It’s now widely accepted that natural daylight not only provides the best environment for
people to work in: it saves energy and reduces CO2 emissions by reducing the need for electric lighting.
However, the major concern with saving energy is not that the artificial lights get switched on; it is that they do not get switched off when they are not needed. For energy saving to be achieved, artificial lighting must be used with automatic control systems.
For this reason it is important to consider automatic lighting controls when specifying rooflights – particularly in applications where building users may not have direct access to manual controls, – or where they may not have responsibility for switching lights on and off.
A control system can turn lights ON or OFF and, in most cases, provide a dimming capability. These functions can be invoked in response to available daylight, occupancy sensing or according to the time of day. The best applied systems will also provide the user with a ready means to request such functions. The dimming and occupancy functions are those that are most affected by the choice of lighting equipment.
Dimming is easily the most effective way of managing artificial light in response to the availability of daylight. As soon as there is a daylight contribution the electricity use can be reduced as the artificial lighting smoothly dims. Dimming, when done effectively, is barely noticed by the occupants and it always ensures that the minimum required lighting level is achieved. In contrast, if the lighting equipment is not compatible with dimming, then daylight linked switching can be used but the switch OFF can only occur when the daylight contribution exceeds the required lighting level by a factor of three (or more). Great care also needs to be taken when setting the switch ON and OFF levels to ensure the lighting does not ‘hunt’. Dimming is also a valuable function when combined with occupancy and manual controls.
In most applications the daylight sensors are either pointed North to take the sample of the current daylight level or internally mounted to sample the integrated light level (i.e. the combination of artificial and natural light). Any such sensors need to be dedicated to this task and be designed to match, as closely as possible, the eye’s sensitivity to the light spectrum. Daylight sensors should not be placed behind (for example) the lens of a passive infra red occupancy detector.
Occupancy sensors (or movement detectors)
Lights are not usually needed if there is no-one present. Effective movement detectors have been available for many years and there are now three technologies used.
• Passive infra red (PIR): This method uses an infra red sensor to ‘see’ the controlled area and keep lighting ON when there is a moving infra red signature present – i.e. there is someone there.
• Microwave: This method is used in larger rooms and detects movement using the
Doppler shift phenomenon to confirm that something is moving within the observed
• Ultrasonic: This method also relies on the Doppler effect but has a smaller range than the microwave units.
Further information about designing with Rooflights and Controlled Artificial Lighting can be obtained in NARM Technical Document NTD 05.