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 (seeLighting 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.
- Variable occupation. Occupants spend part of their time in the space,
and part elsewhere, e.g. an office - Intermittent scheduled occupation, a meeting room
- Full occupation, reception area
- 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).
Energy 41