Added technical information why halogen lighting is a poor choice for plant cultivation by wannabee2004:
Q: Do halogen bulbs have a good spectral output for plant growth?
A: No, the spectral plot output of various halogen bulb types is by far the worst.
Q: Are they efficient at generating useful light for plant growth?
A: No, the best floro/hps/mh lighting each being approx 5X more efficient at plant growth than halogen.
A good way to measure plant growth efficiency of different types of lighting is PAReff. PAR stands for Photosynthetic Active Radiation: the wavelength range of light between 400 and 700 nm that can be absorbed by leafs for plant photosynthesis. PAReff is the PAR/Watt efficiency ratio over a certain area.
Philips Advantage Fluorescent, 5000K: 1.44 PAReff
Philips High Pressure Sodium deluxe: 1.38 PAReff
Philips Dense-Line Emitter Metal Halide, 4100K: 1.38 PAReff
Philips Spot Halogen Masterline, 2950K: 0.263 PAReff
People usually choose halogen for the "low startup" costs of a halogen light. The real costs of halogens result from its low efficiency. 500 Watt halogen lighting produces about 0.08 Watt/s/nm in the green-yellow spectral region which is about the same as 80 watts of cfl's. A 500w halogen grow will cost approx $63 for electricity, whereas a more efficient light source generates the same par value for around $10 (depending on your local prices).
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