Room properties

The shape and design of a room, through reflectivity of walls, floor, ceiling and furniture, has a decisive influence on the dispersion of light and thus the lighting’s efficiency. This means that luminaires should be selected according to their efficiency in applications.

Direct-distribution luminaires are preferred in industry, trade and handcraft applications, where ceiling and walls feature mostly poor
reflectivity, and are assigned to working areas according to EN 12464-1 [48].

Direct-indirect-distribution systems require high ceiling reflectance as well as a limited ceiling height which facilitates utilisation of the reflected light component. Direct-indirect-distribution luminaires with special optics for direct components are particularly desired in office working areas. Reasons for this are high requirements in terms of direct and reflected glare limitation, as well as, high comfort levels achieved by lighting systems with strong ceiling illumination akin to natural lighting as provided by the sky. Where luminaires are directly assigned to working areas, this type of lighting often results in better energy consumption values compared to general lighting for the whole room with direct-distribution luminaires (see also chapter "Lighting concepts").

Indirect-distribution systems, meaning those exclusively directed towards the ceiling, require more energy than comparable direct-distribution systems even with high ceiling reflectance values. They should be restricted to cases where a strong ceiling illumination is required to supplement zonal direct lighting. Examples for the use of entirely indirect lighting are areas such as entrance and banking halls as well as museums, where minimal shadows and reflections on the exhibits are desired. Corridors and common areas, particularly in healthcare sector establishments, can be illuminated adequately and free of glare with indirect-distribution wall luminaires.

Bright surfaces with high reflectance generally increase the utilization factor. This also applies to windows, which only have a reflectance value of about 10%. Bright curtains can increase it to 50%. However, dark ceilings, walls and furniture absorb a lot of light and thus require higher luminous flux levels for the same illuminance.

This particularly applies to compact rooms and narrow corridors. Here, even the light of purely direct-distribution luminaires must be reflected to a great extent – sometimes even multiple times – before it reaches the effective area.

The proportions of a room are accommodated by the room index k in photometric design, which describes the geometric conditions of the room in the following form (see also chapter "Lighting design" and chapter "Luminous flux classification of luminaires and efficiency methods").

In geometries with a small room index, the reflectance values of the boundary surfaces influence the energy consumption for lighting in a particular way. This means more lamps, more luminaires and more energy.

Figure 3.55: Calculation parameters for the room index k:
a  room length in m
b  room width in m
h  = H - lp - e, mounting height in m
H  ceiling height in m
lp  suspension length of the luminaire from the ceiling in m
e  height of the rating level above the floor, e.g. in an office 0,75 m