Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 127

discharge tube

lamp cap outer bulb electrode getter

Fig. 10.14 Main parts of a metal halide lamp in this example with a ceramic gas discharge tube

halide lamps used in road lighting have either a vacuum or an inert-gas filled outer
bulb for heat insulation of the discharge tube. As with most gas-discharge lamps, the
outer bulb contains a getter to maintain the vacuum during the life of the lamp.
Suitable metals that vaporise in the hot discharge tube so as to contribute to the
discharge process cannot be added directly to the mercury in the discharge tube. This
is because metals suitable for this purpose would attack the wall of the tube. The
solution to this problem has been found in adding these metals in the form of their
non-aggressive chemical compounds with a halogen (iodine, bromide or chloride):
hence their name—metal halide lamps. The solid metal halide starts evaporating at a
certain temperature. When this vapour enters the area of the mercury discharge in the
centre of the tube, with its very high temperature (around 3000 C), it dissociates into
its separate elements: metals and halogen. There the metals in their pure, vaporised
state, take part in the discharge process where they are important in determining the
efficacy and colour characteristics of the radiation. The aggressive vaporised metal
cannot reach the tube wall because at the lower wall temperature (some 1000 C),
they recombine again to form the harmless metal halide compound. In fact, so-called
rare-earth metals are used that excite more easily than does mercury. This means that
mercury in a metal halide lamp does not take part in the generation of light but merely
serves to keep the discharge going through a process of heat and voltage regulation
(buffer gas).

10.2.8.2 Lamp Properties

Spectrum and Colour Properties The spectrum of a metal halide lamp is to a
large extent determined by the mixture of rare-earth metals used. In theory, some
fifty different metals are available and different manufacturers have introduced var-
ious combinations of these metals that produce different spectra. The same colour
designation system that is used for fluorescent lamps is used for metal halide lamps.
Figure 10.15 shows the spectra of metal halide lamps used in road lighting with
colour designation 728 and 942. Metal halide lamps are produced in versions in the
colour temperature range of 2700-4500 K and colour rendering indexes from 60 to
90. The S/P ratio of metal halides is for warm-white versions around 1.25 and for
cool-white versions around 1.8.