Before buying lights for an indoor garden, you will need to learn the different light requirements of each vegetable. Leafy and root plants, such as lettuce and radishes, need about 1,000 footcandles (the equivalent of bright shade) for good growth. Vegetables from which we harvest the fruit—such as tomatoes, eggplant and squash—need a minimum of 2,500 footcandles of light to reach maturity.
Few gardeners own a light meter to check their indoor gardens. Plants, however, will become long and spindly or manifest some other abnormality if they are not receiving adequate light (At that point it may be too late to take corrective measures.) One way to measure light without a special meter is to use a camera with a light meter built in to provide exposure settings. Set the film speed at ASA 100 and aim the meter at a white piece of paper placed so as to approximate the position of the plant's surface. The shutter speed reading that appears at F4 will correspond to the approximate foot candles of illumination. For example, if the indicated exposure is l/250th of a second at F4, this means 250 footcandles of light are needed.
There is a light gardening rule of thumb dealing in watts per square foot. For example, a two-lamp fixture holding two 20-watt fluorescent tubes gives off a total of 40 watts. If the fixture and its bulbs cover a two square-foot area when mounted one foot from the surface, then that surface is receiving 20 watts per square foot
44watts/2 sq ft = 20 watts per sq ft.
Most leafy and root vegetables need 20 watts per square foot or more, and most fruit-bearing vegetables need at least 40 watts per square foot.
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