Maybe it would be helpfull at this point to explain a little more about fluorescent tube lighting and it's applications at it relates to our subect here. So, I'll start out
(and Rick, if I say something that's not quite right or should be explained a different way--please correct,etc).
Fluorescent tubes are expressed in terms of 'k', such as 3000k, 5000k, 6500k, etc. The 'k' stand for Kelvin (units) and allows us to measure the wavelength or groups of wavelenghts called 'zones' within a spectrum of wavelenghts and chart them for visual comparison of quantity--often referred to as a color temperature chart. Now, two things here: 'k' values have nothing to do with how strong the bulb is, 6500k is not stronger than 3000k. It is only the wavelength. And the only way to increase the strength is to increase the wattage. And color temperature has nothing to do with 'room temperature' nor how much heat is given off. Color temperature,although derived from kelvin values, is more or less a pshycological expression we use to describe how warm and relaxing the lower wavelenghts are as compared to higher wavelegths being more cold, stark and active. The amount of heat given off by any given tube is determined by wattage and bulb inefficiency but it is not related to color temperature.
So, now then, if we were to draw up a horizontal bar graph or bar chart of a spectrum of say (as an example) a full spectrum tube of 0 to 8000, we might find a fairly even distribution of all of the wavelengths at some volume intensity expressed in terms of lumens (or some numerical value) vertically, and 'k' values straight accross the chart. But, by modifying the phosphors within the tube, certain wavelength groups or zones can also be modified (increased or decreased, more of or less of). Modern biology has determined that plants need and utilize primarily, light wavelengths of two different groups/zones, red with about 3000 or a little less and blue, around the 5000 or a little more. These two groups are the wavelengths best absorbed by chlorophyll in photosynthesis. So then, plant grow light tubes are designed with that in mind and instead of a straight line graph, the line would be greatly elevated in the red and blue zones as compared to the other wavelength groups. Most grow lights still contain a full spectrum of light as well, so they appear as an ordinary but very bright fluorescent light and we are able to see and watch plants grow in a light setting we're accustomed to. In theory, red and blue light (pink light) might be all that's really needed. With artificial lighting we are always working with a constant set kelvin values (k) like 3000 and 6500k. In the world outside, plants experience every possible k value under the sun, literally. And, finally, the sum of all the wavelengths intensities emitted by the tube is expressed as total lumens or peak lumen value
Stay away from incandescent bulbs coated blue or red. All that coating does is filter out the rest of the spectrum and show what appears to be red or blue light.
In general, I've found that young, small seedlings do not require a real intense light setting, but as they grow more, they require more and more light according to size. 5000k is about like midday bright sun in the temperate areas. 6500k (+) is sometimes referred to as super daylight. 3000k is similar to sunrise/sunset light; is sometimes referred to as harvest light or fall light. Some people use 3000k lighting for and during germination as well as during blooming.
Rick, maybe you can add in a little more or restate something to make this a little more understandable..
