Luminous flux or lumen is the quantity of the energy of the light emitted per second in all directions. Its unit is lumen (lm). It is the quantity of the energy of the light emitted per second in all directions. Given here is the simple luminous flux formula to measure the lumen of any given light source based on the values of maximum luminous intensity and cone full angle. First, find the equivalent solid angle and substitute the value in the lumen formula, to get the flux.