Moose101]

Are plants really that straight forward? I mean, do two kinds of frequencies cover all the plants needs for every single kind of plant?

[turtle420]

Are plants really that straight forward? I mean, do two kinds of frequencies cover all the plants needs for every single kind of plant?

Nope.

Cannabis plants.... we're experiementing.

Maybe they'll grow fine...

[Curiosity]

Good info in this artice I believe.
http://ncr101.montana.edu/Light1994C...ula%20text.htm

All of it is interesting, but specifically scroll down to the "Pulsed Lighting" section. Basically, applied light was pulsed at about 6.7kHz, with duty cycles of 1%, 5%, 10%, and the constant 100%. They were all done with the same time-integrated photon flux (ie. the 1% had 100x higher intensity than the 100% one).

Result: for tomato plants, they all gave about the same photosynthetic response. So to match a constant-on LED source, you'd need to be able to drive the peak output 1/(duty cycle) times higher. Seems like one would be better off forgetting about pulsing, and overdriving their LED array as much as their thermal dissipation allows.

thoughts:
*might be different for cannabis vs. tomato leaves
*might get different results pulsing a broader range of wavelengths
*duty cycles might have greater effect if pulsed at lower frequencies. I didn't find anything about constant duty cycle over a range of frequencies, aside from:

http://www.freepatentsonline.com/5012609.html
"It has been experimentally demonstrated that when light flashes of 100 microseconds duration follow each other too quickly, the light is not fully utilized (B. Kok. 1956. Photosynthesis in flashing light. Biochim. et Biophys. Acta, 21:245-258.). Extending the time of the dark period to about 20 milliseconds increased the relative "yield" per flash of light."

I don't know why they used 100us pulses, but here they're suggesting about 0.5% duty cycle at 50Hz as being optimal, with no mention of light intensity affecting photosynthesis. If it's true that the plant sees this case as the same as DC (when both have equal peak outputs), then this would give lots of room to overdrive the LEDs for great gains. But if it still requires equal integrated outputs, then it's not really worth it (100x intensity requires much more than 100x current since relationship gets non-linear at currents ~>100mA, with baseline around 20mA). This is probably something that needs to be found out to get the most out of LEDs.

[knna]

The concept of pulsing light to maximize photosyntesis action is very interesting and promising, but all the studies ive seen performed about concluded that the grow rate is directly dependent of total irradiance. I think a lot of experiments are needed to find a correct pulsing cycle wich achive same growth with less total irradiance.

BTW, i think we miss the main cuestion when talking about growing with leds: whats the adecuate irradiance?
All the experiments ive seen of growing with leds was a bit disappointed at final results. Some parameter fails in order to achieve a correct bloomery. Maybe lacking of far red spectrum (730nm), wich is esential for long day flowering plants and of undetermined effect in short day flowering plants (lika cannabis) or maybe, and i think more probable, insuficient photons density.

For HPS spectrums, we know about 250-350 micromols of photons per second (uE) per sq meter is needed, and about 600-700 uE/m2 for good budding. The experiments ive seen dont double the flux density in bloomery stage, resulting in bad budding, although is enough for vegetative growth.

Comparing with the HPS spectrum and caracteristics, we can reduce the flux requeriments for growing with leds, in about a 25% by best absorbance (of red and blue part of spectrum against the yellow-orange dominated HPS spectrum), another cumulative 25% by higher photosyntesis action (depending of leds spectrum, from 15 to 35%, but not 300 or 400% like many comercial and interested leds applications say) and another 25% comparing with reflectored HID (more usual, and for that standard configuration i stated the 650uE/m2 goal), because leds are directionals, and dont need reflectors (and the light loss implied, about 25% average).

So, result in 0,75%^3=42% of HPS initial flux to achive similar results.

650uE/m2 *42%=275uE/m2

This works fine for red light, but blue light has less photosyntetic action, but is needed if we want a healthy growth, a minimun of 30uE/m2 of blue light, better 40uE. So, including this factor, about 300uE/m2 of led light seems an adecuate irradiance.

And how many watts of leds we need to achieve that irradiance?. It depends of leds efficency, the other issue ussually forgotten when talking about growing with leds.

There is a lot of misconception about this issue. Frecuently, i read that led light are the most efficient, and its a false statement. Probably it would be so in some years, but now, leds have a similar efficiency than halogens, very far of HID lights.

High watage HPS (600w or more) are about 38% (including ballast loses) efficients converting input power into PAR light.

Leds efficiency are very variable, but is very usual find devices with less than 10%, specially the cheap ones. If we go to the best leds, they are at 25% in blue and 30-35% in red, with good thermal design.

So its very important to know what flux emitts the leds we are going to use, in order to obtain conclusions. If we want to use less watts than growing with HPSs, is esential use high efficiency leds, at less 25% wall plug efficiency. I think too many people is missing this question.

[turtle420]

Curiosity, kNNa,

DAMN interesting info... I saved it to my computer... cause I have no time to read it now... I hope I can comment on it soon.

Very good to have some brainiacs running around...

[Moose101]

I seen the webpage at http://www.solaroasis.com/
Am I being dumb or wouldn't it make more sense to suspend the lighting from fixture that can be molded into any form so that you may have invidiual systems for each leaf?

[Moose101]

I'm a bit confused when you mention percentages. I think I'm missing something here. First of all, are we calling light a ray or a wave? Let me get this straight. Each bulb pulses a length of light in frequencies equal to that of the bulb type? That makes it a ray? The light is three dimentional right? Where is the width and height? If you are give a plant 90% red spectrum, what is the actual amount you are giving it?

[Moose101]

err, nm. I just realized how dumb that was. I just figured it out. It's a wave length, meaning it's a wave not a ray. The percentage is of the multiple bulbs? That still doesn't really answer it though. I mean if you had 500 bulbs and 90% were red that would be a solid amount. But that isn't something that scales well. I mean 5000 bulbs of 90% red would mean a lot more red.

[knna]

Led lighting is different to any previous kind of lighting, and if we want to maximize its advantages, we must change our way of thinking about light. Leds dont need to work in bulb shape, in fact, its a very bad idea because thermal considerations, especially important for red leds, wich are very temperature sensitive.

We can put the leds arrays very close to plants and between them, because leds dont give heat enough to damage plants and dont radiate IR and UV (if we dont want to). Using led's reflectored bulbs dont make any sense in horticultural applications. Leds bulbs useful life is below 15000h, because quick degradation due to heat.

Hanging leds strings between plants is the way to go, and use medium-low power leds well spaced, in the way there isnt need of cooling. In this way, the led's low intensity isnt a problem.

When I mention efficiency percentages, it refer at % of light emitted (in watts) of total input power (for horticultural applications, PAR light (400-700nm) is counted). Its very difficult compute leds efficiency, because it depends of ambient temperature, thermal resistance (and how dense are the leds packed) and at what current the led is runned, talking about the same led. And differences between leds are very, very large.