What is luminous flux?
The total visible light a source emits in all directions, and the starting point for the beam a fixture’s optics create.
Luminous flux is the total amount of visible light a source emits in all directions, measured in lumens. It is weighted by the sensitivity of the human eye, so it describes light as it is seen, not electrical power in watts.
In AGL, flux is the starting quantity. A fixture’s optics redirect it into a beam, and what the requirement checks is intensity in candela, which is flux per unit solid angle: 1 cd = 1 lm per steradian. The same flux packed into a narrower beam gives a higher candela value, so a lumen rating on its own does not show whether a fixture meets an AGL photometric requirement.
Frequently Asked Questions
Q1What is luminous flux?
The total amount of visible light a source emits in all directions, measured in lumens. It is weighted by the sensitivity of the human eye, so it describes light as it is seen.
Q2What is the unit of luminous flux?
The lumen (lm). One lumen is the flux a source of one candela sends into one steradian, which is why one candela equals one lumen per steradian.
Q3How is luminous flux different from luminous intensity?
Flux is the total light output of the source. Intensity is the part of that flux going into one direction, per unit of solid angle, measured in candela.
Q4Why is flux not enough to specify an AGL fixture?
The same flux can be spread widely or concentrated into a narrow beam. AGL photometric requirements are set as intensity in candela at defined angles, so the lumen rating alone does not show whether a fixture meets them.
Q5How is luminous flux different from illuminance?
Flux belongs to the source. Illuminance is the flux that lands on a surface, per unit area, measured in lux, so it changes with distance and with the surface.
Q6Is luminous flux the same as power in watts?
No. Watts measure electrical or radiant power. Lumens weight the light by the eye’s response, so two sources of the same wattage can give different luminous flux.
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