Lights are a very complicated subject.
Basically as I can make it:
Lumens is the total light output of the bulb in the visible spectrum. This can vary widely even for bulbs of the same watts.
CRI or color rendition index is a measure of the over all color tone and how accurate it makes colors look compared to sunlight. The higher the number the more accurate the color of a sample under the light. Which gives you a rough idea of how close to natural sunlight the color tone is.
Kelvin Temperature is another measure of the color tone of the bulb. The higher the number the more blue it is.
There is another measure.. PAR which measure the amount of photosynthetic active radiation a bulb puts out. See the problem is plants use light at 420nm and 700nm mostly for photosynthesis. The 420 is down at the bottom end of blue and the 700 is red/far red. These wave lengths dont appear all that bright to us. A light bulb can be talored to put out any wave of light and in any amount up to what the bulb can put out based on it's physical limitations (size, watts etc). A plant bulb looks kind of funky because it has lots of light at both 420 and 700nm and not alot in between. So it's way off on what natural sunlight would measure in CRI and Kelvin but high on the PAR value. To the plant it looks really great. On the other hand a warm white light bulb has a high portion of reddish light and very little blue. It is pretty bright to us but to the plant it has a low PAR value and looks pretty dim.
Without sophisticated equipment you cant really measure PAR value. Odds are you won't find it either from the bulb manufacturers spectral graph. You can check Lumens (the higher the better) CRI (closer to 100 the better because it will be well balanced between red and blue) and Kelvin 5000-6500 or so again so it is well balanced. There are bulbs on the market that produce a larger number of lumens for the same size and wattage compared to the every day cool white. You will pay more for them and odds are you will have to special order them from a lighting store. Is the higher light output worth the extra cost? beats me...
Here are two pages from Philips:
The colortone50 and the ultralume50. Both excellent bulbs at 5000k and high CRI. Note the difference in lumens. (I looked up 20watt T12 2' bulbs)
Colortone 50
Ultralume50
For kicks here is a standard coolwhite in the same size/watts for comparison. Note again the Kelvin, CRI and Lumens between the other 2
coolwhite
Personally I use the Ultralume50 in my lab.
Sylvania makes their own line of comperable bulbs.
Tony