What Is A Foot-Candle?
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Historically, light for plants has been rated in terms of foot-candles. Strictly speaking, a foot-candle is the amount of light cast by a candle 1 foot away.
Plants that adapt to low light will grow with only 100-300 foot-candles of light; those that need medium light need 300-600 foot-candles, while high-light plants need a minimum of 700-1,200 foot-candles. In the interest of comparison, sunlight at noon on a clear summer day is usually in excess of 10,000 foot-candles.
Your plants will usually let you know when they are receiving too little or too much light, but if you like, you can calculate foot-candles using a camera with a built-in light meter, focused on a white piece of paper in the spot you want to measure. If the camera shows an f-stop of f2-f4, with a slow shutter speed to match, the foot-candle rating is probably less than 400, or low light. An f-stop of about 5.6 with a shutter speed above 1/125 second suggests 400-600 foot-candles, or medium light. High light would give an f-stop of 8 or greater, which translates as more than 700 foot-candles.
You can also use the simple shadow test to evaluate light. When the shadow cast by your hand is barely discernible, the site receives low light. A well-defined shadow indicates moderate light, while a sharp, high-contrast shadow is created in high-light conditions.
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