Measuring Light

There are many different units for measuring light and it can get very complicated. Here are a few common measurement terms:

Candela (cd)
Unit of luminous intensity of a light source in a specific direction. Also called candle.
Technically, the radiation intensity in a perpendicular direction of a surface of 1/600000 square metre of a black body at the temperature of solidification platinum under a pressure of 101,325 newtons per square metre.

Footcandle (fc or ftc)
Unit of light intensity, measured in lumens per square foot. The brightness of one candle at a distance of one foot. Approximately 10.7639 lux.

Lumen (lm)
Unit of light flow or luminous flux. The output of artificial lights can be measured in lumens.

Lux (lx)
Unit of illumination equal to one lumen per square metre. The metric equivalent of foot-candles (one lux equals 0.0929 footcandles). Also called metre-candle.