Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 126

Lumen Range High-pressure mercury lamps are available in lumen packages be-
tween some 2000 and 60,000 lm, corresponding to wattages between 50 and 1000 W.

System Efficacy Luminous efficacy varies with lamp wattage and with colour
quality from some 35 to 60 lm/W.

Lifetime Economic life based on L 80 varies according to type between 10,000 and
15,000 h.

Run-Up and Re-ignition The run-up time of a high-pressure mercury lamp to its
operating temperature and corresponding nominal mercury pressure is some 4 min.
The hot lamp will not restart until it has cooled sufficiently to lower the vapour
pressure to the point at which re-strike with the mains voltage is possible. The
re-ignition time is in the order of 5 min.

Dimming High-pressure mercury lamps cannot be dimmed.

10.2.8 Metal Halide Lamps

Metal halide (MH) lamps are high-pressure mercury lamps that contain metal halides
in addition to the mercury. These, too belong to the group of high-intensity dis-
charge lamps. In the heated discharge tube, the metals of the halides take part in the
discharge process and radiate their own spectrum. Compared with high-pressure mer-
cury lamps, both colour properties and efficacy are considerably improved. Thanks
to the fact that no fluorescent powder is needed, the small gas-discharge tube it-
self is the light-emitting surface. This small light-emitting surface makes the lamps
extremely suitable for use in reflector and floodlight luminaires.
Originally, these lamps were produced in extremely-high lumen packages (up to
more than 200,000 lm) for use in sports stadium floodlighting. Their lifetime, at
some 6000 h, was short but more than good enough for stadium and sports lighting
where the annual burning hours are limited. Today, compact metal halide lamps are
available in small lumen packages with much longer lifetimes, which makes them
suitable for use in road lighting. In urban and residential areas they offer white light of
high quality with efficacies slightly higher than those of high pressure-sodium lamps
of comparable lumen output. LED lamps have now become another alternative for
these applications.

10.2.8.1 Construction

Figure 10.14 shows the construction of a metal halide lamp. The discharge tube
is made of either quartz or a ceramic material. Some metals of the metal-halide
compounds have the tendency at high temperatures to migrate slowly through the
quartz wall of the tube. With ceramic discharge tubes this is impossible, so that
the lifetime improves and the colour quality during life remains constant. All metal