Foot candle (sometimes hypenated: foot-candle) is a term of measurement. It indicates the amount of light generated by a single candle that falls on a single square foot of surface no more than a foot away from the candle. As a unit of measurement, foot-candle is rarely used today.
Lumen is the preferred term, and a foot-candle is essentially the same one lumen per square foot of surface. Another similar unit of measurement is lux.
The term foot-candle is somewhat archaic, but it is gaining popularity in many areas, particularly with indoor gardening. This is due to several factors, including the fact that watts and lumens do not actually tell an indoor gardener how much light is being delivered to the plants being grown.
Most of us are accustomed to choosing light bulbs based on wattage – a 60W bulb should emit X amount of light. We know this based on experience with that particular wattage. However, wattage is not a measure of light emission; it’s a measure of energy consumption by the bulb. Replacing a 60W incandescent bulb with a 60W LED or T5 bulb would actually generate a significantly greater amount of light.
Because foot-candles measure the amount of light provided to a specific amount of space, this unit of measurement is actually better suited for indoor gardening than other options, including lumens or lux. Really, one foot-candle equates to a single lumen per square foot of surface space. Based on this knowledge, indoor gardeners can extrapolate how many candle-feet of light output they will need for their space.
With the right foot-candle rating in terms of light output, you can ensure that you’re providing your plants with the optimal light levels for growth and health.