Lighting theory

Luminance in proportion

On the basis of three examples, luminance values are compared with each other in specific applications. The examples are the square in front of a historical function building, a train station concourse and a shop for exclusive accessories in a pedestrian zone.

Example 1: Square in front of a historical function building

Visitors are guided towards the entrance of the building with increasing illumination densities.

The visitor is received on a generously dimensioned square, which one can reach through a small park. Vehicles may only stop briefly in the access road to pick up and drop off guests. The park is framed by unshielded bollards, whose light allows good face recognition on the footpath. At the same time, the pedestrians are clearly visible for the drivers. The square in front of the historical building is illuminated with light building elements. This gives the area a structure. A generously dimensioned staircase, where recessed wall luminaires are installed in the side walls, leads into the building.

The entrance area welcomes the guest with arcades with pillars illuminated with in-ground floodlights. Unshielded ceiling luminaires are used within this covered area. On the roof of the arcades, surface washers emphasise the upper areas of the building façade. In the direction of the entrance, the level of illumination increases more and more. Starting with the lowest luminance at the bollards along the edge of the park, the level increases towards the street to 1 : 2, continuing on the square 1 : 2 : 3 and towards the staircase 1 : 2 : 3 : 5. Directly at the entrance, fifteen times the luminance is achieved compared to the starting position, i.e. 1 : 2 : 3 : 5: 15.

a) Entrance 15-fold luminance b) Stairs 5-fold luminance c) Square 3-fold luminance d) Driveway 2-fold luminance e) Adjacent park 1-fold luminance

The bird’s eye view illustrates the relationships of the luminance values. The lowest luminance value is defined as 1. The result is a ratio of 1 : 2 : 3 : 5: 15.

Example 2: Train station building

Two massive structures are connected with an arched roof. The front sides of this train station concourse are glazed, and provide a clear view as far as the forecourt and the platforms. On the right and on the left, business shops lead into the building. The information desk is in the centre of the hall. In the extension of the hall, at the back, stairs lead to the platforms. The luminance levels are considered from the entrance to the station to the stairs leading to the platforms. The lowest luminance is at the covered area directly in front of the station and is thus “1.” Luminance increases threefold upon entering the entrance hall.

The open information desk in the centre is emphasised with additional light. The result is a ratio of 1 : 3 : 6. The shopping arcades are illuminated slightly more than the station concourse. The illumination of the shop windows has not been taken into account here, since this may vary according to the time of day and is controlled individually. Compared to the entrance, the arcades have a luminance of 1 : 4. The lighting around the stairs leading to the platforms becomes somewhat darker again. Here, luminance is twice as high as in the entrance area, i.e. 1 : 2.

a) Entrance 1-fold luminance b) Station concourse 4-fold luminance c) Information desk 6-fold luminance d) Shopping arcades 4-fold luminance d) Arcades leading to the platforms 3-fold luminance

This view illustrates the relationships of the luminance values. Here too, the lowest luminance value is defined as 1. This results in 1 : 4 : 6 : 4 : 3.

Example 3: Shop for exclusive accessories in a pedestrian zone

The evaluation surface of the shop stretches from the pedestrian zone directly in front of the shop to the interior rear wall, where there is a backlit presentation of goods. Here, too, we begin with the lowest luminance value: directly in front of the shop in the pedestrian zone. The entrance is flanked by two shop windows whose illumination is adjusted to the level of daylight using a light control unit. The figure measured at floor level is 200 lx. The 500 lx mark is reached at the cash desk. This results in a ratio of 1 : 4 : 10.

The goods on the side walls are presented with 20 times the luminance compared to the entrance. Luxury products are presented on the facing wall in built-in and backlit display cabinets. In front of these there are small tables for sales conversations. Here a degree of illuminance of 1800 lx is required. Starting from the lowest luminance value, a ratio of 1 : 36 is obtained. This draws the customers’ attention directly to the goods.

a) Pedestrian zone 1x luminance The luminance in the shop window is controlled depending on the amount of daylight b) Floor 4x luminance c) Counter 10x luminance d) Shelves 20x luminance e) Sales counters 36x luminance

In the shop, the relationships between the luminance levels vary greatly. Here too, the lowest luminance value is defined as 1. This results in 1 : 4 : 10 : 20 : 36.

The examples listed above show possible luminance distributions. From the darkest area to the brightest area, some of the differences are extreme. Accordingly, the light is felt to be pleasant. Why? In the function building (example 1), the entrance is the brightest point. The luminance increases gradually for the visitors as they arrive. The entire situation can be grasped at a glance. These factors convey a feeling of safety. The spatial situation is clear, and is given a clear structure through light. The path towards the event is shown through the increase in luminance. Visitors can easily find their way around, and get the feeling that they are in good hands.

In the station (example 2) soft transitions between the various luminance levels are important. Many travellers are in a hurry. The focus is on information about train connections. Highly contrasting lighting conditions would interfere. Here, a build-up of tension in the illumination has largely been dispensed with. Certain zones are emphasised slightly, by increasing the luminance values at the information desk and in the shop passages. All in all, the light in the station is uniform and conveys a feeling of visual tranquillity. The eye hardly has to adapt.

The illumination in the exclusive shop (example 3), on the other hand, is tense and accentuated. High contrasts attract the attention directly to the presentation of the goods. Despite these differences in light intensity, customers find the light pleasant. The entire lighting situation is easy to absorb.

Very strong light-dark contrasts appear within the human field of vision. It would appear that these contrasts add up to an average value, because they do not lead to rapid fatigue of the eyes. The strain on the eye to adapt to the light conditions would be enormous if it were to experience such strong contrasts in an alternating sequence. So if a lighting situation can be comprehended at a glance, jumps in luminance distribution can be usefully employed. In this way, an arc of tension can be built up with light and persons can be guided and accompanied.

If a room is uniformly illuminated with a certain luminance and the dimensions of this room completely fill the human field of vision, the transition to a more strongly or less strongly illuminated area should only be gradual. In this way, the eye has a good chance to adjust to the change in lighting conditions without becoming tired. Here, jumps between greatly differing luminance levels would lead to glare and rapid eye fatigue. Good illumination makes a significant contribution to our well-being and often leads to business success. In the end, the light designer must decide on site which light is the best in the situation in question.