How to Convert Luminous Flux to Lumens

Use the measurements in watts on a light bulb or bulb package to determine radiant power. Also known as the radiant flux, radiant power is factored by the sensitivity of the human eye to determine luminous flux in lumens. Because most lights used will be within the optical range, the wattage listed for the bulb can be used as the radiant power or flux of the light bulb for the purposes of determining luminous flux.

Determine the luminous efficacy of the light source. The luminous efficacy is equal to the light power or electric power for typical incandescent bulbs. This is determined by dividing the equivalent light power by the electric power for a typical incandescent bulb using the standard for a 100 watt bulb of 1700 lumens, where the light bulb has a lumimous efficacy of 17 lumens per watt. The formula using these values to determine luminous efficacy is 1700 lumens/100 watts = 17 lumens.

Plug in the values to determine the luminous flux of a light source in lumens using the formula: Luminous flux in lumens = Watts x 683 lumens/Watt x Luminous efficacy. The figure of 683 lumens/watt is based upon the assumption that the sensitivity of the eye is 555nm, the peak efficiency of the daylight vision curve where luminous efficacy is at a value of 1 at that frequency.