What is a "footcandle" and why do I need them?

A footcandle is a measure of light intensity. While a lamp (the source of the light energy) produces lumens, footcandles (fc) are measured light intensity at a certain point. A footcandle is scientifically, a unit of measure of the intensity of light falling on a surface, equal to one lumen per square foot and originally defined with reference to a candle burning at one foot from a given surface. Since light intensity is the primary factor in the photosynthesis of plants, horticulturalists often measure and discuss optimum intensity for various plants in foot-candles. Full, unobstructed sunlight has an intensity of approximately 10,000 fc. An overcast day will produce an intensity of around 1,000 fc. The intensity of light near a window can range from 100 to 5,000 fc, depending on the orientation of the window, time of year and latitude. Footcandle measurements will decrease rapidly as the light source is moved further away from your plants.  more