Luminous flux is one of four basic quantities of lighting technology. The others are luminous intensity, illuminance and luminance. The unit for luminous flux, generally for brightness, is the lumen( lm). It is defined as the luminous flux emitted by a point light source in all directions.
The relationship between luminous intensity and luminous flux is given by the beam angle or solid angle. The luminous flux results from the product of luminous intensity and solid angle.
The luminous flux is a physiological quantity that has been determined from the average value of the sensitivity to light and the perception of brightness of many test persons. The luminous flux of an incandescent lamp is between 10 lumens per watt (lm/W) and 20 lm/W, energy-saving lamps and OLEDs have about twice the luminous efficacy, fluorescent lamps bring it to 100 lm/W, and power LEDs achieve over 250 lm/W. This means that LEDs have about 20 times the luminous efficacy of incandescent bulbs.
The amount of light is referred to when a luminous flux is available in a fixed unit of time: lumen-seconds (lms).