Definition of Luminous Flux
Luminous flux is a photometric measure of the total perceived power of light emitted by a light source. Unlike radiant flux, which quantifies the total electromagnetic power, luminous flux is weighted according to the average human eye's sensitivity to different wavelengths of light. This means that green light, to which the human eye is most sensitive, contributes more to the luminous flux than red or blue light of the same physical power.
How it is Measured: The Lumen
The standard unit of luminous flux in the International System of Units (SI) is the lumen (lm). One lumen is defined as the luminous flux of light produced by a point source of one candela luminous intensity over a solid angle of one steradian. Essentially, the lumen quantifies the total amount of visible light produced by a source, encompassing all directions, making it a comprehensive measure of a light source's overall output for human vision.
Distinction from Related Light Concepts
It's crucial to differentiate luminous flux from related concepts such as luminous intensity and illuminance. Luminous intensity, measured in candelas, describes the luminous flux emitted into a specific solid angle or direction, indicating the brightness in a particular beam. Illuminance, measured in lux, describes how much luminous flux falls on a given surface area, reflecting the light received by a surface. Luminous flux, in contrast, focuses solely on the total light output of the source itself.
Importance in Everyday Applications
Luminous flux is a critical specification when selecting light bulbs or designing effective lighting systems. It directly indicates how 'bright' a light source will appear to humans, making it a more practical metric than wattage for comparing the energy efficiency and visible output of various lighting technologies, such as incandescent, fluorescent, or LED lamps. Higher lumen values generally signify greater light output for a given energy consumption.