Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 121

lamp
cap

bi-metal
starter

electrode gas discharge tube

with fluorescent coating

open
bridge

Fig. 10.9 Main parts of a compact fluorescent lamp (welded-bridge version)

and a little mercury. To facilitate starting, the electrodes sealed into each end of the
tube are pre-heated prior to ignition by a high-voltage pulse. Once the mercury is
completely vaporised, an operating gas pressure of slightly less than 1 Pa ( 10 − 5

atm) is reached. The radiation emitted from the low-pressure mercury discharge
contains a large amount of ultraviolet radiation and a small amount of blue light.
The inside of the tube is therefore coated with a mixture of fluorescent powders that
convert the ultraviolet radiation into visible light as it passes through this coating.
The final result is white light, the colour properties of which depend on the mix of
fluorescent powders employed.
The compactness of these lamps is achieved by reducing their length by folding
a longer tube into a U-tube, or by joining together two (or more) parallel tubes by
means of a welded bridge so that one open pathway is obtained where free electrons
and ions can move from one electrode to the other (Fig. 10.9 ). A great variety of
tube shapes are available. Versions with the electric gear integrated in the lamp itself
and with a lamp cap as in normal incandescent lamps are produced as energy-saving
retro-fits for incandescent lamps. The compact fluorescent lamps employed in road
lighting are normally of the non-integrated type and the gear needs to be installed
in the luminaire. They are of the twin-leg or circular versions. The twin leg versions
have a length, depending on wattage, of between 100 and 600 mm.

±

10.2.5.2 Lamp Properties

Spectrum and Colour Properties As has been mentioned, different spectra can
be produced by mixing different fluorescent powders. As with all gas discharge
lamps the spectrum is always discontinuous. The colour-type designation used for
fluorescent lamps has been standardised, with the first digit standing for the colour
rendering index Ra, and the last two digits for the colour temperature T k . A type of
compact fluorescent lamp often-used in street lighting is the colour type 840, which
has a colour rendering index in the 80s and a colour temperature of around 4000 K
(Fig. 10.10 ). The S/P ratio is approximately 1.5 (for colour type 830 it is about 1.3).

Lumen Range Non-integrated compact fluorescent lamps are available in the range
from some 250-6000 lm, corresponding to the wattage range of 5-80 W.