Apples & Oranges
A much heard argument of the criticasters against the use of CFL’s for growing plants is: “if they where really useful/efficient, they would be used in the professional/commercial greenhouses too”. This is however like comparing apples to oranges.
In greenhouses the artificial light is used to supplement the ambient sunlight. Because most of the time the ambient sunlight already contains enough light in the blue spectrum that is necessary for phototropic response1, supplementing light in the photosynthetic range/spectrum is the most efficient. Although HPS lights have a rather narrow spectrum-band with much infrared, the light they do transmit in the PAR-range is mainly in the in the red photosynthetic range. Also, a supplementing light source is preferably of high output and small physical size including its reflector so it can be hung high above plant canopy, and does not block the ambient sunlight. This all makes the HPS light the best choice for a supplementing light source, and is also the reason why CFL’s can not function as an alternative in greenhouses.
But, when using artificial light as a solitary light source, things are a lot different. Not only does the lack of ambient light require a light source with a broader spectrum than HPS lights transmit, it also removes the need for a compact reflector so the complete system (light including the reflector) can be almost as big as the area that’s illuminated. On top of that, lights can be hung as close to the plant canopy as temperature allows, this way we can make use of the good light level in relation to the temperature that the CFL gives us (see fig. 3).
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Fig. 1 Fig. 2 Fig. 3
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