These are some posts I made somewhere else about the same topic, thought I would paste them here too. Might seem a bit discombobulated because I was answering questions not even asked here, but there's still some good info that should be useful to a few, can also answer any of your questions about LEDs. Fire away!
I only read through the first couple pages, and I can see how LEDs are so misunderstood by the vast majority of people here because of marketing.
I work with LEDs every day, doing some pretty interesting things, I understand them intimately. While I do love them, I understand their limits, and I hate to see them misrepresented. They have unbelievable advantages in so many applications, it's so unfortunate to see them bastardized by people trying to make a quick buck.
Over 90% of the LED products I investigate have extremely inadequate thermal consideration. Popular to contrary belief, LEDs do make heat, and quite a bit of it. Similar levels of electricity to heat/light conversion as HPS HID lighting. The difference is that LEDs conduct heat away from the junction(out the rear), whereas HIDs mostly emit the heat as radiation.
They do work, but not as well as HIDs. When manufacturers of LEDs (no, none of those grow lamps actually use their own LEDs) actually start building blue dies that have phosphor wavelength conversion directed at photosynthesis will we have good LED grow lights. (like the pink flruoros but much better)
For now the market for this is far too small to merit the production of LEDs this specific, and this is why all manufacturers use generic LEDs, most often cheap no-name LEDs from China with no datasheet.
If you can't request the manufacturer LED datasheet, and receieve it from the vendor, then the product is most definitely total crap.
If the LEDs aren't CREE, OSRAM, Lumileds, SSC, Nichia, Luminus, Bridgelux, or a few other notables, then they are most likely crap.
Even if they are of good quality, if they do not have an efficient thermal path to dissipate heat, efficiency will go way, way down as the LEDs heat up, and you can say good-bye to that long lifespan you expected.
I've done many tests of commercial LED products falling to 50%, 40% and even under 30% output after only 1000 hours of use in 25C ambient temp. All claiming 50,000 to 100,000 hours.
The LED manufacturers datasheet will state what junction temperature those claims are accurate with. Most often with low quality LEDs this rating will be at 25C, while high quality LEDs will be rated at 60-100C as well.
There isn't less heat, there is less emitted heat. You still need the same ventilation to remove the heat from the LED junctions. Unless you use less overall wattage than you would HID, then your ventilation could be less.
If the advantages of having the LEDs closer, and the ability to place individual smaller light sources for training and such, excede the advantages of an HID light source, then so be it, they have another advantaged application.
I would be careful about putting them too close though. Any blue light not absorbed by the plant will turn into a lot of heat on the leaf surface. Photometric power not absorbed will convert into heat and burn, just like a wide beam laser.
Holding high output LEDs directly over a piece of wood will actually burn the wood. No heat is emitted, it's all the photometric power converting to heat as it hits the surface.
Yes you can place them closer, and they do emit no heat, but you really do still have to understand what you're doing. This doesn't mean there will be no heat.
Some of the photometric power not converted will reflect, but much will turn into heat, especially blue wavelengths.
Plants don't appear green necessarily because they are green, but because they absorb blue and red wavelengths, and reflect back most green. An interesting way to think about it anyway.
As for commercial LED grow lights, I haven't investigated any, but the same engineering approach is applied to any LED array.
I'm not saying that they're all crap either, some may be built very well. Just a word of caution, as most aren't in the general LED world. Ask for the name brand of, and manufacturer datasheet for the LEDs being used in the array. Ask about thermal resistance from junction to ambient. If the commercial supplier cannot readily provide such information, the product is crap.
Taking an LED that says 100,000 hours in the datasheet at 25C and letting it run with a 100C/W to ambient resistance and claiming that it will last 100,000 hours is just like saying your engine oil will last 3000 miles whether you're driving around town or redlining it around 3000 mile endurance race track.
One of the real advantages to LEDs in growing, unutilized to my knowledge so far, would be the ability to passively conduct the heat created out of the room.
Since LEDs create heat at the junction, that needs to be conducted out of the rear, solid heat conductors, or preferably heatpipes could be used to conduct heat out and on top of a small cabnet, taking it completely out of the growing area with no ventilation needed.
