SunnyD,

I think you are confused about what "661nm" means. 661nm is a particular wavelength of electromagnetic radiation. It happens to fall inside the visible light portion of the Electromagnetic Spectrum (Electromagnetic spectrum - Wikipedia, the free encyclopedia).

Plants (any by plants I mean GREEN PLANTS) have a process known as photosynthesis that transforms electromagnetic radiation into energy for the plant. Studies have shown however that it is specific wavelengths of light that the plant uses. Until LED, there were few viable options to test this theory (putting filters on full spectrum lights would be an example). There have been a lot of other posts that describe what wavelengths the chemical chlorophyll (the primary chemical in photosynthesis) respond to most. If you don't want to do your research, there are basically two areas of the spectrum that chlorophyll responds to and they fall in the extreme blue visible and the extreme red visible. These colors appear to humans as deep red, and true blue.

I am sure if I don't say this someone will correct me. Plants also respond to other colors, including green. Blue and Red are the two that plants absorb the most. Colors near green and green itself are the least useful because they are mostly reflected, not absorbed.

I just re read your post before I put this up, and this may all be information that you already know. The BLUE LedEngin that I am looking at for my design emits light in a range from 445-475 nm, and the RED 655-670nm.(EDIT: I MAY STILL USE K2s for the BLUE, I need to sit down and do some serious math before I spend some serious cash)