Luminous flux - symbol f, unit lumen (lm)

Definition of luminous flux (or Light output of a source)

Luminous flux is the concept for the total quantity of light emitted per second by a light source. It is designated by the symbol f. The unit is lumen (lm).

Luminous flux can be defined as: the energy radiated by a light source per second, weighted against the spectral

sensitivity of human eye.

Some example of practical light sources:

Luminous efficacy

As becomes clear from the above table, there is no fixed relationship between the electrical energy dissipated in a lamp and the luminous flux radiated. The ratio between luminous flux and power dissipation is called 'luminous efficacy' and is expressed in lumens per watt (lm/W). Each lamp type has its own specific luminous efficacy.