Overall I think the Dr has done a good job with this article, and I agree with a lot of it. However, there are some problems in the details.
As described in the article, bulb with ratings of 3500K or less are not well suited for growing plants. They CAN be used because they do put a spectrum of light which includes the wavelengths used in photosynthesis. The problem is that much of their energy is wasted. A large percentage of the total output is not used by plants. A grower could use a greater number of bulbs and produce light useful for photosynthesis (as well as waste energy and money).
Bulbs rated at 4100K actually work pretty well for producing plant growth (supplying energy for photosynthesis), and, in fact, their light spectrum contains about as much light in the higher frequency spectrum (useful for plant growth) as do bulbs with higher peak ratings. That's where the K-rating comes from, BTW; it represents the peak of the light spectrum produced by a bulb. The reason that shorter wavelength (higher frequency, higher K) bulbs are not much better for plants is that they put out less light per amount of inputted energy. They do put out a higher percentage of useful light, but the useful spectrum is not significantly (or any, depending on the specifics of the bulb) greater than bulbs in the 4100K to 5000K range. Bulbs with even shorter wavelength (higher K) are even less efficient and may put out about the same or even less (depending on the bulb) useable light.
Finally, mixing bulbs of different wavelengths does not expand your light wave range. As I mentioned above, all the lights put out a fairly broad spectrum of light. The differences are where the peak occurs and how much light is produced per given amount of energy. Because the shorter wavelength bulbs put out less total light per amount of energy, they generally won't add significantly (or any) to the upper spectrum of light as compared to using the same total number of longer wavelength bulbs. In other words, the useful light of two 5000K bulbs will look pretty close to one 5000K bulb plus one 6500K bulb.
If I had time, I'd make a graph, but just think of looking at only the right hand side (representing the useful light range) of a pile of dirt made up of two scoops. The first instance (representing the two 5000K bulbs) is a pile made from dumping two large scoops, one on top of the other. What you will be comparing is one side of the pile. Next, picture a pile made from dumping one big scoop and then dumping another smaller scoop on the right hand side slope of the big scoop (this will represent one 5000K plus one 6500K bulb). What you will see (depending on the bulb/scoop size) is pretty close to the same.
Hope this made sense. It's a little longer than I had planned....
