Hi Shawn,
I actually have a light meter that I needed to use with an old film camera back in the days before digital cameras. It measures in foot candles. I did use it early on to calibrate my plant lighting. I think that zinnias would do OK with 300 foot candles on their leaves. However, I wanted to get as much as 500 foot candles on the leaves of my zinnias. Overdriven T8s, four fixtures per shelf, and with chains putting the 8 tubes only a few inches from my plants raised that intensity to about 1000 foot candles at the leaves. But that was not a requirement.
1000 foot candles sounds like a lot, except that outdoors in the daylight on a sunny day, plants are getting 3000 to 5000 foot candles on their leaves. But apparently my indoor zinnias are reasonably happy with light intensities about one-tenth what they might be receiving outdoors.
There may have been some significant losses with age in the light output of some old T12 fluorescent lights, both in some deterioration of the phosphors in their tubes and deterioration of the analog electric ballasts driving them. But T8 tubes are modernized with improved phosphors and have ballasts with solid state digital circuitry driving them, so loss of light output with age is not a significant problem with my T8 lighting. The tubes themselves do need to be kept clean, and I use Bounty paper towels and Windex to keep my tubes reasonably clean. Over a period of time (months or years), some "stuff" does get onto the tubes.
" I suppose the whole idea, was about exposure to "red" light, but ultimately, for the plants, isn't it more about exposure to Ultraviolet light, from the sun? I know there is UVA & UVB , rays. "
I don't think that plants require ultra-violet light, although there is a significant amount of it in sunlight. I think that plants just require red and blue spectrum content. However, "I" require some green content, so I use white fluorescent tubes for pleasing "natural" light. Plants may like purple light, but I don't. So white light is what they get. Although, I do use different "flavors" of white -- warm white and cool white.
ZM
