Luminance and shade

Luminance describes the brightness impression that the human eye has of a luminous or illuminated surface.

Luminance L is measured in candela per square metre [cd/m²]. Here, the luminous intensity is placed in relation to the illuminated or luminous surface.

To assess the quality of road lighting, the calculation of luminance is mandatory. On account of the standardised reflection properties of road surfaces and the definition of the location of the viewer, the calculation of luminance is an integral part of the planning of road lighting. The planning aid “Light for Europe’s roads“ regulates the lighting of roads, paths and squares in accordance with DIN EN 13201. It makes it clear that increasing luminance from, for example, 1 cd/m² to 2 cd/m² reduces the accident rate by about one third.

The different luminous flux levels of lamps, different light distribution by the luminaires or varying geometry in lighting systems influence the illumination of a road. Another important factor here is the reflection property of the road surface.In order to evaluate the exact luminance of the carriageway, a flat part of the carriageway must be selected with uniform reflection behaviour. A representative luminaire spacing with two luminaires and an observation location 1.5 m above the centreline of the carriageway is used.

Recommendation:

Residential street 7.5 lx

Main road 1.5 cd/m²

Car park 15.0 lx

Light defines our spatial environment. Light and shade give room a structure. Objects appear to be plastic, and surfaces are explained. The interplay between light and shade enables us to determine distances and dimensions. Room situations that are visually easy to understand and grasp give us a feeling of safety. There are now two extremes of illumination that cannot be ignored: Diffused light, which hardly develops any shadows, and extremely directed light, with strong shadows.

In diffused light, the room does not appear plastic, it appears monotonous. Objects and dimensions are hard to detect.

In extremely directed light, individual room elements are strongly emphasised and cast high-contrast, hard shadows. The rest of the room remains unilluminated. Both lighting situations can trigger discomfort and insecurity. A balanced combination of both gives the room dimension and gives objects plasticity. For this reason, many luminaires are developed to combine direct with indirect proportions of light. This leads to a significant increase in their possible applications.