Four lighting characteristics are of primary importance for the selection of suitable luminaires:

• the distribution of luminous flux
• the distribution of luminous intensity
• the light output ratio of the luminaire
• and its glare limitation.
  
  

Luminous flux distribution

Luminaires distribute and direct the light emitted by lamps. Each type of lamp has a particular light output. Lighting technologists speak here of luminous flux, measured in lumens (lm).

The total luminous flux of a luminaire is the sum of the partial luminous flux emitted in the lower and upper halves of the luminaire. Depending on how much of the total luminous flux is emitted in each half, the luminaire is classed as indirect or direct (according to DIN 5044):

Luminous intensity distribution

Luminous intensity distribution describes the way in which luminous intensity is distributed in a room. It is visualised by an intensity distribution curve (IDC) formed by joining the points on a polar diagram showing the luminous intensity generated at different angles.

IDCs show the shape and symmetry of luminous intensity distribution. The most important distinctions are made between luminaires for narrow and wide-angle lighting and luminaires with symmetrical or asymmetrical beams. IDCs provide the basis for planning interior and exterior lighting.

Luminaire efficiency

The efficiency with which a luminaire distributes the light emitted by a lamp is indicated by light output ratios and coefficients of utilisation. The light output ratio indicates what percentage of the luminous flux of one or more unshielded lamps is radiated by a luminaire at an ambient temperature of 25° C. A general-diffuse luminaire radiating light in all directions thus has the best light output ratio – but offers poor lighting quality because it dazzles.

General-diffuse luminaires also have a poor coefficient of utilisation. This indicates what percentage of lamp luminous flux is directed to where it is needed, e.g. on a desk. Optical control elements make for a better coefficient of utilisation: the higher it is, the less energy is needed to achieve the required illuminance.

Glare limitation

Glare – either direct or reflected – can interfere significantly with visual tasks. So glare limitation is one of the most important lighting characteristics. The better a luminaire is shielded, the higher the quality of lighting.