The optical system of a luminaire is used to distribute the luminous flux of the light sources in a way that fulfils the required lighting task as well as possible. For energy considerations, light output ratio and luminous flux distribution as well as illuminance distribution are important luminaire parameters.
The light output ratio LOR specifies what portion of the lamp luminous flux leaves the luminaire. The type of lamp luminous flux direction and the thermal conditions within the luminaire determine the light output ratio. A light output ratio is only specified for luminaires operated with replaceable light sources (lamps), which in turn feature a lamp luminous flux specification. For luminaires with permanently integrated LEDs, the rated luminous flux is specified for the luminaire. Formally, lamp luminous flux is used synonymously with the luminaire’s rated luminous flux in LED luminaires. By definition, the light output ratio is therefore set to the value 1 (see also chapter ).
The luminous efficacy of a luminaire is generally understood to be the product of lamp luminous efficacy and light output ratio. This assumes that the lamps demonstrate the same power consumption when used under the operating conditions within the luminaire as they do under normal conditions outside the luminaire (see chapter "Light output ratio"). The luminous efficacy of LED luminaires is referred to as rated luminous efficacy (see also chapter "Luminous efficacy of LED luminaires").
The luminous flux distribution of a luminaire determines its lighting-related effect in a room. The entire luminaire luminous flux is composed of luminous flux components in the upper and lower segments within the space. The luminous flux component in the upper half of the space is reflected by the ceiling and/or parts of the walls and contributes to indirect lighting. The luminous flux component in the lower half creates direct illuminance on the working plane and additionally increases the indirect component of the illuminance via reflection from floor and walls. Direct-distribution luminaires mainly focus the luminaire luminous flux on the surface to be illuminated and create high illuminance values there.
The utilization factor UF is defined as the ratio of the luminous flux incident on the effective area and the sum of the luminous fluxes of all light sources installed. It increases the higher the light output ratio is. Beyond that, utilization factor – as described above – also depends on the spatial distribution of the luminous flux of the luminaires. It also influences the level of power consumption needed for achieving the illuminance required on the working plane.
The lighting’s energy efficiency therefore depends on the luminaire’s luminous efficacy and its utilization factor in the given application (see also section "Energy labelling" in chapter "Efficient light sources").