Quote:
Originally Posted by Hermitian
Lumens are a measure of human perception of brightness. There is no relevance to plants. They are only on the label because of an outdated US architectural law regarding light bulbs. What matters for plants is spectrum (6400-6500 Kelvin) and Watts/sq.ft. of projected area. Fluorescent bulbs have an output efficiency (initially) in the high 90 percentile, so input Watts is very close to output Watts.
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Actually that looks either like a ballast efficiency or a ballast factor. Fluorescents are in the 25% - 40% range. The main problem is that low pressure mercury emits most of it's energy in the ultraviolet range. That ultra violet is then used to make the phosphors in the tube glow. A photon of ultraviolet light has much more energy that a photon of visible light. The difference in the energy is lost as heat.