IESNA Definition: Standard International (SI) unit of luminous flux. Radiometrically, it is determined from the radiant power. Photometrically, it is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.

The lumen is the time rate flow of light. Whereas a lamp will have many candela values, depending upon the direction of interest, it will have only one lumen output rating. The lumen rating can be considered as the measure of the summation of light output of a lamp. Ratings are determined and published by the lamp manufacturer.

During the design process, lighting specifiers use the lumen ratings of lamps to predict the final illuminance in a space. Since energy-efficient design has become increasingly important to the end user, designers also evaluate the lumen output per watt consumed, or efficacy, of the various lamp choices. Here are a few examples of lumen ratings for some commonly used lamps:

Lamp Lumens
60-W A19 incandescent 750
135-W A21 energy-efficient incandescent 2580
40-W T12 cool white fluorescent 3050
34-W T12 cool white fluorescent 2650
34-W T12 4100 K, CRI 82 2850
32-W T8 4100 K, CRI 85 3050
70-W medium base metal halide, clear 5200
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Glossary Terms: IESNA, luminous flux, solid angle, steradian