Note: This project page has morphed into a design discussion. (Constructive suggestions and collaboration are still welcome. See the comments section at the end.)
**There are no recommended construction-plans here.**
Instead, I recommend shopping around for a good deal on standard White LED Light-Bulbs. Please try using them as grow-lights, and let us all know how it goes. My calculations suggest they should should be economical, energy efficient, and great for plants.
After many hours of research I've concluded that it doesn't make sense to build DIY grow-lights, and I can't honestly recommend it to anyone. This is partly because it is technically difficult (and making it easier makes it more expensive), but more-importantly because commercial products are much less expensive and higher quality in the end.
You're better off buying an off-the-shelf product. And surprisingly, I wouldn't even recommend buying a red & blue "LED grow light", because the best option seems to be conventional white LED lights! Sure, grow-lights are approximately 25% more energy-efficient for growing plants, however, they are priced 2 to 8 times more per unit of power. LED grow lights cost $4/Watt-electric typically, or nearly $2/W-e if made in China. White LED bulbs cost $1/W retail, and sometimes $0.50/W if there is a green-subsidy instant-rebate.
White LEDs emit more green-light than typical "grow-lights", which unfortunately isn't used very efficiently by plants; however, paying for the electricity to emit that inefficient green-light actually costs less than buying a grow-light that doesn't emit green-light! Even if hypothetically you have sky-high highest electricity rates of $0.50/kWh, and zero-percent financing, when I try to amortize the cost of grow-lights, I figure that the LEDs burn-out before accruing enough saved electricity to make up for their high up-front cost.
Step 1: Light-Spectrum Requirements
So what kind of light do plants need? What is best?
Well, in terms of spectrum, the chlorophyll pigments are the main drivers of photosynthesis. These are most active under blue light at ~430 nm, and red light at ~660 nm. According to US patent #6921182, plant health is improved by the presence of light around 612 nm. From the graph below, this corresponds to the peak for "Phycocyanin".
Some of the overall response curves I found, such as for "PAR" are graphed. Also the spectral output of some common White LEDs are graphed. It turns out that the peak sensitivity for chlorophyll from blue light (at ~430 nm) lines up almost exactly with the peak output from a white LED, so this is a fortunate coincidence.