Conclusiones

Rooms that are an everyday experience become a reference for all other interiors. Most familiar of all, for many of us, is the small or medium-sized room with side windows. Its characteristics are listed in Table 7.1. Such a room has been the most common interior form in temperate-climate buildings since the seventeenth century. It was illustrated accurately by painters such as Vermeer and de Hooch, and it remains the type of space most likely to be described as ‘a normal room’: it is the stereotypical living room, or school classroom or small office. It has a complex lighting pattern that we see but take entirely for granted. Figure 7.1 shows the illumination on adjacent surfaces from a small window; in addition to the effects of the flow of light from the sky itself, there is reflection from the ground outside and from many minor areas in and around the window opening. Further back in a room from a side window there is a gradual change in overall

brightness as distance from the window increases, and a differing contrast between horizontal and vertical surfaces.

Now if we enter a room that has a pattern of light and dark significantly different from what we take to be normal, we tend to attribute to the place a specific character. Quite typically:

• surfaces with higher than normal reflectances, especially with light-coloured floors, are associated with interiors that are ‘bright and airy’ (such as the hallway of Ernst Giselbrect’s primary school, Figure 7.2);

• predominantly dark surfaces are associated with ‘enclosure’, so if the upper parts of a room are very dark we might call the place ‘cave-like’; if much of a room is too dark for us to be certain of its nature (as in Figure 7.3), the place seems ‘mysterious’ or

‘frightening’;

• richly decorated surfaces suggest ‘grandeur’, drab surfaces the opposite; if there is uni-form electric lighting that masks the natural variation of daylight the place can be ‘dull’.

Figure 7.2 Primary school. Strass, Austria.

Different building types have different stereotypes, and people’s views (and especially the words they use to describe them) vary from place to place and from time to time. Part of the training of the designer is to sketch and record, building up a repertoire, observing not just the dominant patterns of lightness and darkness that occur in rooms, but also subjective responses to them.

However, an interior brightness distribution that is frequently found is not necessarily one that is preferred. For example, electric lighting in office buildings tends to be used continuously during daytime hours, but often with little room illumination from daylight

58 The design of lighting

Figure 7.3 A man seated at a table in a lofty room, Rembrandt (1606–1669). (The National Gallery, London)

Figure 7.4 Surface brightness in a room with recessed low-brightness luminaires.

Figure 7.5 Office lighting using luminaires with both upward and downward light output.

itself. Many conventional design standards for offices are based on working-plane illuminance (the light falling on the horizontal desk surface), with secondary criteria

60 The design of lighting

intended to limit direct glare and minimize bright reflections in computer display screens.

An outcome from these rules is the common use in offices of ceiling-mounted luminaires with strongly downward output, giving a concentration of light on the working plane, and vertical surfaces that

Table 7.1 A ‘normal’ daylit room Floor

Typically the floor has medium to low reflectance, often the colour and pattern of natural materials.

It is strongly illuminated in areas near the window, but may not be perceived as a bright surface.

Ceiling

The ceiling is usually the surface of highest reflectance, often white. It is lit by reflection from the ground outside and by interreflection in the room.

Vertical surfaces

The walls hold pictures, furniture, curtains; they are punctured by doors and windows; they tend to be decorated in stronger colours; the illuminance can be high in surfaces on which skylight or sunlight falls directly but low in areas hidden from the sky. Vertical surfaces vary in colour and pattern more than floors or ceilings, both within a room and between different rooms.

Directionality of light

Windows form large diffuse sources; their light flows downwards with a strong horizontal component.

are relatively dark. This is shown in Figure 7.4, in comparison with Figure 7.5, which illus-trates an office with luminaires designed specifically to place some light on the ceiling.

Figure 7.6 Schematic relationship between interest and brightness.

Despite adequate task illuminance, a room with only downward lighting is often assessed as ‘under-lit’. In research examining the preferences of offices workers for various room luminance patterns, it was found that the rooms most liked had a ‘bright’

appearance, and this was related particularly to the wall and ceiling surfaces. Typically the cho-sen rooms had a mean wall luminance of at least 30 cd/m² (or about 200 lx illuminance on mid-reflectance colours). Assessments were linked also to the variability of the brightness; this was described in the experiment as the ‘interest’ of the light pattern. The interiors preferred above all others were those with both relatively high luminances and high interest in the broad zone seen above the desk working area, or the area most likely to be seen with a horizontal line of sight. Figure 7.6 shows schematically that, for a particular application, there is pos-sibly a most preferred combination of brightness and interest. It is probably significant that the patterns found to be preferred are similar to those that occur in a daylit space.

Preferences for workplace lighting are therefore affected by more than task illuminance alone. For rooms such as offices it is useful to consider two separate zones, the micro and the macro fields, shown in Figure 7.7. The micro field is the task zone; the macro field is the wider 40° zone around the direction of vision.

Both are continuously in the view of a person engaged in a task. The needs of good task light-ing may require the immediate background to be darker than the central task area (as we shall see in Chapter 9), but these must not override the needs of brightness and interest in the macro field.

Figure 7.7 Macro and micro fields of view.

Judgements related to surface brightness and room character are not dependent on the whole room being in view. We can make inferences from the parts that are visible, and often the clues we use are the patterns of reflected light. When major room surfaces are dark coloured there is little interreflection, and the total quantity of luminous energy within the space is small in relation to the light emitted from the sources. With high reflectances the overall distribution becomes diffuse: there is greater uniformity and greater total surface illuminance.

A high-reflectance ceiling acts as a large source for the light it reflects, noticeable on objects below. The role of the floor of a room is particularly important because often it

62 The design of lighting

receives strong direct illumination, so a light-coloured floor may give clearly apparent upward illumination onto room contents and the ceiling, especially if direct sunlight enters the room. Often the nature of a space can be guessed when only the light falling on people or objects within it is seen—as in Figure 7.8, where the pattern of shading on St Francis’ clothing is enough to describe for us the room as a whole.