Since these lights are all in greenhouses, I don't really use them close to my plants. My greenhouses have wire shelving and there are 3-4 levels of shelving. The closest plants would probably be 24" and the greenhouse flooring (where I have plants as well) will be at least 5' away from those lights. I do use some LED grow lights over specific "baby" orchids, but that's another universe of lighting that I won't get into here. Those LED's are really expensive compared to the basic broad-spectrum T-8's.
If you are growing in a house-setting, with weaker, indirect natural lighting, you could certainly use those T-8 fixtures as good light sources. I would position my plants within 12" of the fluorescent tubing. The tubing produces very little heat.
