This is getting to be an obsession with me! Hours spent on the computer, searching for parts and data, sketching out design layouts, doing calculations, etc. So, I decided to share what I'm doing, hoping that cooler, more experienced heads will speak up where appropriate.
I have a 45 gallon tank, with about a 30 inch by 15 inch footprint, about 20 inches tall, with a PC light fixture mounted about 24 inches above the substrate level. My goal is to replace the PC guts with a LED array that won't break my bank account wide open, and will give me around 50-100 micromols per sq m per sec of PAR at the substrate level.
One approach is to use relatively low output LED's and many of them to get that intensity. Another is to use the fewest number of LED's that will do the job, but using high output units. The first discovery I made is that lower power LED's don't cost 1/10th of what higher power ones cost, so every arrangement I looked at, using many LED's ended up with a higher material cost than using just 3 high output LED's. So, here is the layout I decided to pursue further:
You can get this LED, http://www.luxeonstar.com/endor-rebe...aoe1im80dot2b2
with a lens to concentrate the light, for about $24 each.
Next I figured out the approximate angle of the cone of light I would need to get good coverage of the substrate with these. It turns out to be about 45 degrees. (The LED emits light in about a 160 degree cone, but the intensity drops pretty fast as you move from the centerline - the lens will partially correct that.)
To verify that this would give me enough light I found this chart to use to convert the LED lumens output to a PAR intensity at the substrate:
From this it looks like a good number to use is 70 lux per micromol per sq m per sec, where one lux is one lumen per meter squared. Since I will have most of the substrate getting light from 2 LED's I used 25 PAR units to calculate from: 25 x 70 = 1750 lumens per sq. meter. And, the area each LED will primarily illuminate is about a 20 inch diameter circle, or .3 meters squared. 0.3 x 1750 = 525 lumens. The LED I chose gives about 435 lumens output at 700 milliamps of current, but can be driven to 1000 mA, so this comes close to what I want.