Lighting

Chapter 7
Lighting
Introduction
Illumination and the design of lighting layouts is a subject on its own. There are books
dealing comprehensively with it and it is not proposed to condense the matter into a
single chapter here, but once a lighting layout has been arrived at, it is necessary to
design the circuits, wiring and protection for it; this is an aspect of lighting design which
tends to be overlooked in books on illumination and which we propose to discuss in this
chapter. The electrical requirements of a lighting system depend to a considerable extent
on the kind of lamps used and we shall describe the different available types in turn.
Incandescent lamps
These are also known as tungsten lamps, or GLS general lighting service, and are in fact
the ordinary bulbs still most commonly used in homes. They consist of a thin filament of
tungsten inside a glass bulb. When a current is passed through the filament, heat is
produced and the temperature of the filament rises. The filament is designed so that it
reaches a temperature at which it generates light energy as well as heat, which means that
the filament glows or is incandescent and hence the lamp is called an incandescent one.
The higher the temperature of the filament, the more efficient is the conversion of
electrical energy into light energy, but if the temperature becomes too high the filament
melts and breaks. Tungsten has a melting point of 3382°C, and most modern lamps have
filaments running at about 2800°C, although some special lamps may run at 3220°C.
The colour of the light produced depends on the temperature, becoming whiter as the
temperature rises. At 2800°C, it is rather yellow, but as no material is known which can
be operated at a higher temperature than tungsten, lamps of this type cannot be made to
give a daylight colour.
To prevent the filament from oxidizing, all the air must be evacuated from the bulb,
and the early lamps were of the vacuum type. It was found that in a vacuum tungsten
evaporated and blackened the inside of the bulb. This problem has been solved by filling
the bulb with an inert gas at a pressure such that when the bulb is hot the pressure rises to
about atmospheric pressure. The gas used is generally a mixture consisting of 93 per cent
argon and 7 per cent nitrogen. Unfortunately, the gas conducts heat from the filament to

the bulb, thus lowering the temperature of the filament and reducing the efficiency and
the light output of the lamp. To overcome this effect as much as possible, the filament
must be wound to take up as little space as possible. It is for this reason that gas filled
lamps have the filament either single coiled or coiled coil.
The connections of the filament are brought out to the lamp cap. This is the end of the
lamp which fits into the lampholder when the lamp is put into a luminaires. Lamp caps
are shown in Figure 7.1. The commonest is the two-pin bayonet cap (Figure 7.1a) which
is standard in the UK for all sizes up to and including 150W. It can be inserted into the
lampholder either way around and for applications where the lamp must be fixed in one
position only a three-pin bayonet cap is available. Clearly a special lampholder is needed
for it. Figure 7.1c shows the Edison screw cap, in which the screw, thread forms one of
the terminals. The Edison screw cap is made in five sizes covering all types of lamps
from street lamps to flash bulb lamps, but the only common sizes used in this country are
the Goliath (usually abbreviated to GES), small (SES) and miniature (MES). Bulbs of
200W and over use the GES cap.
It will be noticed that the traditional Edison screw affords slightly greater risk of
accidental contact with the terminal when one is putting a bulb into
Figure 7.1