Handbook of Electrical Installation Practice - BeKnowledge

482 Handbook of Electrical Installation Practice
restored. High-pressure sodium lamps recover a great deal more quickly than
other types.
Fluorescent tubes
A fluorescent tube is a low-pressure mercury discharge lamp. At this pressure, the
mercury arc emits very little visible light and a considerable amount of UV radiation
in two distinct wavebands, one in the harmless near-visible region and the other
in the shorter-wave therapeutic area. This emission is used to excite fluorescence in
the phosphors coating the inner surface of the tube, which may be typically up to
2400mm long and 38mm (T12) or 26mm (T8) in diameter according to the power
rating.
Recently a new range of lamps (T5s in ratings from 14–54W) have been introduced.These
operate only from special electronic control gear and are slimmer than
standard tubes (16mm). Lamp efficacy is much improved. Data for the majority of
types of tube are shown in Table 18.2.
Two recent developments are the introduction of high efficacy fluorescent tubes
utilising the rare earths developed for colour television as well as the conventional
halophosphates, and krypton gas to facilitate starting. Because the light from the
phosphors is produced mainly in the three areas of the spectrum, blue, red and
green, which combine to give the effect of white light, instead of having a virtually
continuous spectrum like the conventional type of tube, they may occasionally give
rise to the effect of ‘metametric mismatching’ of colours. In situations where
accurate colour-matching is essential, therefore, the standard halophosphate tubes
should be used.
In 1981 compact fluorescent lamps, such as GE’s 2D and Philip’s PL and SL, were
introduced as an energy-saving replacement for the gls lamp. The 2D lamp is a
520mm long by 13mm outside diameter tube bent into a tight, double ‘D’ shape
and supported by a central mounting plate which incorporates the starter. Consuming
only 21 circuit watts, the 16W 2D lamp has a light output approaching that
of a 75W lamp, but saves around 70% of the energy. Substantial savings are possible,
especially where lights are in continuous use. But energy saving is not its only
feature. It has a life ten times longer than a filament lamp, is comparatively cool
running and is similar in colour appearance to an incandescent lamp (Table 18.3).
Improvements are growing all the time and while breaking away from the 2D shape,
but still utilising the principle, two limb, four limb and six limb lamps are available
in ratings from 5W to 55W.They have become a compact alterative to conventional
linear fluorescent tubes.
Induction principle
An induction lamp is somewhat like a fluorescent. It contains a low pressure of
mercury which, when excited, radiates UV, which in turn is absorbed by a phosphor
coating and reradiated as visible light. However, the induction lamp has no electrodes
and the discharge is created by a magnetic field generated externally to the
bulb. Lamp life can be 60000 hours and the lamps operate at high frequency so there