A standard four-foot unit with two 40-watt bulbs -- or tubes -- illuminates an area about eight inches in width. A bright but overcast day measures about 1,000 foot-candles. By contrast, a bright, sunshiny day generates about 10,000 foot-candles.

Contrast this to fluorescent lights. At six inches from the source, you'll be receiving about 700 foot-candles. And when you're a foot from the light, the foot-candle measurement drops to 450.
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Carmel Santos
CEO of Herb Gardening Guide
Growing Herbs in Pots
The Australian Guide to Herb Gardening