What is a Lumen?

A lumen is a unit of standard measurement used to describe how much light is contained in a certain area. Lumens are part of a group of standard measurements known as the photometry group, which measures measure different aspects of lights. This group also includes such units as the candela, which measures luminance, and the lux which measures illuminance. Strictly speaking, a lumen is defined as one candela multiplied by one steradian, which can be expressed as 1(lm) = 1(cd) X 1(sr). A related unit of measurement - although not part of the standard units - is the foot-candle, which is often used in photography and film. To really understand what a lumen is, it is important to understand these units: the candela, the foot-candle, the steradian, and the lux.