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.