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