Lumen (unit)

The lumen (symbol: lm) is the SI derived unit of luminous flux, measuring the total quantity of visible light emitted by a source per unit time. Luminous flux differs from power (radiant flux) in that luminous flux is adjusted to reflect the sensitivity of the human eye to different wavelengths of light.

Definition

One lumen is defined as the luminous flux emitted into one steradian by a point source of light having a luminous intensity of one candela. In other words, a one-candela light source radiating uniformly into a solid angle of one steradian emits one lumen.

Mathematically, 1 lm = 1 cd ⋅ sr.

Explanation

The lumen is a measure of the "brightness" perceived by the human eye. It quantifies the total amount of light a source produces in all directions, weighted by the eye's sensitivity to different colors. Because the eye is more sensitive to green light than red or blue light of the same power, a green light source will produce more lumens than a red or blue one of equal power.

Relationship to Other Units

• Candela (cd): The candela is the SI base unit of luminous intensity, measuring the light emitted per unit solid angle in a specific direction.

• Lux (lx): The lux is the SI derived unit of illuminance, measuring the luminous flux per unit area. One lux is equal to one lumen per square meter (1 lx = 1 lm/m²). Illuminance measures the amount of light falling on a surface, while luminous flux measures the total amount of light emitted by a source.

• Watt (W): The watt is the SI unit of power (radiant flux). The relationship between watts and lumens depends on the spectral distribution of the light source. For example, a monochromatic light source at the peak sensitivity of the human eye (555 nm) would have a conversion factor of approximately 683 lumens per watt. This number decreases at other wavelengths where the eye is less sensitive.

Applications

The lumen is used extensively in the lighting industry to specify the brightness of light bulbs, lamps, and other light sources. Consumers use lumen ratings to compare the brightness of different lighting products when making purchasing decisions. It is also used in calculations related to lighting design and photometry.