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: