Radiant power weighted by the photopic luminous efficiency curve, so that each wavelength counts by how bright it looks to a typical eye. It is the headline output number for lamps and luminaires and is measured in an integrating sphere or calculated from goniophotometer data. Flux says nothing about where the light goes: two luminaires with the same lumens can light a room completely differently.
Luminous flux on the test report is 4,200 lumens but that's the bare board, the fixture with its lens is closer to 3,600.
Also heard as
- flux
- light output
- lumen output
Do not mix it up with
- Luminous intensity: Luminous intensity is flux per unit solid angle in one direction, measured in candelas, not the total output.
Related terms
Lumen
The unit of total visible light leaving a source, with each wavelength counted by how bright it looks to the eye.
Luminous efficacy
How many lumens a light source or luminaire produces for each watt of electric power, in lm/W.
V-lambda curve
The standard curve of the eye's daytime sensitivity to each wavelength, peaking in green at 555 nm.
Integrating sphere
A white-lined hollow ball that spreads a source's light evenly so one detector can read its total lumens and color.
Lumen package
Designer and rep shorthand for the delivered lumen output option of a luminaire, chosen from a catalog range.