Lumen refers to the standard unit of measurement, which is used to indicate the amount of light that is contained in a particular space. This unit of measure is under the photometry group of measurements, which is intended for quantifying the various aspects of light. Other standard units of measurements under the photometry group include candela and lux, which describes luminance and illuminance respectively.

Basically, lumen is equivalent to 1 candela times 1 steradian or 1(lm) = 1(cd) x 1(sr). The foot-candle is another unit of measure, which is applied in the field of film and photography. In order to understand lumen, it is necessary to grasp the concepts of lux, candela, steradian, and foot-candle.

The difference between measuring radiance and illuminance is that when radiance is measured, the focus is on the amount of energy that is released by a source of light, and is not concerned with what happens to the light energy as it leaves its origin. On the other hand, when illuminance is measured, the person is interested about the quantity of that energy that makes it to a particular thing.

The foot-candle and lux are two units of measurement, which make use of candela—a unit that describes luminous intensity. Originally called candlepower, the candela indicates the quantity of luminous energy, which is released by a type of candle. The steradian unit of measure, on the other hand, is used to quantify solid angles.

When all of these concepts are incorporated together, it would be much easier to understand the meaning of lumen, which is expressed as 1(lm) = 1(cd) x 1(sr). Simply put, lumen refers to how much luminous flux is released into an area at a specific angle. Typically, the lumen unit of measurement is used together with wattage, in order to identify the luminous efficiency of a certain source of light.