In a lighting system electrical energy is converted into light. In the process of light generation, however, there are certain losses in the lamp, and further losses are caused by the Material Properties of the luminaire components. In addition, not all the light emitted by the luminaires reaches the working plane to be illuminated, because of losses to reflected light at the room perimeters. The various losses are expressed in terms of efficiency using the following parameters.

PEL electrical energy input

Φ luminous flux from the lamp(s)

ΦL luminous flux from the luminaire(s)

ΦN luminous flux at the work plane

η luminous efficacy of the lamp(s)

ηLB light output ratio

ηR room index

ηB utilisation factor

Luminous efficacy is a property of the lamps, while the light output ratio relates to the luminaires, and the room index is determined by the room geometry and surface reflectances.