Lighting theory

Degree of illuminance, reflection and luminance

The degree of illuminance E [ lx ] and the degree of reflection of the surface ρ are included – using the following formula – in the calculation of luminance L [ cd/m2 ]:

L = E · ρ / π

The luminance and thus the effect of the illuminated building or memorial are influenced significantly by the degree of reflection, which is reduced through the accumulation of dirt. However, the effect of the dirt diminishes with the degree of darkness and roughness of the original material. The degree of illuminance must be adjusted accordingly in order to achieve the same luminance or the same perceptive impression.

The effect of the building is all the more impressive the more it stands out from the background. The illumination of a tower in the middle of an illuminated city requires a higher degree of illuminance than if the background of a building is dark, for example a castle standing alone in the landscape.

Influence of reflection surfaces

Despite standardised illumination, the level of illumination may be felt to be too low, depending on the nature of the floor, wall and ceiling surfaces.

In public areas, standards for light planning are very helpful. However, the influence of the spatial conditions must not be neglected. Examples of this are a public hall, a public square and an inner-city main road. Lighting standards for the square and the hall are indicated in lux [ lx ], that is, the degree of illuminance. Standards for inner-city main roads are defined in luminance using candela per square metre [ cd/m2 ].

For example, an average degree of illuminance of 200 lx applies for the public hall. Proceeding from an average room, the standard provides for clearly defined reflection properties of walls, ceilings and floors. If these parameters are changed, the standardised degree of illuminance remains constant at 200 lx. However, the impression of the room is completely different. The illumination level is felt to be too low. From the table it can be seen that the luminance falls to 50% of the initial value. In this case, we recommend raising the degree of illuminance by 100 lx to 300 lx. This way, the illumination in the room would be felt to be more pleasant.

The same largely applies for the space situation in the public square. The value for the degree of illuminance is estimated to be 10 lx. With light ground and a reflection of 30%, the luminance level reaches 1 cd/m2. If the reflection of the ground is reduced to 10%, and the standardised degree of illuminance remains at 10 lx, the luminance is only 0.35 cd/m2. Here we recommend raising the degree of illuminance by 10 lx. This way, the lighting conditions on the square are considered to be more pleasant.

As far as road lighting is concerned, the uniformity of the light on the carriageway and the related distances between the luminaires are interesting for the planners.

The standard specifies values for average luminance. The adjacent table shows the effects of light and dark road surfaces on the distances between luminaires. When the road surface is light ( C1 ), the luminaire spacing is 36 m. On dark road surfaces (C2), the distance is reduced by 10 m to 26 m. Over a distance of one kilometre, this amounts to 28 luminaires instead of 38. The choice of road surface thus has a direct economic effect, which should not be neglected during planning.