Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 124

Lumen Range Induction lamps are produced in the range from 3000 to 12,000 lm.

System Efficacy The luminous efficacy of induction lamps is smaller than that of
normal fluorescent lamps. Depending on the wattage it varies between 65 and 75
lm/W.

Lifetime The lifetime is extremely long, based on L 20 , between 60,000 and 75,000 h.
Run-Up and Re-ignition A high-voltage ignition pulse produced by the HF gen-
erator ignites the lamp within 5 s, after which it emits its full light output within 1
min. Hot re-ignition is immediate.

Dimming Special versions, dimmable to 50 % are available.

10.2.7 High-Pressure Mercury Lamps

High-pressure mercury (HPM) lamps belong to the group of high-intensity discharge
HID lamps. High-pressure mercury lamps, like all high-pressure discharge lamps,
are compact compared to low-pressure discharge lamps. HPM lamps have a moderate
efficacy and moderate colour rendering. With their cool-white light they were exten-
sively used in road lighting, especially in built-up areas. Since the introduction of
the more efficient high-pressure sodium lamps in the late 1960s, these high-pressure
sodium lamps have in most cases replaced high-pressure mercury lamps.

10.2.7.1 Construction

Figure 10.12 shows the construction of the high-pressure mercury lamp. The dis-
charge tube is made of quartz, which has a higher melting point than that of glass
required in view of the high operating temperature of the lamp. An ovoid outer bulb
with an inert gas filling isolates the gas discharge tube so that the ambient tempera-
ture has no influence on its proper functioning. To aid starting, an auxiliary electrode
is placed close to one of the two main electrodes. Because of this, the high-pressure
mercury lamp is one of the few gas discharge lamps that does not need an external
ignition device. The discharge tube contains a small quantity of mercury which com-
pletely evaporates during operation. The operating pressure is 10 6 Pa (10 atm). The
radiation of the gas discharge is mainly in the visible part of the spectrum but lacks
red. In most high-pressure mercury lamps, therefore, a fluorescent powder coating
on the inner surface of the outer bulb is applied. This converts the small ultravio-
let component into visible radiation, predominantly in the red. This does little to
increase the efficacy, but improves the colour quality of the white light.