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nb - I'm certainly not a planted tank expert, but I do have some experience with led lighting systems (for house interiors).

It's great to see people experiementing with LED lighting for tanks. I believe they're the light source of the future for the aquarium.

There are a couple of issues with the white Luxeon LEDs though. Firstly, they're not all that efficient. They beat incandescent light bulbs, including halogens, but frankly don't come close to T5 fluorescents or HPS/MH. Then there's the light spectrum, which is not ideal for plant growth. The problem is that phosphor coatings for LEDs are not particularly advanced, compared to those used in off-the-shelf plant-grown T5 tubes (with good reflectors). This is compounded by the fact that white LEDs are optimised for reasonable colour rendition, not growing aquatic plants.

The solution is to use a combination of descrete colours (Luxeon offer a reasonable range of wavelengths) to create your own customised colour spectrum. Obviously some experimentation would be needed, but as LEDs are very easy to dim, it's within the scope of the DIYer.
 

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PUR-efficiency of Luxeon Stars

I took the white and warm white spectral distributions here:
http://www.luxeon.com/pdfs/DS23.pdf

Question: Is it correct that the warm-white Luxeon has 20 lumens per watt, and the white Luxeon has 45 lumens per watt?

If so, the warm white Luxeon has a very lousy PUR-efficiency of 0.13 microEinsteins per Joule (comparable to an ordinary Halogen), and the white Luxeon has 0.38 microEinsteins per Joule.

The PUR-efficiency was calculated with an action spectrum for green plants (B):
http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantHormones/Nutrition/706.GIF

This means you would need apx 7-8 times as much watts when using Luxeon Stars compared to an efficient T8-bulb like Philips Aquarelle (0.8 microEinsteins per Joule) to achieve the same growth-rate.
 

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Yes I had that into consideration. My guess is that at no depth you easily light-saturate the plants with so much light, so you wouldn't get any significant difference in growth-rates. You have to spread out the LED:s, so you will smear out the wattage over a larger area. A 160% increase from a good reflector on the PC might have changed things also, but all in all I think the plants are pretty much light saturated in both tanks - and if so the nutrients, especially CO2 is the main growth-rate-difference-factor. Breathe a bit more on the LED-tank and it will win.

Also fluorescents gets less efficient with their watt-rating. A 40 watt T8 is for example more efficient than a T8 15 watt of the exact same brand.
 
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