Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 128

200

728 942

150

100

50

0

400 500 550 600 700

λ (nm)

400 450 500 550 600 650 700 450 650

λ (nm)

Fig. 10.15 Spectral power distribution of two metal halide lamps with colour designations of 728
(T k = 2800 K and R a = 70) and 942 (T k = 4200 K and R a = 90) respectively

Lumen Range Typical versions of the metal-halide lamps developed for use in road
lighting are available in lumen packages from some 5000 to 50,000 lm. The versions
for floodlighting range from some 20,000 to more than 200,000 lm.

System Efficacy The road-lighting versions range from 80 to 110 lm/W, and the
floodlight versions from 75 to slightly more than 105 lm/W.

Lifetime The ceramic metal halide lamps specifically developed for use in road
lighting have lifetimes of up to 20,000 h (basis L 80 ). The floodlight types have shorter
lifetimes ranging from 4000 to 10,000 h.

Run-Up and Re-ignition The metal halides in the discharge tube need time to heat
up, evaporate and dissociate into metal and halide. During this process, which takes
about 2-3 min, the light output and colour gradually change until the final stable
condition is reached. If there is an interruption in the power supply, medium and
high-wattage lamps will take approximately 10-20 min for the pressure in the lamp
to decrease enough for it to re-ignite. Compact ceramic lamps reignite faster: after
some 3-10 min.

Dimming Dimming of metal halide is difficult because with the resulting decrease
in temperature some of the metal halides may condense, so changing the colour prop-
erties of the lamp. Only by employing specially shaped burners can this phenomenon
be avoided. Most metal halide lamps specifically developed for road lighting can in-
deed be dimmed to at least 50 % of their light output. The lamp types used mainly
for floodlighting cannot be dimmed.

Bulb Shape Metal halide lamps used for road lighting are produced in different bulb
shapes. The most important ones are the tubular shape similar to the shape of the
tubular high pressure sodium lamp, an ovoid shape (coated with a diffusing powder)
similar to the ovoid high-pressure mercury and sodium lamps and a compact tubular
one with a special bi-pin lamp cap to ensure exact positioning of the burner within
the optical system of a luminaire.