IESNA Definition: the ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

Luminous efficiency expresses the percentage of initial lamp lumens that are ultimately emitted by the luminaire. The efficiency of a luminaire does not necessarily indicate its effectiveness in delivering lumens to the workplane, or its appropriateness for the application.

A porcelain socket, utilizing a 100-W A lamp, has an efficiency of 100%, since no lamp lumens are trapped in the luminaire. A deeply recessed downlight with black multi-groove baffle, utilizing the same 100-W A lamp, may have an efficiency of 70%. Depending upon the application, the less efficient luminaire, because of reduced glare potential, may be the more appropriate choice of the two.

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Glossary Terms: baffle, IESNA