TASK LIGHTING

The principles of task light with daylighting are the same as for electric lighting: a minimum illuminance will be required, and the directionality of the light must enhance visibility. For task performance we are concerned with the absolute quantity of light falling on a surface, whereas for general room lighting what is important is the ratio of the illuminance inside to that outside.

A common method of computing the absolute daylight illuminance is to find the point daylight factor at the task, and then multiply this by the external illuminance and an orientation factor. This is described in Chapter 5.

For people positioned sufficiently near to them, side windows can give excellent work illumination: they act as large sources of medium brightness but with sufficient directionality to enhance modelling. The colour appearance of the light varies (but common experience permits the user to compensate for this), and its colour rendering is excellent.

The most important factor is the relationship between the direction of the light flow and the sightline of the viewer. Two things can go wrong: the view through a window can be too bright in relation to the task luminance; and reflection of the window in the task can cause disability glare. For both reasons the best arrangement is usually a window to the side of the worker, rather than in front or behind; this is especially desirable with display screen tasks.

A downward flow of light from rooflights tends to give less satisfactory modelling than side lighting, with a greater probability of veiling reflections in horizontal work surfaces.

This is similar to the problem of direct downward electric lighting described earlier.

However, the combination of rooflights with side windows in a deep room can be very satisfactory, especially where the rooflights are designed to illuminate vertical surfaces at the back of the room.

SUNLIGHT

Sunshine flowing through a window brings brightness and warmth. It can be welcomed with pleasure in a cold building or can be regarded as a cause of intolerable discomfort in hot weather. It is the most powerful source of light that a lighting designer can choose; it can be interreflected specularly, used directly as a beam, or used to create a bright diffuse reflection. For example:

94 The design of lighting

• Small, changing patches of high brightness, giving sparkle and contrast to an interior. Of all natural images, few match the quality of woodland with sunlight penetrating the canopy, diverse beams making changing shadows on trees and ground, gleaming in patches of water. A shaft of sunlight in a room can be similarly modulated: by the glass of the window, the window framing; by blinds and curtains, semi-transparent or reflecting; by the geometry of room surfaces; by the surface materials—matt or shiny, white or coloured, flat or moulded. There is scope for complexity and surprise—the unexpected reflections on the ceiling, the interaction of surface hue with reflected colour, the drift with time of a sunlight patch.

• Large sunlight patches in a room, probably important as much for thermal reasons as for lighting. In cold and temperate climates, direct sunlight in a room is often welcome. In some conditions it is uncomfortable: visually, when it falls on tasks such as reading and writing; thermally, when the heat gain is excessive. But the use of direct sunlight is a traditional element of housing design, and passive solar heating is now a component of energy-saving architecture.

• Sunlight reflected into an interior from surfaces outside. In sunny regions, light from the diffuse sky is not the main component of interior daylight. Except around the sun, the luminance of a blue sky is often low, and therefore a weak source of light for the room; moreover, the sky itself is often screened from the window to reduce heat gain. The most useful source of interior illumination is sunlight reflected from exterior surfaces—primarily the ground but also from shading devices and other buildings. In this case the flow of light onto a window is predominantly upwards rather than downwards. Most of the light falls first on the ceiling; as a result the overall distribution of light within the space is more even than from skylight.

When people in cool climates have a reasonable expectation of sunlight in a room, both the size of the sunlit patch and the duration of the penetration are significant. Preferences for the area of sunlight tend to form an inverted U-shaped graph: there is an optimum size, typically 15–25% of the floor area, which is preferred to either larger or smaller patches. For a room to be considered a reasonably sunlit space, it was found in research in the UK that sunshine should enter it for at least 25% of the time that the sun is shining outside, with preferably at least one-fifth of this occurring within the winter months. The concept of probable sunlight hours is described in Chapter 5, with a calculation method given in Example (b) of Chapter 16.

Design for good daylighting in climates where the occurrence of sunshine is predictable differs from design for temperate or humid climates: orientation is even more important (because at low latitudes, heat gain is reduced by having windows facing north or south rather than east or west); external shading devices may be essential; and external surfaces should be planned to throw reflected sunlight onto the windows. Figure 10.2 shows how the different nature of daylight in a hot sunny climate induces an entirely different architectural approach.

In all climates, external reflections may be used creatively—sunlight falling onto coloured ground or water beneath a window gives rich hues or moving patterns inside.

The quality of light in a Greek temple depends crucially on upward illumination from sunlit ground; this is shown, too, on the ceiling of the Palladian arcade in Figure 10.3. But the view outwards through a window to a sunlit facade can be glaring. A white surface in sunlight may be a hundred times brighter than an interior room surface; a glass facade,

reflecting sunlight as a mirror, can cause the thermal and visual discomfort that occurs with a direct beam.

Figure 10.3 Cloister of S.Giorgio Maggiore, Venice.

And in all cases, control of sunlight is essential. In dry climates the use of fixed shading devices or a good choice of building form and window orientation may be adequate. In cloudy regions—temperate or tropical—the occurrence of sunlight is rarely predictable so shading must be adjustable if maximum use of daylight is required. It can operate automatically (such as with louvres that move under photoelectric control in response to changing illuminance) or can be operated by occupants. A good solution can be a combination of automatic and manual control, which allows occupants to adjust blinds when they wish but prevents the blinds from remaining closed continuously with unnecessary use of electric light. Curtains or blinds can give the simplest sunlight control, but in warm climates this is inadequate; external shading gives lower internal heat gain.

96 The design of lighting

Fittings such as light-shelves and louvres can do more than exclude unwanted sunlight;

they can be used to redirect light, to increase illuminance at parts of an interior distant from a window. Direct sunlight can be reflected or refracted, channelled through light-pipes or down light-wells to spaces deep inside. Clear glass can be replaced by materials or systems that redirect or attenuate the light falling on them, either passively, as with holographic, prismatic or laser-grooved glazing, or actively with glass that responds to illuminance or to user requirements.

Such systems only control the light entering: they do not increase its total quantity. The amount of light entering a room can never be greater than the amount falling on the corresponding external surface, whether this is a conventional window or the receiver of a solar-tracking device. Furthermore, the incoming energy is reduced by the means of transmission, and the more complicated the system the greater the loss. Clear double glazing with normal dirt can cause 40% attenuation; the loss in a system of mirrors and light-pipes can be far greater.

So under a diffuse sky, the value of light-shelves, louvres or light-directing glazing is only a relative redistribution of light; there is always a loss of total flux. Usually the intention is to increase the light falling at the back of a side-lit room, but such devices can be useful for screening bright views, and can be part of unusual window geometries, such as occur in an atrium where strongly downward light needs to be reflected into surrounding rooms.