The use of LEDs in agricultural applications is expanding along side visual light and light cure technologies. The technology is even more compelling here for its reduction in energy consumption and lack of heat in the light pattern. The key element of LEDs in this application is the ability to create a specific spectral power profile, with none of the peripheral light unnecessary to get the job done. The light plants need is not the same as human vision. In fact, it is almost the opposite. While we humans with our juice camera eyeballs respond to light in the yellow-green spectrum to see by, our blind little green friends use light in the red and blue ends of the spectrum to activate various chemical reactions to generate food, build cells, and dispose of waste.
Living in the northern Midwest, we endure many months of pretty pathetic solar conditions. Short days and cloudy conditions do not support healthy plant growth, indoors or out. In our case, we grow small patches of grass for the cat all year round. The problem is, using the normal rye and oat sprouts, combined with the destruction of the cat puts on these little patches requires regular re-starting and re-growing the crop. This also applied to growing savory herbs for cooking, or starting fresh annual flower seeds for planting in beds or planters around the house all season. For all of these purposes, we’ve experimented with a small light that has been hanging from a shelf in the laundry room connected to a timer, generally getting in the way when it’s not in use, and falling apart over the 6 years its been working. Having the 3D printing and machining equipment around means we can make neat little gadgets at will, without hunting stores, to get exactly what we want in the end.
D12 is essentially the light engine from our experimental rig, tuned up and configured as a portable table light. This is sized specific to our needs, but could easily be scales up to other uses. It also uses some inexpensive Kingbrite RGB LEDs wired unconventionally, to deliver both Red/Green/Blue from 3 of the LEDs (Red being most dominant), and Blue from 2 additional LEDs. This is a combination I came to over time that delivered the greatest result. In the spectral comparisons, you can see the other combinations I tried using the same basic LED setup, changed only by the way the individual colors are wired. The power source is an off-shelf Meanwell 700mA driver. We connect this device to a wall mounted timer to cycle it on and off to suit whatever we are growing at the time.
I am no expert in this field. We came to the spectral distributions through basic research and simple experimentation. I am also aware that our red and blue color choices are not ideal, as those in the business will likely point out. If I were looking for the optimal solution, no compromises, the red would be a bit more red, and the blue a little closer to near UV. However, these colors are not found in inexpensive RGB LED packages, while the cost of blue and red LEDs in more ideal formulas cost many times that of those we used here. The question I had going in, was whether it was possible to achieve reasonably decent results without going off the techno-anal deep end. There is another advantage to using a light source of this type, and that is consistent and extended exposure hours. Daylight delivers and inconsistent 4-8 hours per day, with varying intensity. We used cycle times of consistent, high intensity, focused spectral power light, of 18 hours per 24 hour cycle, with just 6 hours of rest (when plants convert the food created to cell growth.) Using this approach, the performance of the package we put together to be very solid. First and foremost, the combination delivered growth when natural light supported nothing more than moldy potting soil. The time to grow samples from seed to transplant-able maturity, or usage (herbs or cat grass) were reduced to 1/3 that of any other daylight or indoor fluorescent based light source, including CFL grow lights, CFL daylight and full spectrum lamps, incandescent grow lights, and daylight. The LED grow light included in this little portable device blew them all away.
The following are the spectral distributions created and tested over the years as we experimented with various plant projects. Note that the PPFD values indicated are relative to one another (one direct reading taken from a single fixed distance), but do not reflect exact photometric values, as I have not completed a full photometric test of these for total output.