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The careful introduction of lighting controls can ensure that the maximum use is made of the available daylight; so that the amount of artificial light is reduced automatically when all, or most of it, is no longer required to meet the design level.

The term 'daylight linking' has been used already, and this perhaps needs some explanation. It is used in the sense that the artificial lighting in a building is planned and controlled to support the natural light available during the day, to ensure that the combined lighting level meets the desired design level.

This can be done by planning the artificial lighting circuits so as to allow control by simple switching, so that those sources close to the window may be switched on only when required. Such unsophisticated means of control suffer from the human factor, in that once the artificial light close to the window is switched on it tends to be left on all day.

A more sophisticated method known as Permanent Supplementary Artificial Lighting (PSALI) was proposed by Prof. Hopkinson in the late 1950s; the first practical application of the technique being developed for the Esso Building (see Lighting Modern Buildings, p. 89) where there was dual switching for day and night, with the same lamp energy used throughout, but using the daylight available close to the windows to achieve the required design level when available. This still relied on the human factor to turn on the switch.

One of the greatest advances in the technology of lighting is in the development of modern control systems. These will be associated with light fittings which can react by photocell to the level of daylight available outside, enabling the design level to be maintained throughout the day, but offering considerable savings in energy.

The control system should be appropriate to the occupation of a space, and in a leaflet published by the British Research Establishment, Watford, UK, the following are identified.

1. Variable occupation. Occupants spend part of their time in the space, and part elsewhere, e.g. an office

2. Intermittent scheduled occupation, a meeting room

3. Full occupation, reception area

4. Intermittent occupation, storeroom areas.

Before deciding on the appropriate type of control it is useful to analyse the type of 'occupation' as above, as this may help to determine the nature of the control system.

It is unnecessary to dwell on the many types of control system, from 'intelligent' light fittings which react automatically to the ambient light level, adjusting the total light to meet the design level; to systems where each fitting may be controlled individually by an occupant to meet his or her needs by means of a manual controller, or groups of fittings which can be controlled by means of proximity switches, reacting to an occupant's presence.

It should be emphasized that the control system for a particular building or part of a building should be appropriate for its use, for example the control system for a church will be very different to that of an office or a factory. Each programme should be analysed and those areas of buildings where there is intermittent use, such as storage or warehouse, need to be provided with an appropriate control regime; if daylight is available, artificial light may not be required during the day at all, or by some means of occupancy or proximity switching.

Control systems are at the heart of energy savings, and daylight linking is an essential part of the solution, and may be linked into the BEMS (Building Energy Management System).

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