I was going to post this at the Perfect LED Grow Light thread, but as some of what im going to post was posted 2 years ago on the stickied thread about LEDs and people still continue developing lights from wrong ideas, i think a thread about this topic is largelly needed.

The main problem is related to efficacy of spectrums.

When the firsts LED experiments at Overgrow, we work on the hypothesis that blue and red light are more effective. It was an appealing hypothesis that promises large electric savings. But unfortunatelly, it has proven wrong

Now we know both by theoretical research and practice that only red and blue spectrums arnt more effective than complete spectrums, but less.

LED grow light sellers obviously not want to notice it, so they may claim efficacies of their lights 8 or 10X higher than HPS. But thats simply false, and any grower that have checked it got dissapointed.

Does it mean that we cant develop LED lights with higher perfomance than HID? No, we can still develop LED lights with better spectrums, by reducing (not elimitating) cyan, green and yellow light and adding it on the red side of the spectrum. This result on a enhanced photosynthetic efficacy, but its small, from 5 to 15% of enhanced efficacy of same amount of photons.

So any further electric saving from using LEDs must come from other factors: better light distribution and increased energy efficiency.

Better light distribution

HID lights, specially HPSs, are very energy efficients (emit a lot of photons per watt burned) and have good spectrums for growing plants, but have a mayor drawback: they run very hot, so they must work at some distance of plants. This mean all the light of the grow comes for a single point of light, and it creates a very uneven light distribution along the grow. Both horizontally (plants below the bulb have way higher irradiances than the edges) and vertically (bottom of plants gets insufficient light while top of plants are exposed to excess light).

This uneven distribution strongly drops photosynthetic efficacy, because as higher is the irradiance, the lower the photosyntesis rate (per photon). But you must expose the top of the plants to such high irradiances if you want the light reaches some into the canopy.

LEDs dont have this problem. They are solid (no risk of breaking them) and run cool enough to be in contact with plants without problems. So they may be distributed along the grow, both horizontally and vertically (sides and between plants) in a way plants gets the optimun irradiances (those at photosynthetic efficacy is higher) along all their height.

This increases photosynthetic efficacy strongly. When NASA scientistist thought on this, and changed half of the light from top to side and intracanopy (IC) lighting, the yield enhancement was of 35%.

Using LEDs on IC lighting not only enhances productivity, but allows to grow taller plants indoor than any previous type of lighting: the equivalent strategy with HIDs has been vertical growing, wich avoid optical losses at reflectors and achieves more even lighting. We all know how vertical setups increases productivity compared to top reflectorized HIDs. But LEDs allows to do it at any scale, still on very small grows, and control of light delivered at each part of the plant may be controlled accurately, in order to get the max.

Increased energy efficiency

Any enhancement on efficacy may be related to an amount of photons. A better spectrum or a better distribution enhancement is applied to the baseline of PAR photons.

So the first and by far most important factor to determine the growing efficacy is the energy efficiency of the light: how much PAR watts it deliver per each watt burned.

But this factor is almost ignored on most LED grow lights development threads ive read, while is the most critical parameter.

For example, the difference on photosynthetic efficacy from a 635nm peaked led to a 660nm peaked led is around 5%. But most 660nm leds are 50% less energy efficients. Independent of the wavelenght efficacy, using 660nm leds is a bad choice (except if they have same energy efficiency than 635nm, but find me such one and i invite you to dinner ).

In order to compete with large HIDs with energy efficiencies about 36-38% is to use leds at least 30% efficients (an that if the HID is reflectorized). A bit less if the HID has a glass barrier.

But if you use LEDs with energy efficiencies below 25%, and most LEDs on sale are so, there is no way they may offer electric savings (same yield with less watts).

Forgot to analyze this critical factor had led to many dissapointing LED grows. So please, do it yourself a favour and think on the efficiency of the LEDs before buying them.

Happy growing
knna
knna Reviewed by knna on . Building LED lights from facts, no theories I was going to post this at the Perfect LED Grow Light thread, but as some of what im going to post was posted 2 years ago on the stickied thread about LEDs and people still continue developing lights from wrong ideas, i think a thread about this topic is largelly needed. The main problem is related to efficacy of spectrums. When the firsts LED experiments at Overgrow, we work on the hypothesis that blue and red light are more effective. It was an appealing hypothesis that promises large Rating: 5